Lane's: jtallen83 inspired 21.750" 300 AAC Blackouts

[Lane]

I only wanted to build one rifle in 300 AAC Blackout because I thought it was just another data point.

@jtallen83 suggested that there might be a nice barrel length in terms of paper punching accuracy; that should carry well between .22 and .308. I hesitate to use the word magic in this case.

Here is the article linked again with some analysis. 

https://precisionrifleblog.com/2013/10/18/secrets-of-the-houston-warehouse-lessons-in-extreme-rifle-accuracy/

I've dedicated three or more barrels to this project; and just as many total builds. The research suggests that the powder charge doesn't even matter; simply the barrel length, and cartridge/bullet to barrel tuning. There is a supposition of resonance, but I fully intend to refute that during my exploration and analysis of this process. I'll write up a full hypothesis and methodology before the custom 21.750" barrel is finished. I am taking votes on the length of the cutting test barrel... Should it start at 24" or do I simply need to box in from 22"? I plan to build, and tune, ad nauseum until it is appropriate to start swapping barrels between uppers. I have an oscilloscope  just left of my shooting position behind a wall. It will be trivial to measure multiple points of data from that location. Yet another hand full of gas tube roll pins; thank jtallen83...

[Lane]

I'm just going to throw down one more card here. I will commit to four build/barrel minimum on this. The specification is #7 contour; so the first 21.750" is going to be just that. I also want a 24" in #7, a 21.750" in .900-.936", and a 21.5" in .900-.936". Not sure how long to plan on the three fabrications; but one of those shoots in March; well before the snow is gone.

[Lane]

About to get the first two barrels moving for this project. One question I have is whether it's worth getting the local gun shop involved in the process? In the past I've done most of my business without their aid; but it seems like they typically get decent pricing as a dealer, and I pay about the same letting them get a cut of the action. I would be interested to know if anyone thinks it is worth the time to involve a third party.

So the Shilen #7 contour doesn't appear to be a contour at all. It's roughly what Green Mountain sells as their gunsmith edition barrels, which is what I wanted to to test against in the first place. The 7.62x39 barrel measures 0.900", and this specified #7 is 0.890"; I will happily settle for 0.890" on all the barrels. Going to start is with two custom barrels, at 21.750", and 24". Still on the fence about sending bolts for headspacing or not. Will I end up with better consistency if they just make two barrels to specification, and I'll have to check bolts for fitment and headspace? The reason I consider this is trying to introducing as few variables as possible. 

I found an indoor 100 yard range less than two hours away. Unfortunately, I presume they have some kind of ventilation system; so it won't be perfectly still air. It is still worthwhile in my mind to add additional data points. 

[98Z5V]

I'm trying to stop you before you spend a bunch of money that you don't need to.

I think you're going about this on the wrong set of data points (Houston Warehouse).  Barrel length does not affect accuracy, it affects velocity.  Here's a great read on that:

https://www.wideopenspaces.com/the-truth-about-barrel-length-muzzle-velocity-and-accuracy/

In some circumstances, shorter barrels can be more accurate than longer barrels, and that's specifically due to "barrel whip." I can make a 1/4" wooden dowel that's 2 feet long, flex, just by shaking it up and down. I can't make a 1/4" wooden dowel that's 6" long flex, no matter how hard I shake it. 

Here's a decent vid on barrel whip - watch the volume, the music gets loud quick:

Here's another one, really short:

Fact - shorter barrels whip less. 

Here's the best vid out there on barrel vibrations and harmonics:

 

There's alot more in this, that you'd need to look at, before you just spend money, man. The best way to find out the magic barrel length for 300BLK, inch by inch, is to cut it down inch by inch - and make sure you recrown it every time.  You sacrifice ONE longass barrel that way.  Once you find that happy number of "the best accuracy,", you get another barrel, and sacrifice that one, going in 1/4" lengths.  It's just like loading precision ammo.

Don't go down that rabbit hole, thinking you'll make 3 barrels, and one is gonna be the best - you might pick all three lengths that absolutely suck - then you'll think the cartridge sucks, and it'll never be accurate. 

That's my $0.02...

[98Z5V]

I will say this, about barrel length, though...  and I was shocked.

