AR308 Short Barrel, Long Gas? Discussion/Build

[A.James]

Hello everyone! This will be my first post after a lot of research; I’ve found a lot of information regarding my planned goals for this build, but its only helped in narrowing things down, not necessarily getting fully dialed in. But I now know (based on others’ build successes/failures and re-workings of their systems) that my albeit Swiss cheese-like plan will have a probable success after balancing and tuning the system. I’m going to try to bring up some talking points in this thread that I don’t think I’ve found in the archives, perhaps no one has tried this idea yet, so this all might be an entirely theoretical discussion on the physics of a self loading rifle in a hypothetical build. Maybe I’ll end up being the first to try these ideas in the real world, and could potentially offer some anecdotal evidence to prove or disprove our assumed functionality. If this all goes well, I’m sure others may want to try what I’m going for.

 

I’ll first outline my objectives before asking any what-to-buy advice, listing any parts I plan on moving forward with. From there we can discuss compatibility of parts, and the likelihood that said combinations will achieve the objectives outlined first. From there I’ll begin a discussion regarding differing gas systems for a set barrel length with a full-time suppressor, highlighting internal pressures and dwell time… the system has to operate, and operate well, always. I’ll finish up with some parts I’m selecting for what-to-buy advice. NOTHING has been purchased yet.

 

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OBJECTIVES:

 

1. ***CHAMBER 7.62/.308***

 

Researching ballistics, I have found that up to 400-500-600m (load dependent), 7.62x51mmNATO A.K.A. 0.308 WIN carry the most energy downrange at supersonic speeds (when compared with .556NATO or 6.5CM or .300BLK etc.). It would seem that as we surpass 500-600m range, favor goes to 6.5CM, producing better numbers in velocity AND energy than 7.62/.308, but doesn’t have as much punch within the short-mid range. I see my proposed system as one that won’t be needed past that 500m range, and I’d rather have a second system chambered in 6.5CM for long range accuracy. So I’ve decided to chamber in 7.62/.308 for my needs. I also plan on using a DPMS pattern barrel and upper receiver - based on what appears to be more available and have [slightly] more cross-manufacturer compatibility.

 

2. ***FIND OPTIMAL BALANCE/TRADE-OFF BETWEEN BARREL CLEARANCE (CQB) AND VELOCITY/DOWNRANGE ENERGY***

 

In my research of SBR AR-10/308LR, I’ve found that 12.5” length doesn’t always “stretch” to those longer range shots, 16” may be great but I’d rather save the extra length since a lot of people on here have touted their 14.5” .308’s as their favorite guns. I also think the 14.5” can “stretch” out past the 500m mark, though with lesser accuracy than the longer barrel. Again, looking for optimal balance here, I don’t think I’m too far off wanting a 14.5” - including that length being the minimum barrel length supported by the manufacturers warranty on the suppressor I plan to utilize.

 

3. ***MAINTAIN MY HEARING FOR AS LONG AS POSSIBLE, WITHOUT ABANDONING OBJECTIVE #2***

 

I want a suppressor on this barrel 100% of the time, except for cleaning - I’m not going to pin a suppressor to the barrel, but I do plan on using a brake or flash hider with a threaded suppressor mount. Looking at the Ultra 7 CB, and I hear that their flash hider with CB mount is long enough to pin, avoiding an SBR stamp. Might do that. This can adds a lot of length, but is still a relatively lighter can, aiding in handling - and hell! Soldiers used to be issued 20” M16’s and trained for CQB with them back in the day… probably had a 2” flash hider on the end? A full length rifle stock? Maybe? Either way, I’m looking at 20.9” OAL with suppressor mounted, so still shorter than these guys had to deal with. I think I’m pushing away from my goal of short length now, but the suppressor is non negotiable, and technique can help to mitigate a longer stock-muzzle distance. If you can point me toward a device that can get down below 135 db on .30 cal, that is as light as the Ultra series, has a threaded mount AND is less than 7” - I’d love to hear more! Very interested in shaving length, and mass for that matter - especially at the muzzle end.

 

4. ***KEEP OVERALL MASS AS LOW AS POSSIBLE***

 

I’m not trying to pay premium prices for space-grade materials… at least not on every part… but yes, I’ll pay a couple extra bucks if I can save a few ounces…maybe…? Gotta keep it light out in the bush!

