Benelli M4 Super 90

The Benelli M4 Super 90, also known as the M1014 JSCS, is a gas operated, semi-automatic combat shotgun currently used by Takashi Komuro in place of the Ithaca M37. While it is heavier than the Ithaca, it is more efficient in combat.

Overview
As what Kohta Hirano said, it was basically used by the US Marines and British Army, it is heavier but its easier to use in combat

Plot
Acquired on Chapter 27 when Kohta Hirano opened the evidence stockroom to look for ammunitions and guns, and tells Takashi to use it