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New 20" barrel, now it short strokes


Dracomeister
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2 hours ago, jtallen83 said:

And that begs the question, which rifle length gas tube came first? Is it possible the AR-15 tube was shortened from the original AR-10 and when 308AR's became popular makers just made adjustments for the already in production shorter tubes.

   Its all about timing , pretty much what 98 was talking about in his Sermon , Gas port size matched to Gas system length & also to some extent Buffer Spring ,to keep Bolt locked to for a fraction of a second , not to mention the weight of the Buffer its self ,& to some extent BCG weight , all have an effect on Timing . As most know , if the timing is too long or short , it can cause malfunctions .

 Armalite made their Gas system to match their Buffer system ,its a balance of pressure ( Gas impulse ) weight & Spring pressure , Its just a Number they came up with for the AR 10 at the time . As long as the Balance of these things create proper timing , the system works , get one out of balance , you will most likely have issues .

   I believe DPMS made or designed their's differently, to keep Armalites legal team off them , enough difference to to stand apart , use of already plentiful Gas Tube lengths , Receiver Extensions , all from the already plentiful M16/AR15 at the time . You can't argue with results , both Armalite & ( proper ) DPMS systems work , with DPMS LR 308 Rifles , (probably because of price ), have the largest market for the 308 version , also for a couple of other reasons , one , type of Magazines used. Armalite now with the "A " model is real late in that department & definitely cost them some market share of rifles sold .

We know they had reasons , but why they waited so long , was a bad decision in my book .

  If you have a properly balanced system , you should not need an Adj. Gas Block , its the way the systems are designed . You want a little over pressure or what some call Recoil impulse , to make sure the system will function with a dirty system , remember these rifles were originally made for Combat , not the Bench .

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98, took the second tube back to the shop where I bought it, sales weenie didn't know there was a difference between Armalite and DPMS tubes. Then went to my local shop to get an adjustable block, talked to the 'smith about the different length tubes. He basically said ... "Crap, how the @$#!& did I miss that!" then proceeded to discount the new block by 50%. I think he won't forget to look at the tube length in the future!

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On 4/13/2017 at 7:29 AM, survivalshop said:

...type of Magazines used. Armalite now with the "A " model is real late in that department & definitely cost them some market share of rifles sold .

I've often wondered about this. EVERYONE missed the boat. Should have made the lowers compatable with the existing M14 magazines. Already mil-spec, proven design and readily available, Would have saved everyone a lot of trouble.

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30 minutes ago, mineralman55 said:

I've often wondered about this. EVERYONE missed the boat. Should have made the lowers compatable with the existing M14 magazines. Already mil-spec, proven design and readily available, Would have saved everyone a lot of trouble.

In a way Armalite did. They needed to change up a few things, like the follower, to get reliability. They sold conversion kits for those old M-14 mags for a long time when the ban hit.

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I really started thinking about this, and I think I have the answers.  I'm pretty certain I know why the Armalite AR-10 gas tubes are longer.

 

So, as I mentioned before, the carrier gas key will sit further back on the AR-10 than it will on any AR15.  It's mounted the exact same distance back from the front of the carrier face, but the AR-10 bolt is larger, thus the gas key is further back when the bolt is closed, in the firing position.  As it turns out, it's about 3/16" further back.

308AR BCG on the left, AR15 BCG on the right.  Both bolts are pushed into the carrier, in the position they would be in when a round is fired.

 

Armalite gas tube length reason sm.jpg

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In order for Armalite to give the bolt carrier group the proper amount of gas, for the proper amount of time, and generate the proper amount of force in order to cycle the weapon, they had two choices:

1.  Move the gas port in the barrel, which will affect dwell time,

2.  Use a longer gas tube and keep the gas port in the barrel in the same place, with the properly developed dwell time.

Eugene Stoner did the math on it all, and developed this.  I think this is the why on this one.

Actually, the AR-10 was developed first, and all this was "mathed out."  I think the gas port location and gas tube length on the AR15 was changed, from the AR-10 design...

That's gotta be it.  There is no other explanation.

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Every other manufacturer out there just uses the shorter AR15 gas systems on the Big ARs, and says "It'll be good enough," rather than buy proprietary gas tubes from Armalite, or violate Armalite's patent on it all.

