if you do it have your paperwork straight because it will become a SBR and require a NFA tax stamp to be legal (prior to installing the stock)

What is the best brace on the market for this gun? Or, what brace fits the gun right out of the box? Is an adaptor needed? Thanks in advance for your help.

you can add the pistol brace and its just like a stock and no tax stamp it needed. also the atf said shouldering it is not illegal any longer.

It is Matt but it will go from being classified as a "Pistol" to an NFA "SBR" Short Barreled Rifle and will need a TAX Stamp and Registration as an NFA firearm.