shotgun (smooth-bore)
Portable firearm where the inside of the barrel has no grooves; it can shoot a number of lead, copper or nickel pellets at a time.
butt plate 
Metal, rubber or plastic covering attached to the end of the stock to reinforce and protect it.
breechblock 
Movable part that closes the chamber where the cartridges are loaded.
hammer 
Part that triggers the shot by striking the firing pin, which in turn strikes the cartridge primer causing the powder charge to explode.