Always wear safety glasses! A few videos from YouTube illustrate why.

[ Definition: Firing out of battery: when a round is fired before it is fully chambered. Instead of the expanding gas being forced out the barrel it wrecks havoc inside the gun) ]

AR-15 blows up, not sure that happened. Out of battery?

12 Gauge single shot shotgun blows up. A home made shell apparently caused the problem.

He seems to have a problem fully closing or cocking the shotgun. Maybe the shell was to long and it protruded slightly out of the chamber, causing the explosion. Anyone got a better explanation?

The slide is blasted off a blank firing pistol.

A homemade pistol blows up. The barrel shoots off and the shooter gets the steel primer cap embedded in his arm.

AK-47 blowing up. Out of battery. This is an old one I have posted before on the blog.

Safety glasses are your friends. These are WileyX tactical glasses.

UPDATE: A bonus video. The bolt action barrel is plugged causing the barrel to split. [ Thanks to UraniumHead for the link ]