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So then I wonder what the other manufacturers do to correct any of the issues? 

DPMS actually has an AR15 carbine length LR308 whereas Armalite's carbine is actually an AR15 mid-length setup.  I'm wondering now if this 3/16" difference only applies to Armalite or if all the others are also all off that are using the AR15 tubes and have simply been speaking by?

Edited by GRA
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8 hours ago, 98Z5V said:

I really started thinking about this, and I think I have the answers.  I'm pretty certain I know why the Armalite AR-10 gas tubes are longer.

 

So, as I mentioned before, the carrier gas key will sit further back on the AR-10 than it will on any AR15.  It's mounted the exact same distance back from the front of the carrier face, but the AR-10 bolt is larger, thus the gas key is further back when the bolt is closed, in the firing position.  As it turns out, it's about 3/16" further back.

308AR BCG on the left, AR15 BCG on the right.  Both bolts are pushed into the carrier, in the position they would be in when a round is fired.

 

Armalite gas tube length reason sm.jpg

 Someone show a measurement of a real Armalite AR 10 BCG next to a DPMS one & see if there is any Difference & then also the AR 15 . Its all about where you install the Barrels Gas Port .

 On another note , where does the Gas tube on an AR 10 ( Armalite ) sit inside the Upper Receiver , is it sitting deeper in the Upper then in a DPMS LR 308 ? 

 

DPMS LR 308 Gas Tube & /AR 15/16M4

 

  Pistol             6-5/8

  Carbine          9-3/4

  Mid-Length  11-3/4

  Rifle             15-1/4

 

 

 

 

Armalite lengths,

Rifle ( 308 ) ----- 15.5 "

Mid length 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"

 National Match---------------  ?

 

 

 

Edited by survivalshop
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14 hours ago, survivalshop said:

 Someone show a measurement of a real Armalite AR 10 BCG next to a DPMS one & see if there is any Difference & then also the AR 15 . Its all about where you install the Barrels Gas Port .

 

 

Didn't measure but the pictures tell the story, part of it. Left is an AR-10 BCG and right is a d. wilson DPMS style BCG. Maybe we need to start another pinned thread for this topic, I'll try to get more detailed photos and include an AR-15 BCG and shots of the gas tube inside the receiver.

100_5556.thumb.JPG.b7c269cdbfc1ea7e458fb7b2c5670940.JPG100_5557.thumb.JPG.ea5e65b25cac61c5158e27580ac6ecdb.JPG

and yes the wilson BCG is NIB and dirty as all get out but in the middle of a lube test after numerous rounds of suppressed fire.

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^^^  Interesting, Jim - big time.  In MY pics, that 308 BCG was a DWilson, as well, compared to the AR15.

Here, in yours, it's Armalite AR-10 on left and DWilson DPMS-based on the right - and the Armalite is even further back than the DWilson.  So, with the Armalite AR-10 compared to the AR15, there would be a bigger difference...  and a greater need for a longer gas tube, in order to properly cycle the weapon.

The bottom line of "normal" in all this was before everybody and their brother decided to jump into the Big AR World here.  People mostly did it right, on parts.  Doesn't matter is a barrel maker drilled the gas port in the DPMS position, or the Armalite position, on the barrel - they told you if it was out of the ordinary, and make certain to use "xy" gas tube.

Now, people that are making barrels, and gunsmiths in shops, don't even know there's a difference to begin with - it's gotten that bad.

"Normal" was that the gas tube comes back to the center of the cam pin cutout in the upper receiver.  If you had a DPMS-based barrel, it just uses AR15 gas tubes and comes back to that location.  If you had an Armalite AR-10 based barrel, and used a short gas tube, it doesn't extend to the correct position.  The fix is easy enough - after diagnosing why your rifle runs like shiit sometimes, but works great with other ammo, and drilling gas ports up, and adding adjustable gas blocks (which DO NOT help this situation...).  You get my point...

I think we should move this thread to one of the technical sections, then pin it...

Edited by 98Z5V
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1 hour ago, jtallen83 said:

100_5556.thumb.JPG.b7c269cdbfc1ea7e458fb7b2c5670940.JPG

 

Of interest - on both your flavors of "BIG BCG" here, the front of the carrier, the bolt height when in the firing position, AND the shelf that the charging handle grabs onto - are all in the very same position. 

