Is a crossbow considered to be a firearm? Prob not. So.........

Use a tube as a crossbow bolt with sufficient ID to hold a 410 shotgun
shell. Inside the tube is a lead slug with a firing pin on its front
almost touching the primer of the shell. The shell is prevented from
sliding abckward against the pin by a lip so it cannot fire during
launch. However, the sudden stop of the bolt on reaching the target
causes the lead slug to slide forward firing the shell. Your bolts
would be pre-loaded but "safed" by a removeable piece that prevents
the lead slug from sliding. A hand cocked crossbow would be
sufficient since you are not relying on the velocity of the bolt to do
much.