I built a pistol caliber AR, an AR45, with a 16" barrel.  When I went out to finally shoot the thing, I shot it at 25 yards, and it was dead-on.  So I shot it at 50 yards - dead on.  I had steel at 100, 150 and 200 (and way beyond that).  I decided to try it at those steel targets, and I was shocked.  It nailed al IPSC steel target at 100, so I went 150, held about the head - hit it. Went to 200, held top of head, and hit that one, too.  Went to the 250, and I couldn't figure it out, but it didn't matter...  I just hit something at 200 yards with a .45ACP!!!  

I went back to the 200 yard target, and kept hitting and hitting...  It was SO WEIRD to shoot a .45ACP at 200 yards and hit the target everytime.   Well, unless you're Larry - he can do that with a 1911...      In the throat...  

I could never do that with a 5" 1911 - but the 16" barrel made the difference.  On a round that starts out as subsonic.

My point in that is this, and it's simple - it's the cartridge.  You need to look at that cartridge that the guys in the Houston Warehouse were shooting, before you try to make a 300BLK perform like that cartridge.  

[Lane]

13 minutes ago, 98Z5V said:

I think you're going about this on the wrong set of data points (Houston Warehouse).  Barrel length does not affect accuracy, it affects velocity.

I'm in agreement here from a scientific perspective. It was the barrel length that started this whole discussion though; and was clearly stated as some kind sweet spot in the article. It was further supported by saying "the best 30 caliber, and best 22 both had 21.75" barrels".

11 minutes ago, 98Z5V said:

inch by inch, is to cut it down inch by inch - and make sure you recrown it every time.  You sacrifice ONE longass barrel that way.

That's what the 24" barrel is intended for in this first order. The 21.75" is of course a reference to test that against during the process. I haven't fully crafted a hypothesis on this, but I don't think these first two are a waste of money. I'll absolutely listen to any comments, and read more about this. I'm well aware that I'll need to take chamber castings, and spend a lot of time developing my reloading to make this a realistic investigation. Once I feel like I've learned something from cutting up that 24"; I will consider future barrel options.

Clearly my sense of humor about this matter doesn't always come through this medium. I had a 300 blackout build in need of a barrel. This is at the very least motivation to finish that/them, and start recording data about the accuracy of my builds. I have no illusions that my small sample size can't really add scientific value; but it is still worthy of rigorous inspection. I will always strive to be a better shooter, and build more accurate firearms. I am curious to see if these thicker barrels flex in any appreciable way when fired.

[98Z5V]

Make sure you do it in a supersonic load  - for sure.  Something in the 110~125gr loading, or the data might not matter.  Here's my $0.02 on that part:

I have a solid copper, 911gr projectile, in .510 caliber, that was designed as a subsonic projectile, for the cartridge it was designed for.  It was never meant to go supersonic - and I have no idea what kind of casing or charge that you would have to design to make it do that.  It was strictly designed as a subsonic cartridge that would have an accurate effective range of 1000 meters, quiet, delivering a payload that will crush an engine block.

When we look at the ballistic coefficient of projectiles, you'll see people say "500" or "600" - those are DAMN good BCs!  Those are usually G1 BC drag models, and the real number is (for example) a 506 G1 BC for the Hornady 6.5mm 123gr ELD-M projectile - for it's sectional density, that is one badass projectile.  But the G1 BC is ZERO POINT 506.    It's a .506 BC.

This 911gr Projo in .510, if it was ever designed for supersonic, or would GO supersonic, is a 1600 BC.  1.600 ballistic coefficient.

The cartridge, and the load, is everything. To get accuracy out of the 300BLK, and determine it's max-best, it will definitely need to be a supersonic load. 

@Robocop1051, for your 500 Blackout comments, my brother... 

[Lane]

40 minutes ago, 98Z5V said:

My point in that is this, and it's simple - it's the cartridge.  You need to look at that cartridge that the guys in the Houston Warehouse were shooting, before you try to make a 300BLK perform like that cartridge.  

It is unfortunate that Virgil doesn't specify what the 30 caliber was that shot so well, only that it was his own gun. His 22PPC was apparently the best; just a necked down 7.62x39 from what I can see. The fact the both of those were his guns leads me to believe his case prep may be a huge part of that accuracy. 

