What is the only U.S. military pump shotgun?

Mossberg is the main brand of shotgun widely used in all branches of the United States military, but three different models are in active service: the M500, M590 and M590A1. The Remington M870 pump-action shotgun is also used by certain branches in limited roles.

The Mossberg Model 590 and 590A1 are upgrades from the Model 500 and are more commonly used. Though fundamentally similar, the 590 models have some improvements such as additional reinforcement, the ability to equip a vented heat shield and an extended magazine. The Remington M870 has been in use by the Marine Corps since 1966 and by other services since the 1970s.