Brandon, You would have to file to register this firearm as a SBR (short barrel rifle) and pay a $200 tax fee and wait to be approved if you want to put a stock on this firearm. You can get a pistol brace. There are many different brands of pistol braces. Even if you SBR this and pay the tax this firearms commercial buffer tube is not really set up for a folding stock. You can buy a 300 blackout upper to put on any AR 15 lower. The 8.5 inch barrel is threaded to accept the muzzle device that is on the firearm.

IF you put a "stock" on it you would need to have done the paperwork been approved and have paid the $200. tax stamp to make it a SBR (short barrel rifle)

Brandon, as is or with a "Brace" this is classified as a pistol. With any type of shoulder stock, folding or otherwise, the classification then becomes a Short Barreled Rifle "SBR" and would then become an NFA firearm requiring a $200 Tax Stamp and approval by the ATF for legal ownership.