If you've read all the fine print and you don't see anything about a buttstock in the definition for a handgun it should be fine. In New York the only thing that defines a handgun is a barrel length of less than 16" I think. If they make no mention of it than it should be fine to use one. If they go so far as to state that you can't use explosive arrows, I think they would have already considered something relatively simple such as a buttstock.