It's called a "Bullet Button", the idea is you use the bullet of a round to press the button to release the mag, that's what makes it legal. I have 3 that are "featureless", meaning they don't have the evil features which include pistol grip, adjustable stock, no vertical foregrip. I use the Thordsen FRS gen II stocks, it's pretty comfortable and is not considered a pistol group. There are some with fins on the rear of the pistol grip to allow you to hold it from the front so it can't be considered a pistol grip. And there are various wrist supports and short devices to get around the pistol law, Cali doesn't allow any AR pistols. I have a few, but none are assembled. I milled them prior to pistols being illegal.

We should take this to the off topic forum when it gets setup.

TT
Keroppi - 1946 Chevy 1/2 Ton Pickup