I think there's a grenade launcher is a good idea. M32 XM25 M79 can be.
And one more thing,When M249 using the 5.56mm magazine , the magazine is not inserted in the gun underneath, but next to the bullet supplying mouth.If the author can modify it,it's Ok(But I think it is a little difficult.)
In the end, USAS-12 is made in Korea.