I didn't want to yell this out in the other forum; but I don't actually believe any part of this 21.750" barrel length business. But I'm game to put my money where my mouth is, have fun with it; and see what the data says. I won't be upset regardless of what I learn in this process.

[Lane]

9 minutes ago, 98Z5V said:

Make sure you do it in a supersonic load  - for sure.  Something in the 110~125gr loading, or the data might not matter.

I will look into this a bit further; the twist on these will be 1:7. The loading data I have here on paper shows even 150s and 168s can launch supersonic. I'll run the numbers to see what's still hitting targets above the speed of sound. I knew I needed to test a range of projectile weights; so I'll start my reloads small and fast. The box of factory 300 blackout I have on hand are 125 grain; so I'll see how those land right out of the gate.

I appreciate all the input on this. 

[98Z5V]

8 minutes ago, Lane said:

 The fact the both of those were his guns leads me to believe his case prep may be a huge part of that accuracy. 

 

His rifle prep was the huge part of the accuracy - to include the prep of the barrel threads with lapping compound, for true barrel mating to the receiver.  Those guys were the best "accurate shooters" that there were, with the most handloading knowledge, and the most (unheard of) gunsmithing knowledge, at the time - that's the only way they got "invited to the warehouse" in the first place.  And the produced groups in the tens.  Repeatedly.  Shot after shot.  They could even tell you which shot was the "fouler" and would be off by so much, after cleaning the barrel.

Shot groups that are 0.010" in difference.   That's all in the same hole... 

 

[98Z5V]

4 minutes ago, Lane said:

The loading data I have here on paper shows even 150s and 168s can launch supersonic.

My 150 loads are about 1800fps. Not enough to set off Tannerite at 100 yards.  Been down that road, with solid hits.  Takes 2000fps to blow it. 150s are not what you should load for accuracy testing.  They're badass, for General Purpose use, and all I'll load - but they're not for accuracy.  They won't help you, and nothing heavier will help you.  This is 300BLK, here.  Get that projo weight down, WAY down...

Edited March 1, 2019 by 98Z5V

[Lane]

9 minutes ago, 98Z5V said:

This is 300BLK, here.  Get that projo weight down, WAY down...

This is the kind of advice that will save me money down the road. I was shocked when my reloading dies showed up and I saw the range of projectile weights for this cartridge. I initially took an interest in this as a stepping stone towards making .260 Rem from .308 Win brass. 

[98Z5V]

31 minutes ago, Lane said:

This is the kind of advice that will save me money down the road. I was shocked when my reloading dies showed up and I saw the range of projectile weights for this cartridge. I initially took an interest in this as a stepping stone towards making .260 Rem from .308 Win brass. 

30BLK is low 100s to 220s, man.  It does everything.  Supersonic to the-most-quiet-subsonic, at 220s. It was designed to battle-back against a 7.62x39 - and do alot more than the 7.62x39 could ever dream of.  It excels at that - but you need to pick the projo that will do what you intend to do - accurate - and go with one of the light ones.  Very light.  Who knows what kind of accuracy you'd get from the 220s - but the range on them was never designed to go 100 yards.  That was for max-quiet, at room-distance. 

This is a tough cartridge, man.  Just because of everything it will do, from far extreme to the other far extreme. 

[98Z5V]

One bad thing you're facing on the .308 diameter projectiles - sectional density drives accuracy at range.  The lighter .308 projos suck at sectional density numbers. Most "awesome" .308 projectiles that people think of are 168s and 175s - and those SUCK at sectional density.  The lightest .308 Win loads that I load are Hornady 178gr HPBT projos, because the BC is higher, but the sectional density starts to climb there...  I'm loading 195gr HPBTs now, just due to the BC  - but it's really about the sectional density difference.  There's a giant difference on the 178s and 195s. 

At either of those weights, on 300BLK, you're approaching subsonic, and defeating the purpose of accuracy testing. 

