If you’re gonna choose a shorter barrel, you have to know how to use it. Or more accurately, you need to consider some ballistic pros and cons of different bullet weights and caliber options available to you.
AR-15 pistols are super handy, especially when equipped with the right gear like a proper brace. Two of my favorite AR pistol braces are the Maxim Defense CQB, 4-Position Adjustable on the new Springfield Armory SAINT Edge and the Gearhead Works Tailhook Mod 2. Both are rock solid, work exceptionally well for one-armed support, and serve admirably if you choose to raise the pistol closer to your face and shoulder. But I digress.
Here’s the thing. With most calibers, for every inch you shorten the barrel, you’ll lose somewhere between 30 and 50 feet per second of muzzle velocity, all else being equal. So, if you choose an AR pistol a barrel six inches shorter than its rifle counterpart, your muzzle velocity may be 300 feet per second less, give or take. If your intended use is to ring steel or make holes in paper targets, that’s no big deal. If your use is hunting or defense related, you need to make sure that your ammo will do the desired job at in that velocity band.
When professional door kickers started to use more compact AR-type rifles for urban combat, one of the complaints was a reduction in effectiveness on target. That’s not entirely surprising as the “standard” 55-grain AR load was originally designed to operate properly at a 3,000 foot per second speed. At high velocity, that light full metal jacket bullet will tumble and fragment when it hits the target, thereby causing more fight-stopping damage. At slower speeds from shorter-barreled rifles, users found that it was more likely to just create small holes. That’s certainly bad for the recipient but not optimal.