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View Full Version : Best barrel length for SBR DI or GP


cmace22
05-31-2011, 6:06 AM
Im building an AR pistol and Im wondering what you think would be a good choice for upper.

To me a pistol AR is as close to an SBR as I can get here in cali and If I were to ever move out of state it wouldnt be a pistol for very long. So for the purpose of this thread consider it an SBR build instead of a pistol :)

Im looking at a few different lengths: 12.5, 10.5 and 7.5 seem to be the most common. Im familiar with the velocity drop between barrel lengths and what that does to the effectiveness of 556. It seems the word on the street is GP is a better performer than DI when it comes to barrels under 14". Cost isnt an issue.

So given the choices what would you use on an SBR and why. 12.5 vs 10.5 vs 7.5 and DI vs GP.

The pole are the ones Im currently looking at.

LMT 10.5" Mk18 clone build
http://i19.photobucket.com/albums/b164/cmace22/MK18.jpg

LMT MRP 10.5"
http://i19.photobucket.com/albums/b164/cmace22/lmt105cqb.jpg

LMT MRP 12.5" piston
http://i19.photobucket.com/albums/b164/cmace22/CQBPiston2.jpg

PWS Diablo 7.5"
http://i19.photobucket.com/albums/b164/cmace22/MK107u_556_L_1.jpg

PWS MK110 10.5
http://i19.photobucket.com/albums/b164/cmace22/MK110u_556_L_1.jpg

I know there are others that make an MK18 type upper (DD and Noveske come to mind) Basically its a 10.5" DI with a rail.

FeuerFrei
05-31-2011, 3:58 PM
For overall 556 accuracy I choose an pistol upper with a 10.5" barrel w/heavy contour. Mine isn't on your list. RRA barrel-chrome lined-heavy contour w/Adams GP kit on it. Also a 7.5" DI I assembled and another 7.5" GP.
All are accurate and reliable with all ammo I've fed it including Wolf.
You can build up a short upper for cheap too. I also use GSE short buffers to keep lengths manageable.

Stone
05-31-2011, 4:53 PM
I prefer DI for mid/long guns, but I vote 12.5" piston for the SBR...because the accuracy of a free floating barrel isn't needed, the weight will already be low, and the advantage of not having to clean it will be rewarding on something that's designed for more rapid shooting. I wouldn't go lower than 12.5" because of the loudness and recoil, unless of course you are going to rechamber it for handgun ammunition.

cmace22
05-31-2011, 5:21 PM
For overall 556 accuracy I choose an pistol upper with a 10.5" barrel w/heavy contour. Mine isn't on your list. RRA barrel-chrome lined-heavy contour w/Adams GP kit on it. Also a 7.5" DI I assembled and another 7.5" GP.
All are accurate and reliable with all ammo I've fed it including Wolf.
You can build up a short upper for cheap too. I also use GSE short buffers to keep lengths manageable.

Didnt even think of RRA. Ill have to check them out. How do you like the AA kit?

I prefer DI for mid/long guns, but I vote 12.5" piston for the SBR...because the accuracy of a free floating barrel isn't needed, the weight will already be low, and the advantage of not having to clean it will be rewarding on something that's designed for more rapid shooting. I wouldn't go lower than 12.5" because of the loudness and recoil, unless of course you are going to rechamber it for handgun ammunition.

I think Im going to toss out the 7.5" idea.

Since my goal isnt to have the smallest package available I dont think the added noise and muzzle blast is worth it for me.

10.5 seems to be the shortest length thats recommended for a suppressor if I ever wanted to add one in a free state.

12.5 adds velocity which would make a better performing platform in 556.

Is there any legal reason I can not shoulder the pistol buffer tube? It would seem that if its a pistol I could shoot it however I want as long as its in a safe manner. The act of shouldering woudnt make the tube a stock would it since its integral to the pistol functioning unlike a stock.

tomd1584
05-31-2011, 5:32 PM
I'd go with the BCM 12.5" kino upper. you get that added 2" of velocity, with the longer handguard/sight radius of the midlength gas system. Perfect for adding a suppressor.

Thats my plan once i'm in AZ permanently.

