“That is not an arm brace.
That’s an adjustable stock.”
NO. It’s definitely a brace—you can clearly see the strap tight around the butt of the brace, drawing the brace arms together so that it can be shouldered as if it is a stock. Braces can also be adjustable, resembling an adjustable stock, so the adjustability lever shouldn’t fool you.
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.