Why would anyone put an “arm brace” on an AR15?
So it resembles an adjustable stock, has an adjustability lever, it’s on an AR15, but just because it looks like and adjustable stock, it’s on an AR15 which most have an adjustable stock, it has and adjustablity lever...but it’s not and adjustable stock.
OK then.
Clearly, you didn’t know why an arm brace is used, and what a stock is.
These arm braces are not on an AR-15 RIFLE, they’re on an AR-15 PISTOL. The barrel for a rifle is 16 inches or longer, and most of the AR-15 pistols have barrels less than 10 inches.
Pistols are harder to shoot accurately, being a one-handed and non-shouldered weapon with a much shorter sight radius, but using a brace, even a short one, lets the gun gain much stability in aiming, and thus in accuracy, whether the brace is just securing the pistol to the shooter’s forearm (the end of the brace splits into two pieces to go around the forearm, and the strap is secured around the pieces and the forearm, basically to soft-weld the gun into the forearm), or whether the end of the bound-up (unopened) brace is held against the shoulder, similar to a real stock butt.
Originally, the braces were for Veterans and others without the full use of both arms to aid their firing of a pistol—they couldn’t hold up a full length weapon (rifle with stock) and couldn’t use the non-firing hand to help hold up a pistol accurately.
But with the terrific buffering power of an AR-15, making the gun have so little recoil while still delivering a very flat-shooting effective weapon at 50 yards or less, the AR-15 pistols could be easily used with an arm brace with only one functioning upper limb. And when the BTFAE said the arm braces did NOT turn the pistols into short-barreled rifles (SBR’s—look it up), then even those with two good upper extremities wanted in on the wholesome goodness of these fun little pistols.
And no, most AR-15 rifles are NOT with adjustable stocks. Many are, especially those used in close quarters combat, but the original Colt AR-15 and many other use a full fixed stock. A real stock, by the way, has a much taller and wider surface area where the stock rests against the shoulder, compared to the arm braces, and are much longer than the “adjustable” arm braces even at adjusted full extent, and so the arm braces when shouldered help with accuracy but are very uncomfortable to use for an extended shooting session.
Next time, leave the oh-so-wise-sounding gun pronouncements to those who actually own, understand, and use such weapons.