 

5. ***ENSURE SMOOTHEST OPERATION***

 

For accuracy/follow up shots, failure prevention, longevity/reliability/dependability, operating bandwidth in extreme conditions, etc. Let’s talk gas systems!

 

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After researching gas system lengths, reading as much info as I possibly could find, it seems to me that any given gas length with port less than ~5” away from the muzzle will not have enough dwell time to cycle smoothly… at least on the “medium-length” barrels, like the 14.5” we are discussing here. On the 14.5” length, this would imply that a mid gas system would have an optimal pressure for cycling a moderate-mass buffer weight. I have reason to believe that in my goal to use a suppressor 100% of the time, in conjunction with my browsing of the forums that all seem to echo each other, that a longer gas length than the aforementioned mid length may in fact cycle even smoother. I’ve heard anecdotes hinting that suppressors on the already “over-gassed” AR308 platform almost always produce significant back pressure; and increase dwell time, though not as much as trading your suppressor length for additional barrel inches. I’ve read many people address this issue by increasing buffer weight mass and/or utilizing an adjustable gas block (which I also intend to implement on this build) in order to tune the rifle into operating parameters. From what I understand from my research and interpretation, the longer gas system will allow more pressure dissipation before the round is extracted. This lower pressure wouldn’t move a high-mass buffer weight, and switching in a couple steels for tungstens might have a desired effect. This is what I think of when people say longer gas lengths are “softer shooting” than shorter gas systems, due to lower pressure pushing lesser mass at a slower speed of travel. All of these comparisons may be minute, but it all adds up in the end, perhaps with longer gas putting less long term wear on the components…? Shorter gas systems require heavier buffers and operate with a lot of force compared to longer gas systems that require lower mass buffers and more dwell time. Please correct me if I’m wrong or misguided here.

 

I have a dream to have rifle gas on the 14.5” barrel, but that 2.5” of barrel after port wouldn’t give enough dwell time for that lower pressure rifle gas to do its thing… but with the suppressor on there? My main theoretical question regarding my gas length to barrel + suppressor length ratio vs. dwell time is this:

 

Would rifle gas (12” port distance) on a 14.5” barrel + 6.4” of suppressor = 20.9” OAL have enough dwell time to function and cycle? Any drawbacks here? My train of thought = less pressure, less mass, smoother shooting - might last longer but might jam easier (thinking of dirt/mud/rain)?

 

I’m still open to the idea of implementing a mid gas system with a buffer weight of greater mass to produce higher internal pressures to assist “dependability” factor in inclement and/or “gunky” conditions. My train of thought = more pressure, more mass, harder shooting - might be rough operating, wear faster, etc., but might easily push through jams/operate in less-than-optimal conditions?

 

Please advise!

 

- - - - - - - - - - -

 

So after we talk a bit here and theorize the probability of system operation, I’ll be able to decide on more parts, but we gotta start somewhere, so here we go. Feel free to critique my choices, but please cite my objectives above - explain how your recommendation helps to achieve the objective better than my initial choice.

 

***Need barrel recommendations that shoot well but don’t break the bank… Ranier, Criterion, X-Caliber??? I’m ready to pay $200 to $350-ish on a 14.5” - we will decide on gas length via our discussion started above… Feel free to justify slightly more expensive barrels, just aim for my price range, please! Custom is not off the table.

 

Looking into Aero Precision M5E1 Enhanced upper (stripped?), enhanced M-Lok 15” hand guard, and (stripped?) lower… curious about using carbine length receiver extension to grab a shorter stock for shortest end to end length (objective #2 and #4…might compromise objective #5?). Thoughts? RE: Stripped upper, I feel that if I’m doing some experimental short barrel / long gas thing that hasn’t been attempted much before, it may be smarter for me to choose compatible parts to balance my wonky mismatched barrel configuration, rather than take the pre-fab which may or may not like my barrel/gas choice. I don’t want to be switching parts, I’d rather calculate component choices and buy once. Plus if I switch out parts that I don’t end up selling, they’ll probably start talking to me and convince me, “Build me! Build me!” Then I’ll have to sell the farm… If we conclude that mid gas is best, I have no worries grabbing an assembled upper from Aero because I know that a 14.5” mid length operates as expected out of this platform based on anecdotal evidence.

