Ithaca M37

The Ithaca M37 is a pump action shotgun belonging to Rika Minami. It is primarily used by Takashi Komuro until he finds the Benelli M4 Super 90. Rika modified the Ithaca by attaching an Aimpoint CompM2 dot sight.

Overview
''"Everything Kohta said in the story about this being a pump action shotgun and whatnot is all true, but whenever I think of the Ithaca M37 I just think of the American soldiers in Vietnam who would use the gun in place of a bong to smoke marijuana." ''-Daisuke Sato

Plot
The M37 is in Rika's locker of weapons. Kohta and Takashi break open the lockers with a crowbar and find some weapons, including the M37. When rescuing Alice, Kohta uses it against the hoards of "them". After crossing the river, he gives it to Takashi, instructing him how to fire and reload. Takashi uses it as his main weapon until he finds the Benneli M4 Super 90 in the evidence locker.

Trivia

 * Real world - The Ithaca M37 is a pump-action shotgun developed in 1937, and despite losing out in military contracts to the Winchester M1897 and M1912 during World War II, would see widespread use during the Vietnam War. It is nicknamed "Featherlight" for being lighter than other shotguns. It is also very reliable, and by having both its loading and ejection port under the gun, is ambidextrous.
 * The effect of the 12 gauge shotgun pellets is drastically upscaled in both anime and manga, to the point where its possible to fire a hole big enough to climb through into the ceiling with just one magazine load, and where a single shot knocks over more than a dozen of "them" at a time.