From The TimeSplitters Wiki

Easily the best close quarter weapon in the game, the double shotguns can mince oposition with four shotgun rounds in a little over a second. On a small map with this weapon, nothing short of a rocket/grenade launcher is going to stop you from winning a fire-fight. Even armoured targets cannot withstand two vollies.

These are a must have weapon for those wishing to achive dominance in the hospital mexican mission levels.