 

I’ll look into triggers and other components of the lower as my barrel and upper start to come together, as well as fielding recommendations from my new friends. I hear good things about Magpul and SLR, Giselle… Thanks in advance!

 

Haven’t taken the time to look into adjustable gas blocks yet, have seen a lot of people succeed with SLR R.W. brand… let me know if you’ve had good experiences with this company, or suggest other manufacturers. I’ll get there, still really interested on designing the barrel first, then everything else will fall into place from there based on the needs/balancing of the entire firing system. Just ran into a road block with my research vs. my goals/dreams regarding this barrel configuration I’m proposing.

 

TBAC Ultra 7 CB, like I said, PLEASE point me toward a suppressor that upholds my objectives and is comparable to the Ultra 7, without as much length. Otherwise, my research points me toward this one. Pretty sure this is what I’ll end up with, but I’m COMPLETELY open minded to other suggestions. Again, please, show me something I missed and I’d love to find a shorter alternative. KAC has great reviews and built like tanks, have a great QDC platform (many say best in industry), but the cost and weight doesn’t justify shaving 1/2” on the QDC/CRS. Even the cost of the QDC flash hider is considerably more than what I’m looking at with TBAC. Speaking of which, trying to decide between the CB brake + SBR stamp (OAL would only come out to 15.63 with their brake) vs 16” OAL with the CB flash hider, no stamp = I’m happier +1 for the flash hider. The suppressor mount of these muzzle devices is at the same location so the OAL only differs when suppressor is not installed, which in my case will be NEVER. Alternative option to pin brake to a 14.9” barrel, but then we lose another 1/2” of clearance. +2 flash hider. My barrel will be shorter than my hand guard, the brake would banana that biotch, the flash hider *MIGHT* be safe to fire with the overhang from the long hand guard. Another alternative would be to grab two hand guards, 12” for unsuppressed (I might want to try this if I can build the thing before NFA clears a suppressor), and the 15” when suppressed. Assuming I stick to 14.5”, not 14.9” then that means another stamp if I opt for the brake, so if the flash hider is good to go under a 15” hand guard, that’s +3 flash hider. From my understanding, brakes assist in slowing wear to the baffles of suppressors, whereas the flash hiders inside our suppressors might mitigate powder wear to certain baffles, but concentrate the blast between the flash hider’s tines. +1 brake. Unless this damage mitigation is truly worth +3… or more… I don’t see the point of spending another chunk of change for an SBR stamp and a second hand guard to run this brake… I’ll be suppressed, that’s more important. Agree with me, or change my mind. I just need some back up on this call to pin a flash hider to my 14.5” to reach 16” OAL and avoid a stamp and forget about a shorter hand guard. I’m tall and lanky, I like a longer grip, and the hand guard over suppressor look is definitely something I’m into. Nuff said.

 

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Thank you so much for taking the time to open this can of worms, I can’t wait to get this thread roaring with opinions and experiences of using longer gas lengths in shorter barrels running a suppressor, especially on the 14.5” barrel (fingers crossed that someone has tried suppressed / rifle gas already). Let me know if there’s anything I can elaborate on or improve with my post, and thank you in advance for any advice you can give! Wanting to have this thing finished by 2021, assuming ATF “does their part…” LOLOLOLOL!!!

 

-A. James

[DNP]

You can go way less on the dwell time. I run  16” Fulton (criterion) with rifle gas. I forget the number but the dwell is like 2” or less. It cycles a lot of ammo just fine. Gas port size matters. Couple that with the fact that you’ll be running a suppressor 100% of the time....dwell ain’t gonna be an issue for you. 
 

the barrel I’m running is not light. I have a PRI tube on it, again not light. Unfortunately I live in a communist no suppressor state, so I have no help for you there. 

[98Z5V]

Post an intro in the Intro Section and tell us about yourself.  You made one HELL of a first post, and you need alot of information from this board, it appears - so reciprocate.

Go hit this, tell us about yourself.

https://forum.308ar.com/forum/22-introductions/

Smack away about YOU...  give and take here.  Become part of the place, and you'll get all the info that you'll need. 

Welcome aboard. 

