Ah, the ole AR pistol. Nothing like taking an already marginal rifle round and shortening the ever loving crap out of the barrel.
But hey, to each his own.
To be fair, sometimes the "pistol" designation is being used as a work-around for imported firearms that are blocked by the current "semi-auto rifle" import bans (going all the way back to Bush 41). Change the configuration a bit to remove the buttstock and shorten the barrel to, say, 14" - presto, it's importable as a "pistol". It's an option for manufacturers who don't (yet) have a manufacturing facility in the U.S.
Of course, the owner can have a longer flash hider pinned and welded to the muzzle to get to the 16" minimum, and add the requisite number of American-made components and a buttstock. Voila! It's a legal rifle (carbine).
So there is a legitimate reason for some of those firearms to be sold as pistols - thanks to our screwed-up government.
The 5.56 NATO does give up a lot of velocity out of the shortest of the “pistol” at the 7.5” it’s down right a fireball maker the concussion of the round going off makes the eyes water. At the 14.5” length its much more manageable and give up little to the 16” M4 length.
Where the short barrel ARs really shine is calibers that were designed to burn fast powders such as the 300AAC it reaches peak efficiency at 10” or 8” with a suppressor. The 6.5 grendal also is very usable at the 12.5” lenght carrying well over a 1000ftlbs and 1800fps past 300m that’s solid class 2 game power up top 500lbs. The 458 socom in a 10” will lauch a 350gr flat nose hard cast round with 2000+ ft lbs that will penetratw stem to stern on any feral hog imaginable I have seen this round in a 12.5” barrel exit a bison that was a cull having broken the shoulder going in and ribs going out that was a 1500lb bison. Short barrel AR have a place the 6.5 and 458 are both game killing machines , in a kight compact perfect truck gun sized package. They also make the best tree stand guns for anything under 300m in the 6.5 and in the brush at 150 and under the 458 is thors hammer with 350 , 450 or 500 gr 45 caliber hard cast.