Didn't the SCOTUS decision in the 1934 Miller case set precedence that the 2nd protected arms in current employ by military/militia? I think that it goes without saying, and one can fairly assume, that applies to ammunition feeding devices as well. Then again, most gun related regulations passed since have violated this precedence in one way or another (IE: M4A1, technically, according to the 1934 SCOTUS decision, is legal and protected by the 2nd).

