The slowest firing full-auto weapon in the game alongside the DP27, the M12 is a weak submachine gun in any scenario, and is only somewhat suitable in mid-range engagements, as its slow rate of fire and low recoil do make it easy to control. Attaching the Extended Barrel will also eliminate damage drop-off, allowing it to take down any target with 3 shots at any range. It's suited for a more methodical playstyle thanks to having the tightest hipfire accuracy out of all submachine guns.
The M12 in reality fires in an open bolt configuration, meaning that one in the chamber would be impossible, unlike the in-game counterpart, which holds one in the chamber. This error is shared with other guns with an open-bolt configuration.
Despite the reload animations correctly depicting the weapon with an open bolt by showing the operator locking the bolt to the rear, during firing, the reciprocating bolt is shown not locking back with the bolt, and instead reciprocates in a closed bolt fashion during firing.