^
Theoretically, I'd assume you shoot the centre barrel(s), then move forward after it explodes and pushes the surrounding barrels away. By the time the chain-reaction of explosions works it's way around to behind you, you're well out of harm's way.
Theoretically, remember. Explosion power is erratic, and such a tactic is a pointless endeavour when an easier (and far safer) option is available.
Sorry, getting all strategic. That's what happens when you play Survival with cack-handed mugs though; you end up becoming a Skulltag Sargeant and spout advice at every god-damn opportunity.
The noblest of all dogs is the hot dog: it feeds the hand that bites it.
DoomBears!: http://www.youtube.com/watch?v=fU2xFxdK1t4