Free: Contests & Raffles.
Wanna know something scarier? For the new 320 grenade launchers the military adopted (replaced the 203, can be shot standalone or mounted on the m4/m16) there is a metal and plastic cap that snaps onto the barrel and it lanyarded to the weapon. They say you can shoot it with the cap on and the cap moves harmlessly from the barrel before the grenade hits it.
I just stuff my muzzle in the dirt to get a good plug in there.