Well, seeing a M60 was never mated with a shotgun. It is heavy enough as it is, and no one in their right mind would go and purposefully add more weight to it and still able to use it, effectively in combat. I'd say go with a scifi kitbash, say.... a iron man from the 12" line repainted and some other stuff added on so it wouldn't look like you just repainted a iron man and added a large gun.