I see handguard from HK (or M4 mod) and mag but proportions of body, stock and pistol grip are more like in M60. I think the base was HK but now this is mix. I thikn that ponies don't need real guns, they have their own.
I can safely assure you that the whole thing is an AR-15 platform rifle, with no M60 at all. The entire base picture is pure M4 mod. Feel free to take a look yourself: [link]. It also seems your head canon greatly differs from mine, but c'est la vie. You probably wouldn't like any of my work then xD