I'll let others comment on the proper load, but I thought I'd mention that years ago I made concrete projectiles in cans. The cans would often rip off of the concrete and then the concrete would scratch up the barrel. I would look into casting lead or zinc, or maybe fill cans with something else. Also, it probably won't be too long before you want to switch to something that is more accurate when the fun of just making noise and not hitting much wears off. You might want to consider skipping a step.