The whole point of AR pistols is to be able to have a 10.5in AR without a $200 fee.
The idea of a pistol round in a carbine these days for any serious use is laughable. No matter the platform or barrel length 9mm, 40 S&W, 45ACP, etc. are just pistol rounds and as such suck.... at pretty much everything.
I have used 10.5in barreled 5.56's with good effects out to just shy of 400m qith proper ammo. With Greentip good terminal effects are shy of 75m. With 77gr SMK's it's not hard to keep them on a 12in plate at 600, but it acts like an ice pick short of 150m. Some of the bonded/barrier blind rounds designed specifically for terminal effects will show good performance out past 300m with shorty's.
Short (i.e. under 14.5in M4's) fill very specific niches. As far muzzle blast and noise? BFD. You have to wear ear pro with ANY gun. They have some limitations but you deal with them. Pick good bonded ammo designed to upset at low velocities and move on. I've seen and produced both and I would rather have the wounding effects of a 10.5in barreled 5.56 with good ammo at 300m than the effects of good 40S&W at 3m.
Don't go below 10.5in on 5.56 M4's. 11.5's are easier to get and keep running. Everything becomes more critical as you shorten the barrel but anything less than 10.5in barrels is just about useless.
If you need/want shorter than a 10in barrel, get a 300BO or 6.8SPC. These days the Blackout makes more sense. In an 8in barrel, Barnes Blacktip 110gr TTSX upsets completely past 300m.
