A friend has old pump gun (I think it's a Mossberg 500, not sure, haven't seen it) that has been barrel whacked, but it is over 16" from the bolt face to the end of the barrel, he says. The butt stock has been whacked to a pistol grip. Wanted to check for him and ask is this a legal configuration or does he need to find a new full-length buttstock for it to make it legal?