commonly used deer slug ( most likely 12 guage as it the most popular size deer gun --could be 10 gauge but there not as common))--- foster type (note hollow base) -- brennake are solid based * --foster took the earlier brennake design and hollowed out the base -- hoping to get a better gas seal -- the hollow base expands similar to the way older black powder minie balls did -- making for a tighter gas seal to the barrel-- the fins built into it make it spin in flight to make it more accurite