[A.James]

19 hours ago, DNP said:

You can go way less on the dwell time. I run  16” Fulton (criterion) with rifle gas. I forget the number but the dwell is like 2” or less. It cycles a lot of ammo just fine. Gas port size matters. Couple that with the fact that you’ll be running a suppressor 100% of the time....dwell ain’t gonna be an issue for you. 
 

the barrel I’m running is not light. I have a PRI tube on it, again not light. Unfortunately I live in a communist no suppressor state, so I have no help for you there. 

Thanks for your response! I appreciate the accumulation of information in a centralized place... That's my main goal in posting, not solely to find help with my own build - but to provide a discussion for all time to come that anyone researching similar topics may find useful in a context of 14.5" rifle gas... I haven't found a single thread on this topic. We will ALL get to the bottom of this. Your experience echoes my theoretical knowledge of this platform. Despite feeling like I had no other unraveling threads to follow yesterday when I posted, I've gathered more information about the entire system functionality based on buffer systems. I feel like just about anything can be tuned to work together... especially when having an adjustable gas block. My research today is centered around buffer springs and mass and how they relate to gas lengths. I may be on to something here.

And LOL @ your state of residency comment... I know the word "communist" is thrown around a lot, but... I'd say communism means = EVERYONE GETS A SUPPRESSOR!!! (although the shittiest one you can imagine hahahah)

[DNP]

I would gladly take the shittiest one I could get over none at all. Bernie didn’t tell me we’d all get suppressors..  I might have to change my vote. 

[A.James]

59 minutes ago, DNP said:

I would gladly take the shittiest one I could get over none at all. Bernie didn’t tell me we’d all get suppressors..  I might have to change my vote. 

Hahahahah I certainly don’t think the 2nd amendment is in jeopardy regardless of who sits at the end of the puppeteer’s strings 😉 love your energy though!

Let’s see if we can’t find anymore stories from other users! I’ll make a more in depth post with links when I get home to show what I’ve been reading into today!

Thanks again for your participation, you’re my first replied, I owe you now... lol

[98Z5V]

On 3/19/2020 at 4:00 PM, A.James said:

Would rifle gas (12” port distance) on a 14.5” barrel + 6.4” of suppressor = 20.9” OAL have enough dwell time to function and cycle? Any drawbacks here? My train of thought = less pressure, less mass, smoother shooting - might last longer but might jam easier (thinking of dirt/mud/rain)?

You're not going to be able to run a rifle gas system on a 14.5" barrel.  I think when you reference the 12" number, that was a number for common rifle length handguards, but it's not gas tube lengths. 

An Armalite AR-10 Rifle gas tube is 15.5" long.  That's more gas tube than you have barrel length, which would be measured from breach-face to barrel end.  The gas tube extends into the upper, yes, but even if it's close, that places your gas port right at the end of the barrel (if it's not beyond the end of the barrel).

If you're dead set on 14.5" barrel, you'll have to have the muzzle device pinned and welded, or silver-soldered, or Tax-Stamp it as an SBR.  If you don't Tax Stamp is as a legal SBR, and you don't permanently attached the muzzle device, then you've just created an illegal SBR.  That's Felon Status, right there.  Not worth it.

Here's a pic of a 16" rifle-gas .308AR - look closely at how close that gas block (and the gas port) is to the end of that barrel.  There's not another 1.5" of barrel to lose on a rifle gas setup.

 

Here's a 13.5" barrel,  AR-10 Carbine gas system (a touch longer than AR15 midlength gas systems), and a permanently attached muzzle device, bring total barrel length to 16.1".

Here's gas tube length information:

Posted October 12, 2018 (edited)

Gas tube lengths, courtesy of @survivalshop:

DPMS Gas Tube/AR 15/16

 

  Pistol             6-5/8

  Carbine          9-3/4

  Mid-Length  11-3/4

  Rifle             15-1/4

 

 

 

 

Armalite lengths,

 

Rifle ( 308 ) ----- 15.5 "

Carbine ( 308 )--12 1/16"

super SASS Carbine ( 308 )--11"

Super SASS< Rife (308 )   ?

Mid ,( 223 )------------------ 11 3/4"

Carbine ( 223) --------------- 9 3/4 "

Rifle ( 223 ) ------------------- 15 3/16"

AR15 National Match---------15 3/16"   (That's myinformation, from the one I have here...)

