The Locomotive 12G is a sawn-off version of the Reinfeld 880. In many respects the two weapons are quite similar, with similar damage, accuracy, stability, and magazine and ammo pool.
The primary difference between the two weapons is that the Locomotive 12G takes up the secondary slot, and the locomotive's much higher rate of fire. The Locomotive 12G, like the Reinfeld, can kill most enemies with a headshot at close range, and its heightened accuracy can make it more effective out to longer distances.
The Locomotive 12G is an excellent compliment for a ranged or heavy primary weapon, such as the M308 or any of the LMGs, and can be a viable backup should the primary weapon be empty or on the reload. It really comes into its element when used on heists such as Bank Heist or Framing Frame, being able to clear rooms of enemies in very little time. It is a good secondary for an Enforcer with shotgun boosting skills, such as Shotgun CQB or Shotgun Impact.
The Locomotive 12G can also be suppressed and used as an effective stealth weapon. While it obviously lacks the range and accuracy of other silenced sidearms, its excessive power can send a guard flying several metres away, depending on how close one is. This can prove to be either an advantage or a detriment in a stealth scenario, as the player can knock guards clean away from door ways and out of sight of other guards, but must make sure not to propel the guard's body out into the open where other guards and civilians can see it, or so far away that reaching the pager is impossible.
The suppressor obscures the sight post, and several of the stocks also add an accessory rail that entirely hides it. The Muldon Stock however, will give the sight picture a rectangular frame in which it's easy to center the target. (This stock also increases concealment slightly, and the stability malus is usually inconsequential given the weapon's fire rate.)
When using ammunition types other than AP Slugs or HE Rounds, the Locomotive 12G fires 6 projectiles per shot.
Coupled with the GL40, the Locomotive's high damage and fast pump time allows for effortless dispatching of Bulldozers.
The Assault Light works very well for estimating where shots will hit due to the spread.
Since the stated damage in the menu is applied to each pellet fired and will not stack up on the same target, it may be disadvantageous to increase accuracy, as it will limit the gun's ability to hit multiple targets.
When using a sight with the Locomotive 12G, keep in mind that pellets will instead hit around and below the optic's indicated point of impact. To make the most out of a sight, it may be worth using one of the circle reticles.
The game files name it after the Super-Shorty, however this is obviously incorrect as the barrel and magazine tube are longer than a Super-Shorty, the pump and grip are different, and the Locomotive has sights.
The words "Buff The Loco" are graffitied on a wall in the backyard of the Safehouse. This refers to the substantial amount of complaints from PAYDAY: The Heist players that the Locomotive 12G was too weak.
The Locomotive 12G was later nerfed for being too effective at longer ranges.
The Locomotive 12G has the words "Reinbeck Tactical" inscribed on the side.
A base model Locomotive 12G is actually more powerful than an unmodified full sized Reinbeck 880 despite having a much shorter barrel, in real life this would not be the case as even shotgun pellets benefit from higher muzzle velocity from longer barreled shotguns and thus inflict more damage.
It has an unusually high rate of fire for a pump-action shotgun, able to shoot 2 bullets every second.
Like the Reinfeld, when using some stocks, (The Standard, Tactical, Wide, etc.) there will be a rail added along with it. However, if one looks closely, they would notice that the rail isn't actually connected to the stock - it's a purely visual addition, and it's position is rather arbitrary, as it could perform the same function further up the receiver.
It is apparent that the Locomotive originally had a stock, before being unanimously removed in the final version. This was evident in the beta release, when weapon mods used to have descriptions, with that of the Tactical Shorty mentioning the removal of a stock to improve Concealment.