[gyvel]

Well, since they KNOW the dimensions of any factory-produced M1891 or 91/30, and the markings that they would have borne, I'd say they'd go over it with a fine-toothed comb to see if any metal or markings have been removed, and if there is, then the presumption would be that that you stripped down an existing rifle, and therefore, have an SBR.

If that were the case, the same would apply to an AR 15 receiver that was purpose built to be a pistol, no?