[edgecrusher]

So, I read your post and didn’t see you planned shooting range desired. I saw the bit about energy, but not your purpose for this weapon. I will also ask your experience in shooting as well, for my own info to assist you. Nothing wrong with a 12.5” out to 400. 16” to 800. The thing I will correct you on is your statement on accuracy. The shorter barrels are no less accurate but suffer from velocity loss resulting in drop, not a loss in accuracy. Wind will affect a projectile more with less weight and velocity. 

[DNP]

To add to what edge was saying, wind will affect it more due to the slow velocity. It takes longer to reach the target, therefore the wind has more time to affect it. 

[A.James]

16 hours ago, 98Z5V said:

You're not going to be able to run a rifle gas system on a 14.5" barrel.  I think when you reference the 12" number, that was a number for common rifle length handguards, but it's not gas tube lengths. 

An Armalite AR-10 Rifle gas tube is 15.5" long.  That's more gas tube than you have barrel length, which would be measured from breach-face to barrel end.  The gas tube extends into the upper, yes, but even if it's close, that places your gas port right at the end of the barrel (if it's not beyond the end of the barrel).

If you're dead set on 14.5" barrel, you'll have to have the muzzle device pinned and welded, or silver-soldered, or Tax-Stamp it as an SBR.  If you don't Tax Stamp is as a legal SBR, and you don't permanently attached the muzzle device, then you've just created an illegal SBR.  That's Felon Status, right there.  Not worth it.

Here's a pic of a 16" rifle-gas .308AR - look closely at how close that gas block (and the gas port) is to the end of that barrel.  There's not another 1.5" of barrel to lose on a rifle gas setup.

 

Here's a 13.5" barrel,  AR-10 Carbine gas system (a touch longer than AR15 midlength gas systems), and a permanently attached muzzle device, bring total barrel length to 16.1".

 

 

This is an awesome post! Thank you!

I believe I was referencing this page, https://www.at3tactical.com/blogs/news/9298047-carbine-mid-or-rifle-a-beginner-s-guide-to-ar15-gas-systems and now I realize this is describing AR15 gas lengths... I thought when comparing "rifle" vs "mid-length" vs "carbine" we were talking about set distances, regardless of the platform... I stand corrected. But it seems the page I cited in this post is just UTTERLY wrong? Based on your measurements, the .308 and .223 have the same numbers? Then WTF is up with this info I've found. This is precisely why I needed some insight from other members... contradicting info on the interwebs... nothing new.

I think I am dead set on the 14.5" with welded device to bring to 16" OAL, avoid the stamp and be legal. I never intended to make an SBR without a stamp. Thank you for your concern though, always good to stay legal! Thumbs UP!

Thanks for those pics! It really helps to know the measurements of pictures I've looked at (especially since most pics are AR15's) - speaking of which, what are the lengths of your hand guards on the black vs FDE?

Still a little confused on my contradicting research, but the measurements you provided would make me want to amend my original post. Any time I mentioned using rifle gas, the port distance I had in mind was 12" - like the article told me (face palm) - so I was referring to the "mid-length" 11 3/4", which still puts my question into perspective. Based on your 13.5" barrel AR10 with carbine 9 3/4" gas, do you think I can squeeze the mid-length gas system into a 14.5" barrel, physical distance / space? If so, back to my original question, is that 2 3/4" of barrel after port gonna provide enough dwell time to function? Adding a suppressor gonna help me "cheat" those last couple inches? @DNP seems to think so. Anybody got his back?

Thank you again for correcting my measurements... please let me know what you think of that webpage I linked.

 

[A.James]

3 hours ago, edgecrusher said:

So, I read your post and didn’t see you planned shooting range desired. I saw the bit about energy, but not your purpose for this weapon. I will also ask your experience in shooting as well, for my own info to assist you. Nothing wrong with a 12.5” out to 400. 16” to 800. The thing I will correct you on is your statement on accuracy. The shorter barrels are no less accurate but suffer from velocity loss resulting in drop, not a loss in accuracy. Wind will affect a projectile more with less weight and velocity. 

Sorry about my massive post, I tried to make it as organized as possible, but it is so long that anyone could have easily read something at the beginning and forgotten it by the time they get to the last sentence - LOL my apologies!

I did mention it really quick in my thought process on deciding on 7.62/.308 (objective #1), and again in objective #2 discussing barrel length vs VELOCITY. I don't intend to use this over 500m, and I'll build up a 6.5CM if I want a long range pony in the stable.

