The 12.5" AR Performance barrels in 6.8 are only a 100-150fps slower than the 16" versions and are amazingly accurate with 110 Prohunters. I built one and it has become my "go to" over the other (3) 6.8's. I put a full length buffer tube with a rubber end cap on it and it works very well, resting it against my shoulder.

 

Yep what ^^ he ^^ said. The 12.5" barrel only gives up 125 to 175 fps compared to the 16" barrel, if I remember correctly.

 

While there are some out there that have enjoyed the 7.62x39 in an AR platform, there are too many downsides for me. The case taper of the x39 is greater than 5.56, so they don't feed properly in the longer straight section of magwell in the AR. They also leave little meat in the bolt once they're bored out to 0.447+" from 0.376+"(good luck finding a x39 bolt, btw).

It seems to me that in a pistol platform, the 6.8 gives up the least in terms of velocity. The 300BO and 7.62x40wt also do well with short barrels. In my opinion, you're giving up a lot more in a 5.56 with a short barrel.

Since you already have and load for a 6.8, I would build your pistol with that. Wilson Combat, AR15 Performance, Black Hole Weaponry, and Bison all make great barrels. Just need to wait for them to be in stock.

 

12.5" would be ideal for velocity/terminal ballistics, but does it diminish the returns of a compact pistol configuration? I guess what I'm looking to establish is the relationship between barrel length to terminal ballistics on a thin skinned animal. Where would I drop below that 800-1000 ft lbs of energy threshold/effective range threshold for a 12.5", 10", 8", etc? This would obviously vary some with bullet and load, but just looking for general consensus. I don't see many opportunities where this platform would be pushed much past 100yds, and more realistically would probably find it's niche well within that range.

 

There's a big thread on the 8" barrels from Bison. None available now, but maybe in the future. I believe there are some velocity numbers in that thread for 8" barrel. Seems like it was mentioned that it shortened the effective range to 100-150 yards max.

Edit with links to threads:

http://68forums.com/forums/showthre...read.php?31392-8-5-quot-Pistol-Barrels-Available&highlight=bison+pistol+barrels

http://68forums.com/forums/showthre.../showthread.php?42594-AR-Pistol-caliber-choices!&highlight=bison+pistol+barrels

 

Black Hole will do a pistol barrel in what lenght you choose. The finsih choices are nice and velocites out of the true ploygonal rifling will exceed the normal rifling types for a given lenght.

Greg