The Model 77E is a reliable combat shotgun which boasts very high damage at a good range with a low jam chance. However, the pump-action makes this gun very slow, and the small clip size means you will need to reload frequently. For these reasons, it is recommended to have a high combat skill troop operating this weapon. This weapon is one of the starter weapons, along with the M16 and M1 Carbine.
It is important to note that shotguns follow a special range calculation mechanic, in that their range malus is far worse than other weapons, meaning they get much more inaccurate outside of effective range, than, say, a rifle.