For the 300BLK, go as light as you can, and get the speed up as much as you can, because you're going to be fighting sectional density.  If accuracy is the goal. The biggest thing in this fight will be "what distance?" Just due to the huge variance on projo weights.    Determine "how far" you want to be accurate, then look at the projectile weight in 300BLK that will get you there.  It's a balance, with this cartridge, lighters flying faster, but having shiity sectional density.  It's really gonna be a trade off...  

Sorry to be that guy that pees in the pool...   ?

[Lane]

5 minutes ago, 98Z5V said:

300BLK is low 100s to 220s, man.  It does everything.  Supersonic to the-most-quiet-subsonic, at 220s. It was designed to battle-back against a 7.62x39 - and do alot more than the 7.62x39 could ever dream of.

So; given your earlier stated logic, I should be loading my 7.62x39 with 108 grains for speed and accuracy? Save the 150s for doing damage?

5 minutes ago, 98Z5V said:

This is a tough cartridge, man.  Just because of everything it will do, from far extreme to the other far extreme. 

This is part of what made it easy to decide say yes to more than one of these builds; and the experiments to go along with it. That; and the fact that I was unable to buy a 22" barrel off the shelf.

[Lane]

12 minutes ago, 98Z5V said:

shiity sectional density.  It's really gonna be a trade off...  

Sorry to be that guy that pees in the pool...   ?

Piss away man. In this case; I have some more reading to do... In short; heavier bullets just can't perform as well as the lightest in the same caliber because of the larger SD? More obviously; smaller calibers have a lower SD (or heavier projectile weight for same SD). 

So when ladder loading for accuracy testing; I will likely land on the high end in terms of powder charge?

[Lane]

I ordered the two initial barrels for this project and had them drop shipped to a fabricator for chambering, etc. A slightly longer lead time than I anticipated; but still shorter than most anywhere else I checked. Four to six weeks (or less), is what I was quoted. I'll probably be dormant for a while in this thread as a result. I have a lot of things to consider in the mean time; and I'm a bit nervous about chamber casting. Casting can be fun; but pouring into a small opening with a low melting point alloy will be interesting to say the least. I doubt my first attempt will be perfect.

@98Z5V I was thinking more about our earlier discussion in regards to .260 Rem neck turning. In this case; the necks were turned down only 0.0007" below the chamber size. I'm guessing you leave a bit more clearance with a .260 in a semi-automatic? I don't even have tools to measure quite that accurately yet; but I can imagine that being an important factor in terms of accuracy. I remember seeing hints elsewhere that resizing dies should be lapped to get wider necks (for better fit)...

In other news I got a reply from Armalite today. Apparently the AR-10 A uppers will be back in stock later this month; that will allow me to kick of my mostly Armalite .260 build.

[98Z5V]

18 hours ago, Lane said:

Piss away man. In this case; I have some more reading to do... In short; heavier bullets just can't perform as well as the lightest in the same caliber because of the larger SD? More obviously; smaller calibers have a lower SD (or heavier projectile weight for same SD). 

So when ladder loading for accuracy testing; I will likely land on the high end in terms of powder charge?

Just the opposite, for distance.   Sectional Density ties in with ballistic coefficient, to get those projectiles downrange with accuracy.  You're constrained to a fixed diameter, per caliber.  Heavier projectiles have to be longer, considering the same projectile construction (with a few exceptions).  Lighter projectiles will weight less.  Sectional density it a projectiles weight for it's diameter.  Ballistic coefficient is the drag on the projectile as it flies through the air - it's shape is the biggest factor.

Here's probably the best articles on sectional density and ballistic coefficient:

https://www.chuckhawks.com/sd.htm

https://www.chuckhawks.com/bc.htm

For a short distance, and accuracy testing, the lighter projectiles will get it done - if they don't have a terrible ballistic coefficient.  It has to be something decent.  .45ACP has the ballistic coefficient of a brick.  Not what one would use to determine accuracy at any kind of distance, outside of pistol-range.

[Lane]

1 hour ago, 98Z5V said:

Ballistic coefficient is the drag on the projectile as it flies through the air - it's shape is the biggest factor.

