A hammer cap, also called a hammer stall, is used as a safety device. Half cock on the lock is the safety, but the hammer cap is a secondary safety device just in case the gun should "go off half cocked". In which case the gun flint would simply hit the leather hammer cap & the gun would not fire. Here the hammer is not closed over the pan as it would be if the gun was loaded & primed, but you can clearly see the leather hammer cap in place covering the locks hammer. Here you can see the hammer cap for my pistol securely tied to the trigger guard. Keith.