From The TimeSplitters Wiki

The Tactical 12-Gauge shotgun has replaced the double-barrel shotguns of TimeSplitters 2 as the best shotgun around. It deals a Great amount of damage with each shot and can easily dessicate oppositions at close range with its relatively short (compared to the double-barrel shotgun) wait between shots. The double-barrel is better for maps with few robots, since there is more time to reload between battles, but on a well populated or smaller map, the Tactical 12-Gauge comes into a league of its own, being able to cut 8 players to ribbons before needing to reload. The reloading on the shotgun is fairly unique among weapons, for instead of loading a magazine of a set amount or rounds taking a set amount of time, you reload the shotgun 1 round at a time. Although it takes longer to reload the entire clip, you can fire instantly while reloading instead of having to wait (pressing the trigger stops the reload and fires a shot as long as there is at least one round in the weapon).

This weapon is not recommended for medium or long range engagements, as the buckshot is fired so the damage spreads over a wide area. Stick to close quarters for best results.