The gas key is in a slightly different location, and the cam pin channel in the carrier body appears to be in a slightly different position.   This all affects timing in the direct impingement system, and it's just two different ways or techniques to accomplish the same thing...

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  So much for anyone to say the two BCG's are the same , regardless of a Spring loaded Firing Pin .

    This thread has been enlightening , but I would like to see some some what precise measurements of the differences or comparisons . I can get the DPMS Measurements , but hopefully one of you guys can measure a real AR 10 BCG  for us /me .

   I have two DPMS 308AR BCG's & one SI Defense 308AR BCG's to measure , 

 BC face to Cam Pin slot .

BC face to end of Gas Key ( front where as Tube enters )

BC face to rear base of Gas Key 

Complete length of Gas Key , also the Base length where it attaches to BC .

Is the complete Bolt any longer ?

 Anything else we should compare ?

 

  It may just be the Photo , but the Bolts seem to be seated  (or bottomed out ) at different depths ? 

 

Edited by survivalshop
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22 minutes ago, survivalshop said:

   It may just be the Photo , but the Bolts seem to be seated  (or bottomed out ) at different depths ?

My take on that is no, but there is a difference in the bolt, and that's giving the perception of what you see.

My reasoning for that is:  Both those bolt faces are flat on the table, and both those carrier body fronts are at the same height.  So, the bolt drops back in the carrier, into the firing position, to the same depth.  If they didn't, then some serious other things would have to come into account, primarily manufacturing barrels for headspace...

However, that cam pin slot is definitely different on the two types. Only way for that to happen is: that cam pin hole n the bolt is in a different place on the Armalite  and the DPMS-based.  No other way around that, I'm thinking.  If the hole was not in a different place, and the same on both those bolts, you couldn't get away with having the cam pin slot in a different place, or different depth (front to back).

My reasoning sound logical? I'm guessing, based on what I see, but it makes sense in my mind...

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1 hour ago, 98Z5V said:

My take on that is no, but there is a difference in the bolt, and that's giving the perception of what you see.

However, that cam pin slot is definitely different on the two types. Only way for that to happen is: that cam pin hole n the bolt is in a different place on the Armalite  and the DPMS-based. 

NOTE: This may factor in to what I was once told by a barrel-maker that you can use an LR-308 barrel in an Armalite upper IF you also use the LR-308 BCG, but you cannot mix and match.  I guess this is where some of the differences and variations are between the 2 different manufacturers ... (???)

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My matrix is an armalite pattern upper with DPMS pattern guts. I've seen threads where people posted pics of guns using mixed barrel/bolt setups that headspaced and ran just fine. I've seen posts of people with both platforms who micd them out and found no appreciable difference, and others who swear the lugs are longer, or the face is deeper, or the lug spacing is different, or they know a guy who knows a guy at armalite. 

If at all possible I'll try and keep my parts the same because I know it works. If I find myself in a position to have a mixed setup, I'll headspace it and then go shoot it. I'm willing to bet the difference between AR10 and LR308 bolts is no worse than the tolerance stacking you end up with mixing manufacturers of LR stuff. 

Edited by blue109
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3 hours ago, blue109 said:

My matrix is an armalite pattern upper with DPMS pattern guts. I've seen threads where people posted pics of guns using mixed barrel/bolt setups that headspaced and ran just fine. I've seen posts of people with both platforms who micd them out and found no appreciable difference, and others who swear the lugs are longer, or the face is deeper, or the lug spacing is different, or they know a guy who knows a guy at armalite. 

If at all possible I'll try and keep my parts the same because I know it works. If I find myself in a position to have a mixed setup, I'll headspace it and then go shoot it. I'm willing to bet the difference between AR10 and LR308 bolts is no worse than the tolerance stacking you end up with mixing manufacturers of LR stuff. 

  I read where there were small dimensional differences in the Bolt Lugs also & he gave the measurements . I read that post in another Forum , a long time ago , don't remember where ( ARFCOM ? ) As will the measurements of the Bolt length & Bolt Face to BC face when Bolt fully into BC .

  Measurements will at least confirm a difference or not . Maybe a Measurement from the face of the BC & the charging Handle Notch , just for curiosity .

 I will Take measurements with Photos of where or how I'm measuring , just for clarification .:thumbup:

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