I read a post yesterday that makes me realize I'm not using the word "accurate" properly, and I should probably use the word "precision." Accuracy is hitting the bullseye (which you can do with a shorty, I agree) and precision is the size of the grouping (longer barrel = more velocity = less windage, drop). We're on the same page here, now? I was using the wrong term, but do understand the correlation between velocity and precision, and that's what I was getting at. Thank you for the correction!

[A.James]

@edgecrusher I'll make a quick intro post in the proper section, but to answer your experience question in a nutshell...

I've been shooting rifles and shotguns since I was a kid, though not so much lately. Trying to get back into it. I've shot 20ga, 12ga, trap and skeet, .22 (of course), black powder rifles (I think .50cal), various 9mm, Glock .40 and 45ACP pistols, random small caliber lever action rifle (can't even recall what that was), a few bolt action rifles (.243 and .247 if I recall correctly, as well as a few 30/06 sporadically). First time I shot an AR15 I was stoked, and I've shot them a handful of times in the last decade. I like the AR's a lot. But I'm definitely rusty. 50% of wanting to assemble an AR is just cuz I like puzzles and legos and tuning explosive chambers (like turbo charged boxer engines). Its all about balance. Let me know if you want any more info - I'll start a thread in the intro section and we can carry on there, and keep the posts here about the weapon design / theory.

Thank you again for everything offered so far! This is getting good already!

[DNP]

DNP also has zero personal experience owning suppressors. So take any of my thoughts related to them with dinosaur sized chunks of salt. 

[A.James]

8 minutes ago, DNP said:

DNP also has zero personal experience owning suppressors. So take any of my thoughts related to them with dinosaur sized chunks of salt. 

HAHA I love this guy.

NOTED!

I was referencing your post speaking of your experience with "notta lotta length after port = seems to be working." 

Can I ask, that 16" Fulton with rifle gas, is that .308? If so, can you confirm your rifle length gas with the measurements that @98Z5V posted? 

I also found this, its got a nice chart: https://truenortharms.com/ar15_default_store_view/gas-tubes-for-ar-10-dpms-308.html - Do different manufacturers use different measurements but still call them all "rifle" or "carbine" without any standardization for these gas-length terms?

Edited March 21, 2020 by A.James

[A.James]

On 3/19/2020 at 11:33 PM, 98Z5V said:

Post an intro in the Intro Section and tell us about yourself.  You made one HELL of a first post, and you need alot of information from this board, it appears - so reciprocate.

Go hit this, tell us about yourself.

https://forum.308ar.com/forum/22-introductions/

Smack away about YOU...  give and take here.  Become part of the place, and you'll get all the info that you'll need. 

Welcome aboard. 

Been finding some more of your older posts... you are definitely experienced in the areas I'm looking into at the moment. Hopefully we can keep talking! Thank you!

[DNP]

My 16 is a 308 and I’d bet my left nut that 98 is pretty damn close.  You can see how close to the end of the barrel the block is. 

[A.James]

16 minutes ago, DNP said:

My 16 is a 308 and I’d bet my left nut that 98 is pretty damn close.  You can see how close to the end of the barrel the block is. 

These pics are great! Nice rifles everyone! You two have really helped me visualize the dimensions. So its confirmed by you both, rifle gas is gonna be longer than the barrel hahaha!

So I'm gonna go for the 14.5" mid-length, unless... intermediate gas? Anyone got a measurement on this length? Or is the "intermediate" for the .223 only? Just trying to wrap it up with a bow on it. I think pushing that gas length as long as possible is still a desirable goal, no? Especially when considering additional back pressure from the suppressor? Plus, I like seeing your success with having a gas block waaaaaaay out there! You like how that thing feels to shoot, @DNP?

Thanks so much again for the help, definitely gets our discussion filed to a point!

[98Z5V]

4 hours ago, DNP said:

My 16 is a 308 and I’d bet my left nut that 98 is pretty damn close.  You can see how close to the end of the barrel the block is. 

The gas port diameter on that particular barrel is 0.105" - and the max you can go on a gas port diameter is 0.125", because that's the internal diameter of a gas tube.  Going larger than 0.125" on a port wouldn't gain anything, because of the ID of the gas tube. And that's with a 0.936" gas block diameter, at the gas port location, on that barrel @DNP posted. 