I stumbled across the chuck hawks on sd last night, but didn't finish reading up on bc. These often get lumped together, or set as constants in simple projectile math. I did pick up a usable chronograph to inspect what I have here right now in more depth. It actually looks easy to modify (very easy); so I can drill down to the highest accuracy the software supports pretty quickly. Unfortunately it may not work well in the extreme cold; so that remains to be tested. It was dusk by the time I got to shoot today; so I wasn't able to try it.

1 hour ago, 98Z5V said:

.45ACP has the ballistic coefficient of a brick.  Not what one would use to determine accuracy at any kind of distance, outside of pistol-range.

That's something that made me think after your earlier comments... If you were nailing 200 with a 16" barrel; a long barrel on a 300 blackout shouldn't be a terrible idea. I've yet to try and project what to expect in terms of FPS from a 108 grain in a 24" barrel; but it seems like it should be fast.

[98Z5V]

7 minutes ago, Lane said:

That's something that made me think after your earlier comments... If you were nailing 200 with a 16" barrel; a long barrel on a 300 blackout shouldn't be a terrible idea. I've yet to try and project what to expect in terms of FPS from a 108 grain in a 24" barrel; but it seems like it should be fast.

It all comes down to the selected projectile, brother - and 300BLK has a WIDE weight range to choose from.  Anywhere from 110-grain supers to 220-grain subs. 

The .45ACP doesn't have alot, not like that. 185~230, basically, and most are subsonic.  Barrel length is what got that fat flying pig to 200 yards.  16" did it, but you wouldn't expect to repeat that all the time with a 5" pistol.  Unless - Larry... 

That leads me into another part of this - 300BLK ammo. It's only .308 - but the higher you go in .308 Win projos, the better they get.  208 ELDs and 208 BTHPs from Hornady are unreal - in .308 Win and .300 Win Mag and 30-06.  If I really push a 208 BTHP with RL15, I can get it to 2400fps, with a .620 BC.  It flies forever.  The BC on my 178 BTHP Match projos is .530 - and THEY fly forever, at my 2400fps loading. 

You're WAY into subsonic  with those, on the 300BLK.  Deep into subsonic. Those projos aren't going to work as well on an accuracy-over barrel length test, in that caliber.  You'll need to balance a .308 projectile weight for the 300BLK that you can keep supersonic for your testing, but with good BC and SD numbers.  That's gonna be a tough selection, but it's gotta be the first selection...  

[Lane]

48 minutes ago, 98Z5V said:

.45ACP doesn't have alot, not like that. 185~230

As I mentioned earlier; that's what made this challenge so attractive. There is a huge range of variables in this particular 30 caliber cartridge. I've not seen anything like it in my previous reloading experience. And now I have to build a castle out of this particular build set... I need the experience; and I personally learn/understand best through testing. Book work can make me gloss over the important facts in terms of math very easily. I simply want to build and test ad nauseum.

I understand that the .308 Win cartridge might be superior in that matter. That's why I want a .260. You sold me a long time a long time ago @98Z5V ; but I didn't fully understand why until very recently. 

I've been considering various methods of barrel bedding; but I'm not 100% sold on its use, or necessity in the AR platform. I would be easily swayed by some experiential testing though. There may need to be a side by side 24" test if this becomes and issue. That is one of the reasons why I specified no less than four barrels for this experiment.

[ARTrooper]

I can't fathom why anyone would want to build a 300blk with a barrel longer than 16 inches. Hell, I didn't even build mine, I bought it because it was an amazing deal I couldn't pass up. when I plan on building one, I will probably use around a 10 inch barrel and make it a pistol. the nice thing about such a slow moving round, is it is less affected by shortening the barrel compared to very fast rounds. if you wanted to make an ar15 with a 21.75 in barrel, I would suggest it be chambered in 224 Valkyrie. but the extra velocity will help .300 blk, I just think it would help other calibers more efficiently.

[Lane]

7 hours ago, ARTrooper said:

I can't fathom why anyone would want to build a 300blk with a barrel longer than 16 inches.

I'll try to keep this succinct... I live in NY; so far as I know, anything below 16" is considered a pistol by state law. That leads me to ask; why doesn't someone create a thread entitled: "Show me your attempts to concealed carry a 300blk pistol".

