The 10mm SMG is most effective at close-range by unleashing a hail of bullets. However, the SMG is next to useless at longer ranges unless assisted by the perk Concentrated Fire. The SMG simply has too much spread to be of much use beyond close-range combat. Another downfall of the SMG is its ammunition consumption; if ammunition is not properly managed, the SMG can eat through 10mm rounds at alarming speeds. With full automatic fire and a 30-round magazine, the SMG is a decent emergency sidearm when enemies start to get too close, particularly given that 10mm ammo is plentiful. The SMG however is very common, making it easy to repair and is recommended to lower level Small Guns users.
Sydney's 10mm "Ultra" SMG - Relic hunter Sydney's weapon of choice, the Ultra SMG holds an astounding 50 10mm rounds in a single magazine, and deals more damage than the standard SMG.
Sim version - A sim-only version of the weapon which differs only in health, with a whopping 999,250 hit points. Essentially preventing the gun from ever degrading, well over 1500 rounds of ammunition can be fired without it degrading 1%. The weapon is not accessible outside of the simulation without the use of the Gary 23 exploit or console commands. It can be obtained in the sim through certain equipment packs on the equipment terminal.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
10mm Alloy Steel Submachinegun - Another version of the weapon was cut from the Operation: Anchorage add-on. Called the 10mm Alloy Steel Submachinegun, it is otherwise identical to the standard 10mm SMG.
End of information based on Fallout 3 cut content.
Several SMGs can be found inside the L.O.B. Enterprises building. There's one in the main floor's cafeteria, one on a box in the East Wing, and several in the Archives section. All are in poor condition.