Utilizing the same spring loaded mechanism as Tokyo Marui's own Benelli and SPAS 12, the new M203 uses the same shotgun shells (pictured green shell) to load 30 rounds into the chamber, with each cock ejecting 3 BBs at a time. Check out our review in our reviews section. No additional mounts are needed to install this on the M4A1 or M4A1 RIS. All required mounts already included!
Be sure to buy more Marui shotgun shells. Gun already comes with one shell. Each shell holds 30 rounds. Also be sure to stock up on extra BBs. For skirmishing, why not consider also getting a shotgun sling that allows you to store your shotgun shells for easy access?