You can add what ever you want to what ever rifle you want. The laws that you are talking about are clearly unconstitutional. You might want to read the 2nd Amendment. And while your at it, might as well read the whole constitution, the Federalist Papers, the Anti Federalist papers, and any and all writings my the Founders. Of course, state laws may vary, but if your in one of the northern states, the 14th amendment applies(it doesn't in the South, since they never properly ratified it), and states can't violate rkba, or any related rights.