What is up with the "pistols" such as the P516, P716, PM400, P556xi? It seems that they shoot rifle ammo (5.56 or 7.62) and in profile look like AR style SBRs but in the description I see that what appears to be a shoulder stock is actually a forearms brace. Seems a little inconvenient to me to have to walk around with that strapped to my arm. Is that how people really shoot them? Does weak hand grab in front of the mag well? Does the brace also function as a shoulder stock? Is there a legal issue with just putting a shoulder stock on?