AR Pistol barrel length

[Refleks]

My next project is an AR pistol ("PDW" type firearm in concept) which may be used for home defense in a pinch but will most likely spend it's life as a fun range toy.

 

.300 blackout is prohibitively expensive for me, even reloading (since I don't cast bullets) so I've decided to go for 5.56mm for now.   Of course velocity is important in 5.56 -- and unfortunately neither the 10.5" nor the 7.5" barrels will offer sufficient velocity for M193 to reliably fragment even at the muzzle from what I understand, much less M855.   Since I'm not limited to FMJ like the military, that's not a deal killer since commercial expanding ammunition should help -- Hornady 55gr Zmax apparently expands and still achieves good penetration (~19.5") in gel even out of the shorter 7.5" barrel.  I can still use the cheaper FMJ at the range.

 

 

So my question is more about usability.  An AR pistol is apparently pretty unpleasant to shoot, especially indoors, even with earpro.  So would it be better to get a 7.5" barrel with KX3 flaming pig to send the concussion downrange away from the shooter, or stick with the 10.5" barrel but with standard birdcage flashhider?  They both have about the same overall length, and I'm not wanting to go any longer than that -- I just don't know how much of a difference the KX3 would make.

Edited April 26, 2015 by Refleks

[Dolomite_supafly]

Missouri Bullet Company sells some 245 grain cast, and coated, bullets for not a lot. Something like .20 each shipped and they do shoot really well.

 

If you are sticking with 5.56 then barrel length is king. A 7.5" barrel is barely over 2,500 fps and the extra 3" can bring the velocity up into fragmentation velocities. I would also try to find a 9 twist over a 7 twist. A 7 twist with such a short barrel is a very bad combination. Not only will accuracy suffer with anything but the heaviest bullets the bullets will not tumble, or fragment, limiting the effectiveness of the round.

 

The next shorty I build will have a 12 twist barrel to ensure the bullets tumble.

[Clod Stomper]

The next shorty I build will have a 12 twist barrel to ensure the bullets tumble.

That's what I'm gonna look for when I build another pistol upper. Mostly because it will be used mainly with a .22 or conversion, but also for some instability with 5.56.

Do you know of a source? Edited April 27, 2015 by Clod Stomper

[Dolomite_supafly]

That's what I'm gonna look for when I build another pistol upper. Mostly because it will be used mainly with a .22 or conversion, but also for some instability with 5.56.

Do you know of a source?

I am going to make my own. I will likely use a Green Mountain blank.

[nightrunner]

70-77gr is best for the shorter barrel 5.56

Maybe consider a 5.56 upper for the range and put a .300blk upper on it if used for HD.

[Dolomite_supafly]

You can buy 7.62x39 upper, or barrels, pretty cheap these days. If I wanted a supersonic only 30 caliber in an AR it would be 7.62x39 over 300 Blackout. 7.62x39 will feed fine from 5.56 magazines as long as you don't load them all the way up. I would load 15 rounds in 20 round magazines and never had an issue. I bet the AK magazine lowers will become available before too long.

The ONLY thing 300 Blackout does better than 7.62x39 is shoot heavy subsonic bullets through a silencer.

For home defense I would also consider a 9mm blow back gun. You get 357 magnum performance out of it. And being blow back it is the most reliable setup, infinitely more reliable than any locked breech gun.

[atlas3025]

Missouri Bullet Company sells some 245 grain cast, and coated, bullets for not a lot. Something like .20 each shipped and they do shoot really well.

 

Not to derail, but will those feed well in an AR?

 

Also, if there is no desire to suppress, I would agree completely on either 7.62x39 or 9mm for a short home defense gun.  If you want to suppress, 9mm will still work, but to get the good stopping power you need a barrel length that is likely going to put you supersonic.  

 

The mag-wells are pricey, but you can get a modular lower/mag-well combo from MGI-Military that with two different mag-wells, can accommodate Colt SMG mags in 9mm, and/or AK Mags for 7.62x39.  They do make them for Glock Mags too when you can catch them in stock.  They used to make a 45 ACP mag-well for Grease Gun Mags too...been waiting on one of them since 2012.   

 

I recently got my SBR stamp back on one of these.  I have mag-wells for standard AR mags, 9mm Colt SMG, and soon am getting an AK mag-well.  One stamp, but can run anything that takes any of those styles of magazines now as an SBR.  

 

Lower: https://www.mgi-military.com/store/index.php?product_id=80&type=&category=20

 

Mag-wells: https://www.mgi-military.com/store/index.php?category=9 

Edited April 27, 2015 by atlas3025

[Refleks]

lol now I have more to think about!

[Refleks]

While lighter, shorter bullets like 55gr M193 out of a 1/12 twist barrel is fine, shooting heavier or longer rounds like M855 out of a 1/12 twist barrel is a bad idea...