(courtesy of Retreathell)

http://i685.photobucket.com/albums/vv219/RetreatHell/BCM%20Kino%20SBR/DSCF1680.jpg


http://i685.photobucket.com/albums/vv219/RetreatHell/BCM%20Kino%20SBR/DSCF1703.jpg

cmace22
05-31-2011, 5:33 PM
I'd go with the BCM 12.5" kino upper. you get that added 2" of velocity, with the longer handguard/sight radius of the midlength gas system. Perfect for adding a suppressor.

Thats my plan once i'm in AZ permanently.

(courtesy of Retreathell)

http://i685.photobucket.com/albums/vv219/RetreatHell/BCM%20Kino%20SBR/DSCF1680.jpg


http://i685.photobucket.com/albums/vv219/RetreatHell/BCM%20Kino%20SBR/DSCF1703.jpg


OHHHHHHHH!

I likey!

cmace22
05-31-2011, 5:38 PM
I'd go with the BCM 12.5" kino upper. you get that added 2" of velocity, with the longer handguard/sight radius of the midlength gas system. Perfect for adding a suppressor.

Thats my plan once i'm in AZ permanently.

(courtesy of Retreathell)



It looks like it retains th carbine gas system but moves the FSB farther forward.

http://www.bravocompanyusa.com/v/vspfiles/photos/BCM-URG-M4-12%20KD-2.jpg

tomd1584
05-31-2011, 5:46 PM
It looks like it retains th carbine gas system but moves the FSB farther forward.

correct. longer sight radius, long handguards = better control of the weapon! :D

Stone
05-31-2011, 7:59 PM
12.5 adds velocity which would make a better performing platform in 556.

Meh, not really. Velocity only matters if you want to maximize your long range capability or if you think you can achieve fragmentation. Even with a 12.5 inch barrel you'll be below the fragmentation threshold beyond after probably 15-20 yards at best. Beyond that point velocity doesn't matter much. I think you'd be better off with 5.45x39, or rechambering into a handgun round like .45 ACP, with a really short barrel.

cmace22
05-31-2011, 8:01 PM
Meh, not really. Velocity only matters if you want to maximize your long range capability or if you think you can achieve fragmentation. Even with a 12.5 inch barrel you'll be below the fragmentation threshold beyond after probably 15-20 yards at best. Beyond that point velocity doesn't matter much. I think you'd be better off with 5.45x39, or rechambering into a handgun round like .45 ACP, with a really short barrel.

True.

Stone
05-31-2011, 8:12 PM
Honestly, I think short barreled AR's are more of a novelty item than a practicality. The AR platform was designed around rifle ammunition that doesn't perform well from short barrels, so if you want a short barrel, you really should be looking at SMG's firing handgun ammunition, for which there are much better options...at least for LEO :P

bluebird
05-31-2011, 8:24 PM
I voted "other" because I think 5.56 itself is not sufficient as an SBR round.

There are much better alternatives such as the 6.8SPC and 300BLK because they perform way better than 5.56 in ballistics and if you are concerned about velocity (which causes bullet drop), in sbr length a 77gr .223 has similar velocity comparing to 300BLK and 6.8SPC. If you are concerned about compatibility, 300BLK can run with ALL 5.56 parts except the barrel and flash hider.

Correct me if I'm wrong.

JayBeeJay
05-31-2011, 8:35 PM
I'd go with the BCM 12.5" kino upper. you get that added 2" of velocity, with the longer handguard/sight radius of the midlength gas system. Perfect for adding a suppressor.

Thats my plan once i'm in AZ permanently.

(courtesy of Retreathell)

http://i685.photobucket.com/albums/vv219/RetreatHell/BCM%20Kino%20SBR/DSCF1680.jpg


http://i685.photobucket.com/albums/vv219/RetreatHell/BCM%20Kino%20SBR/DSCF1703.jpg

WOW that BCM is a beauty!!! I wish I could turn my BCM into an SBR as well, maybe someday if I ever move to my Las Vegas property.

Back on topic I vote for the 7.5" Diablo because I have one and it's pretty damn fun to shoot. I don't really care, but it's a head turner as well :43:. If the funds are right after a couple other toys I have lined up I'll be getting a full DD pistol with the DD MK18 upper.

http://i1195.photobucket.com/albums/aa393/JReezelle/IMG_0656.jpg
http://i1195.photobucket.com/albums/aa393/JReezelle/IMG_0657.jpg