The Matsutani Mechanics B331 "Lobster" Grenade Launcher was a single shot, shoulder fired, breech-loaded grenade launcher created by Matsutani Mechanics for use by the United States Military in 2024. The grenade launcher was a compact, shoulder-fired weapon with a collapsible stock for quick storage, being a compact grenade launcher, compared to other standalone single-fire grenade launchers like the M79. The B331 was breech-loaded, the user able to unlock a circular bolt and pull a latch off of the rear upper receiver, inserting the grenade into the breech.

The B331 used 40mm grenade ammunition, as was common with most grenade launchers of its time. The B331 became known by users as the "Lobster", due to the fact that it "lobbed" grenades, and had a flat, armored look to it. The Lobster was often used by the U.S. Army in the 2530s, but soon became phased out when a newer, revolving grenade launcher similar to the Milkor MGL was sent into service. However, it was not as durable as the Lobster, which continued to be used by the Army along with the other, revolving grenade launcher.

The Lobster had typical leaf sights, as seen on most standalone grenade launchers, and was not attachment-friendly, as it had no rail systems, and the fact that the collapsible stock on the Lobster was built in.

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