If the player didn't pick it up in the RPD, the shotgun will instead be found in the Security Room in NEST where the player would normally find Shotgun Shells.
"4-round capacity 12-gauge pump-action shotgun. Its sturdy steel action makes this popular model reliable and easy to control."
The Long Barrel will increase the shotgun's size, taking two slots in the inventory from then on. It can, however, be taken on and off at will to accomodate for Leon's inventory management needs. The Stock, on the other hand, won't increase size, and will neither give the option to be taken off once applied.
The Shotgun will most likely be Leon's main weapon for taking down zombies, as it possesses the ability to nearly guarantee a critical headshot to a zombie's head on a pointblank range. Using the Long Barrel will increase the range from which this can be done, reducing risk for the player.
Even when not aiming for the head, the shotgun possesses acceptable stopping power to stun regular enemies and good damage; capable of taking down a Cerberus in one shot, all the while a reliable weapon against tougher enemies such as Lickers and Ivies. Ammo for it is likewise relatively plentiful; being easily crafted from the abbundant gunpowders in the game.