I went out as elmer fudd a few years ago, and had a "shotgun" I used.
I just took an old, junk '03 sporter stock, a piece of black 1" pipe about 25" long, sunk 2 bolts through the stock/"barrel", and voila, i had a perfect halloween prop.
I caught a little flak for it looking to realistic, but surprisingly, no one cared that much.
Makes for one helluva roman candle launcher too