7 hours ago, ARTrooper said:

the extra velocity will help .300 blk

I thought this was true as well; and was building a 300 Blackout rifle anyway. It was the lack of options for long barrels that made me choose this route explicitly.

As as far as the 21.75" length in particular; I found the Houston Warehouse testing to be a very interesting and thought provoking venture. Much of the information provided goes a long way in describing accuracy tuning of some great shooters. I suspect, if the results as published were to undergo the scrutiny of peer review; that bit about 21.75" barrel resonance would have been called into question immediately. But; if the best guns in .22 and 30 caliber that shot there had barrels of exactly that length, it is in my mind a worthwhile reference point in testing 300 AAC Blackout with barrels 24" and under.

[ARTrooper]

6 hours ago, Lane said:

I'll try to keep this succinct... I live in NY; so far as I know, anything below 16" is considered a pistol by state law. That leads me to ask; why doesn't someone create a thread entitled: "Show me your attempts to concealed carry a 300blk pistol".

not exactly. we are getting into NFA (national firearms act), federal laws. in an AR style rifle, anything below 16" is a short barreled rifle and requires a tax stamp. the exception is if it is a "pistol." this would require no stock or an arm brace in place of a stock. also there are some other restrictions. As for your state laws and local laws, that I do not know because I live in a rather free area (for the time being). 

Now idk anyone that tries to conceal carry a 300blk pistol. that is not the point of having a 300blk pistol, same as most people wouldn't try and conceal carry a 44 magnum revolver with a 5 in barrel and many other types of pistols. the point of a 300blk pistol is it is short, light, easy to maneuver, and great for home defense or as a truck gun. also it is fun. and one huge reason why long barrels were never a thought in the minds of the developers of the 300blk is because it was made to put a subsonic round in an ar15 with the same capacity and without replacing anything but the barrel. now, you can get supersonic rounds flying at reasonable speeds for a rifle, but subsonic always has to be slower so that it doesn't break the sound barrier. so doesn't matter if the barrel is 26 inches or 6 inches, it still needs to remain under those supersonic speeds, so that's really where your barrel length is not going to matter much. 

for most people, 300blk is a round that can be chambered in an ar15 with only changing the barrel and keeping the same mag capacity. it is also a round that can be shot supersonic or subsonic with a can. now if you will always be using supersonic ammo, than I see the benefit of the longer barrel, but if you want it for its subsonic capabilities, in my opinion I see more benefits from a pistol or short barreled rifle setup. 

either way, any gun is a thing of beauty to me and I'll support your 100% in what you choose and I look forward to seeing what you build. ?

[Lane]

26 minutes ago, ARTrooper said:

not exactly. we are getting into NFA (national firearms act), federal laws. in an AR style rifle, anything below 16" is a short barreled rifle and requires a tax stamp.

This is all true; and so far as I know, entirely legal get a tax stamp for an SBR in NY. That is where the NY State law picks up though; anything below 16" barrel length must be put on your pistol permit, and thus treated exactly like a pistol. NY doesn't issue open to carry permits to anyone without serious high ranking connections. LE, judges, very wealth people, etc. Even then; those people are instructed to concealed carry for the most part, because civilians are so nosey they will call the police any time they see one. I have never seen a pistol displayed in NY without a badge next to it on the belt. So far as I know, there is no law forbidding the open carry of a rifle NY; yet I've never seen that either.

All you free Americans are welcome to play with short ARs from 1" to 15.9"; here in NY it's less hassle for me to stick with the rifle length, between 16"-24" in this case. 98Z5V says "stick with supersonics for accuracy in this build", I'm inclined to believe his advice on the matter. The 24" barrel I ordered is intended to be chopped down bit my bit. It may end up short enough to play with subsonics at some point; but of course suppressors are entirely illegal here. 

34 minutes ago, ARTrooper said:

either way, any gun is a thing of beauty to me and I'll support your 100% in what you choose and I look forward to seeing what you build.

I'm quite fond of the heavy barrel on an AR; I think it looks slick and feels great in my hand. My 7.62x39 build has one in 0.900" diameter; and the accuracy thoroughly impressed me. These 300 Blackout builds will be experiments in terms of accuracy tuning. I expect the education I get in the process will carry over well into my past and future AR builds.