There's not alot of dwell time on a 16" rifle gas barrel.  Almost nothing.  That's why the gas port is huge. 

[98Z5V]

I'll hit the rest of this stuff later - going over to the Gun Pusher's house now for a Coronavirus BBQ.   I'll be primed for conversation when I return.

[DNP]

It’s a pretty soft shooter, but you still feel it. I think as long as you have a balanced system, you’d be fine with anything long, medium or short. 

[98Z5V]

11 hours ago, A.James said:

This is an awesome post! Thank you!

I believe I was referencing this page, https://www.at3tactical.com/blogs/news/9298047-carbine-mid-or-rifle-a-beginner-s-guide-to-ar15-gas-systems and now I realize this is describing AR15 gas lengths... I thought when comparing "rifle" vs "mid-length" vs "carbine" we were talking about set distances, regardless of the platform... I stand corrected. But it seems the page I cited in this post is just UTTERLY wrong? Based on your measurements, the .308 and .223 have the same numbers? Then WTF is up with this info I've found. This is precisely why I needed some insight from other members... contradicting info on the interwebs... nothing new.

I think I am dead set on the 14.5" with welded device to bring to 16" OAL, avoid the stamp and be legal. I never intended to make an SBR without a stamp. Thank you for your concern though, always good to stay legal! Thumbs UP!

Thanks for those pics! It really helps to know the measurements of pictures I've looked at (especially since most pics are AR15's) - speaking of which, what are the lengths of your hand guards on the black vs FDE?

Still a little confused on my contradicting research, but the measurements you provided would make me want to amend my original post. Any time I mentioned using rifle gas, the port distance I had in mind was 12" - like the article told me (face palm) - so I was referring to the "mid-length" 11 3/4", which still puts my question into perspective. Based on your 13.5" barrel AR10 with carbine 9 3/4" gas, do you think I can squeeze the mid-length gas system into a 14.5" barrel, physical distance / space? If so, back to my original question, is that 2 3/4" of barrel after port gonna provide enough dwell time to function? Adding a suppressor gonna help me "cheat" those last couple inches? @DNP seems to think so. Anybody got his back?

Thank you again for correcting my measurements... please let me know what you think of that webpage I linked.

 

Addressing the two things quoted in red above.

#1.  They have to be referencing gas port position after the chamber - no other way they came up with that number.  Gas tubes are not the lengths that they are quoting in that article.

#2.  I never stated that I had a AR15 Carbine gas system on my 13.5" gun, but the number that you put in there was for an AR15 carbine gas tube length - I specifically stated that I had an Armalite AR-10 Carbine gas system on that gun, and I said that it's a touch longer than AR15 midlength gas. Details...

Look again at the gas tube lengths I quoted above.  AR15 midlength gas tube is 11 3/4" long. Armalite AR-10 Carbine gas tube is 12 1/16" long...   Armalite only does two gas systems for AR-10s.  Carbine and Rifle gas. Armalite AR-10 gas tubes are 3/16" longer than comparable AR15 gas tubes, and they have different bends to them to account for height-over-bore. They're different from AR15 parts, totally.

Details like this are why it sucks, for us here, when someone jumps on the board and tells us all about their AR-10 build, and we start asking questions, only to find out that it's really a DPMS LR-308-based build, and nothing to do with Armalite AR-10 pattern at all...   The two are not identical, and there are differences...  There are even bigger differences once you toss in the Bushmaster BAR-10/RRA LAR-10 design. 

Both those handguards, black gun and fde gun, are 12" versions.

Edited March 22, 2020 by 98Z5V

[98Z5V]

11 hours ago, A.James said:

do you think I can squeeze the mid-length gas system into a 14.5" barrel, physical distance / space? If so, back to my original question, is that 2 3/4" of barrel after port gonna provide enough dwell time to function?

 

Yes, you can.  For a 14.5" barrel, you can either run AR15 midlength gas system or Armalite AR-10 Carbine gas system on a 14.5" barrel - I have it on a 13.5" barrel, the fde gun I posted above.  You have another inch of dwell time over what I have on mine.  Will it work?  Yes, if your gas port is the proper diameter, and you're running the proper recoil system.  It'll work like a champ.

