Depends on MOS and billet and sometimes mission. But the primary weapon in the Marine Corps is the service rifle. M16/AR15 family. In an infantry rifle platoon. The T/O (table of organization), everything works in groups of 1 and 3. 1 Platoon Commander for 3 Rifle Squads. 1 Squad Leader with 3 Fireteams. 1 Fireteam consist of the Team leader and three other members, an Automatic Rifleman, Assitant Auto Rifleman, and a Rifleman. The The Team leader, usually carries a service rifle with a grenade launcher attachment. The Auto Riflemen carries and fully automatic rifle M249 (until the it gets replaced). The Assitant Auto and the Rifleman has the service Rifle. And depending on the mission each member will have a sidearm. Sometimes shotguns are added and it's ussually the Rifleman or Assistant Auto Rifleman will carry that.
Again situation dependent, the rifle squad or Rifle plattoon can be augmented with attachments. So going back to the 1 to 3 chain of command, each rifle company has three Rifle platoons (known as Line platoons) and a weapons platoon. The weapons platoon augments the other platoons because they have machinegunnners, mortarmen, anti armor assualtmen with in that platoon. They carry what is known has crew served weapons where it takes a few people to operate the system.
You 1 to 3 further up and you have the A Battalion with 3 Line Companies and a heavy weapons Company. This is my experience as a Marine Rifleman. Not sure about the Army's T/O.