 

Any spitzer type FMJ (basically any pointy modern rifle round) is going to tumble once it hits a person, no matter if you're using 1/14 all the way down to 1/7.   But what you don't want is it tumbling through the air!  1/12 is fine for 50-55gr but it won't won't stabilize M855 (much less longer and heavier 75 / 77 gr type OTM bullets) and beyond a certain point it's going to start tumbling through the air, reducing accuracy / opening up groups and keyholing targets.

 

If you're relying on fragmentation as a wounding mechanism, then you want to overcome the structural integrity of the jacket and round so that when it starts to tumble it comes apart.   ~2,700 +/- is the magic number for 55gr M193 and M855, but a faster twist like 1/9" and 1/7" can only help as long as you're above that velocity -- it puts significantly more stress on the round because of how quickly it's spinning, which will encourage fragmentation -- it's fast enough to stabilize the round in the air, but not nearly fast enough to stabilize it inside a much denser medium like a human, so it's still going to tumble.

The problem in Afghanistan and Iraq is not the twist rate, it's the bullet they're using.  M855 / SS109 steel core wasn't designed to shoot through unarmored skinny guys, it was designed to defeat soviet helmets out to about 600 meters as part of the SAW program, the rifles followed suit to maintain ammunition standardization.   It will fragment past 2,700 FPS like the 55gr M193, but the issue is the penetration depth before the round starts to tumble and fragment.  With M193 it's consistent and will start to tumble and fragment very quickly, often within a few inches of penetration.   With M855, because of it's construction it's inconsistent (depending highly on the angle of the round as it strikes the target, so I guess the yawing through the air thing isn't SO bad in a sense if you're using green tip ;)  ).  Sometimes it works great, other times it penetrates further into the target before upset and may even pass through before it gets a chance to tumble and fragment, which yields poor results.

In Afghanistan and Iraq 55gr M193 would have been a far superior round to use against the threats we're facing there, even with existing twist rates for the M4.  Or even better, the the heavier and longer 75/77 gr OTM bullets that will still fragment at lower velocities.

Edited April 28, 2015 by Refleks

[Ron Padilla]

to add another question to the above, where would one fing a bcg for 7.62*39 for an ar configuration.  I am thinking of getting one for either my ar pistol or carbine.

 

thanks

[atlas3025]

to add another question to the above, where would one fing a bcg for 7.62*39 for an ar configuration.  I am thinking of getting one for either my ar pistol or carbine.

 

thanks

 

http://palmettostatearmory.com/index.php/ar47/psa-7-62x39-bcg-phosphate.html

[Ron Padilla]

Thanks for the reply

[Dolomite_supafly]

You can also just buy the bolt and swap carriers.

 

http://www.tacticalambush.com/AR-15-762X39-MPI-Bolt-By-Tactical-Ambush_p_141.html

[Ron Padilla]

thanks dolo you da man

[79troublehead]

Thanks. Now I gotta build a 7.62x39 ar. 

[Dustbuster]

I built a simple small ar pistol w a 7,5 faxon stainless barrel dda side charger and red dot sight. If they come through the hall it'll work, especially w a kx3 noveske to blow noise towards the fools that hit the wrong house. The 7.5 is short enough to maneuver and stash as needed. Am not concerned about frang attributes as I have no neighbors to be concerned w. But take what dolomite says as gospel

[Jct1911]

thanks dolo you da man

Before ordering from tactical ambush, be aware of slow shipping times. They've been hit by a rush of customers. I've also read a post about a broken 7.62x39 bolt. Not all bolts are the same.

[Ron Padilla]

Oh ok cool thanks for the heads up

[Ranger431]

I have 5.56 10.5" barrel and put a Claymore linear comp on it right out of the box, it's not unpleasant to shoot at all...

[ehull20000]

223 Federal Fusion MSR 62gr works really well in my home defense 10.5 AR with a 1:7 twist barrel. It precludes the fragmentation concern associated with lower velocities and instead offer controlled expansion down to around 200 meters. I feel that is a very acceptable effective range for civilians in a urban/suburban setting.

Without a muzzle device it is loud and there is considerable muzzle flash. With a brake it is loud and shoots great. With a can it is less loud and shoots excellent. I have no experience with a KX3 product.

[10-Ring]

On a pretty much related topic what are the pros and cons for different barrel lengths if you are going for a 9mm AR pistol? 

[atlas3025]

On a pretty much related topic what are the pros and cons for different barrel lengths if you are going for a 9mm AR pistol? 

 

I went 5.5 inches so that with my suppressor it is still REALLY short.  I also wanted to make sure 147gr ammo stayed subsonic, which may not have been an issue even at longer length, but in cold weather, another couple of inches could put me trans-sonic.  If you don't care about suppression, and instead want to get as much ooomph as you can out of a load, then I suspect 8 to 10 inches would be a pretty sweet spot.  Most 9mm loads don't tend to pick up much steam after about 10.  

 

If you are running 115gr +P ammo, 8 to 10 inches should get you somewhere around 1450 fps.