[A.James]

1 hour ago, 98Z5V said:

Addressing the two things quoted in red above.

#1.  They have to be referencing gas port position after the chamber - no other way they came up with that number.  Gas tubes are not the lengths that they are quoting in that article.

Exactly what I was thinking, and probably the main source of my confusion, port position vs tube length. Thank you for clarifying this!

1 hour ago, 98Z5V said:

#2.  I never stated that I had a AR15 Carbine gas system on my 13.5" gun, but the number that you put in there was for an AR15 carbine gas tube length - I specifically stated that I had an Armalite AR-10 Carbine gas system on that gun, and I said that it's a touch longer than AR15 midlength gas. Details...

I know you didn't, and that is totally my mistake. I've been diligently reading all the posts here and in other threads, especially yours. Your responses to my questions are concise and full of information, I read them very carefully to make sure I'm getting things straight. When I was replying to your earlier post, I had to scroll back and forth to re-read those numbers you listed and I must have just grabbed the wrong number while I was composing my response. My apologies, and thank you for your patience. Im learning a lot and humbled by the experience of others here and the guidance I've already received.

1 hour ago, 98Z5V said:

Look again at the gas tube lengths I quoted above.  AR15 midlength gas tube is 11 3/4" long. Armalite AR-10 Carbine gas tube is 12 1/16" long...   Armalite only does two gas systems for AR-10s.  Carbine and Rifle gas. Armalite AR-10 gas tubes are 3/16" longer than comparable AR15 gas tubes, and they have different bends to them to account for height-over-bore. They're different from AR15 parts, totally.

Details like this are why it sucks, for us here, when someone jumps on the board and tells us all about their AR-10 build, and we start asking questions, only to find out that it's really a DPMS LR-308-based build, and nothing to do with Armalite AR-10 pattern at all...   The two are not identical, and there are differences...  There are even bigger differences once you toss in the Bushmaster BAR-10/RRA LAR-10 design. 

This is definitely another factor in confusion, and I'm aware of that pattern difference. I think the names used between Armalite and DPMS got me spun. Is it safe to assume that something called AR10 would be Armalite vs. LR308 meaning DPMS? I was calling the DPMS pattern "AR308" before, and I want to refer to each properly. Also for clarification, does the DPMS pattern use AR15 gas tubes on .308 barrels - and Armalite uses AR10 gas tubes on .308 barrels? That's what I'm getting from your last post and I don't want to misinterpret good advice.

Just for the record in case it was missed in my first post, I plan on building the DPMS pattern, probably Aero Precision M5E1.

1 hour ago, 98Z5V said:

Both those handguards, black gun and fde gun, are 12" versions.

Thank you, sir! Always good to know what I'm looking at for reference.

 

[98Z5V]

18 hours ago, A.James said:

 I think the names used between Armalite and DPMS got me spun. Is it safe to assume that something called AR10 would be Armalite vs. LR308 meaning DPMS? I was calling the DPMS pattern "AR308" before, and I want to refer to each properly. Also for clarification, does the DPMS pattern use AR15 gas tubes on .308 barrels - and Armalite uses AR10 gas tubes on .308 barrels? That's what I'm getting from your last post and I don't want to misinterpret good advice.

Just for the record in case it was missed in my first post, I plan on building the DPMS pattern, probably Aero Precision M5E1.

This is correct.  If it's really AR-10, it's the Armalite pattern.  If it's stated as DPMS or LR-308, it's DPMS-based in design.  That's why we call everything a .308AR.  That covers them all.

The most common pattern out there for building your own rifles from parts is definitely the DPMS pattern.  There are just more companies that went that route. 

Gas tubes, length, proper gas port position - is a giant clusterfuck in the Big-AR industry.  It either uses AR15 gas tubes, or Armalite AR-10 gas tubes (EDIT - Wilson Combat is doing some of their own gas port positions on their own excellent barrels, and special-length gas tubes that they make in-house).  Thank barrel sellers for that one, with funky gas port positioning.  You really need the barrel in your hands to see what you've got - and an appropriate AR15 gas tube.  If it's too short, you wait, and buy the Armalite AR-10 gas tube to solve the problem.

Edited March 23, 2020 by 98Z5V

[98Z5V]

Hit this thread  - there's a ton of info in here, with numbers, details, explanations, etc.