It's easy to be an arms dealer when the writer decides you simply have the better weapon.
Somehow 5.56 out of a carbine at point blank range can't penetrate armor that an MP5 and then a P90 can.
Two of those are 5.56. Two of those are 7.62x51. One of those is an AK.
The g3 platform has several firearms based off it, (I'm blanking on the names right now.), and those weapons have at least several parts that ware transferable. (The bolt and carrier I think.)