Why you say 10.5" run the best?
5.56 or .300 blk? Does it matter?
I'm prolly gon get 1 of both, or 1 pistol (buffer tube) with 2 uppers (one 5.56, one .300blk).
I want 1 AR pistol & 1 AR rifle
What I said was my 10.5 runs the best out of the piston uppers I have. I'm talking in terms of maneuverability, reliability, and being suppressor friendly. I have had issues with gas blow back with my D.I. Ar's, when used in conjunction with suppressors. Piston driven uppers gas systems are adjustable, they can be set to the shooters liking.
I have a 16" and 8.5" 300 BLK, both are D.I., so I can't speak for the piston variants.
I have owned AR and AK pistols, they aren't practical in my experiences. They are meant to be shouldered, which doesn't lend well to pistols.....