The machine pistol fires a shot every time the player presses the fire key. Holding down the fire key will make Sam rapid-fire it, but not as fast a hitting the fire key as fast as possible.
Each shot is very weak; it takes multiple shots to kill anything weaker than a Beheaded-type enemy, the weakest enemy type in the game. It has infinite ammo, giving the player a ranged weapon they can always use if they run out of ammo for the rest of their weapons.
The machine pistol is borderline useless; it does a pitiful amount of damage per shot, and its rate of fire does not compensate for this. The only good thing about it is that it has infinite ammo, but its pathetic amount of damage per shot makes this a very small advantage. It should be removed from a stack as soon as possible.
The machine pistol's infinite ammo makes it useful as an emergency weapon if the player is out of ammo, but it should only be used for self-defense while running to more ammo. Taking on hordes with this weapon is suicide.
The upgrades in Serious Sam: DD XXL to the machine pistol actually makes it semi-useful in a stack. There are four upgrades available:
A jetpack addon (air buffer) that lets you slowly decend while firing. This is useful while getting to high to reach places with the jump pad.
A health restoration modification that regenerates health while firing (approx 1 point a second).
A damage boost that increases the damage of all guns in the stack.
A shuriken modification that flings out throwing stars.