??????

[Edward Tinker]

It is illegal because the US gov't says it is so, in the USA

Common perception is this, if you own a stock, you better own one of the types that it fits it.

Are you going to be visited by a member of gov't? Probably not, but on the odd chance you display it, or someone comes by and reports you, (say you are at a range shooting with the stock on), then you'd better be able to "show" them. If not, then you might become a felon and a felon can not own a gun...