10.5” Aero Precision upper.

quote:

Originally posted by Kevbo:
Under 11.5” I would go with the .300 over 5.56

The missing question in this discussion thus far is what do you intend to do with the gun?
If it’s just to impress and/or annoy the other shooters at the range, and keep them from crowding too close, then 5.56 is superior. Even with a can, the one SBR I’ve seen fired at night produced an awesome muzzle blast. And of course 223/5.56 ammunition is less expensive in general and far more available.

For serious or semiserious purposes, though, the 300 Blackout cartridge in a short barreled AR type gun is far superior.

The wounding effects of the 5.56 round are largely due to its velocity from carbine or longer barrels. When shortened to even 11.5 inches it loses a lot, and going shorter is even worse. That doesn’t mean its bullets won’t poke a hole in a BG from a short barrel (a 2" 22 LR revolver will do that), but with typical loads the 300 BLK from a pistol length barrel produces as much kinetic energy as the 5.56 does from 16", and of course fires a larger, heavier bullet.

Unless we’re hoping for an intimidation effect if we don’t actually hit the threat, the muzzle blast of 5.56 from a short barrel can actually be counterproductive for the shooter in a defensive situation.
“Well, just put a can on it!” Sure, if you don’t mind the trouble and expense of doing that (and assuming it’s even legal where one lives), and then what do we have? Add 6 inches to 10 inches and it’s the 16 inch carbine, except one that must be carefully tuned so it doesn’t blow gas and other stuff back into one’s face and eyes when shooting, and still suffers from the inferior ballistic performance of a short barrel.

I have an MCX with 9 inch barrel in 300 BLK and despite having much less experience handling and shooting it than a conventional AR carbine in 223/5.56, my performance with it to 50 yards during a comprehensive skills course I developed some years ago is virtually the same as with my Colt 6920s. The recoil of the MCX is slightly greater and the controls are a little different, but none of that affects speed and accuracy to any noticeable degree.

I bought a Colt commando 11.5 upper a few weeks ago for $405 shipped from Brownells. I specifically sought out a 11.5 inch upper for the 40% better dwell time of a 11.5 vs a 10.5 upper.

I put 200 rounds down range last week and it worked perfect and wasn't all that obnoxious, honestly.