Sirex
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Colt made 7.62x39 sporter ARs, so I would not be jump to hop on that train due to clone interpretation concerns. I also have concerns about .300AAC due to the fact that it's 100% parts interchangeable with a normal AR (as in, you could swap anything and wind up with a working rifle, albeit in a different caliber). I don't know about 6.5G. Bolt and barrel are different, which does seem to kill interchangeability.
It seems like the MSP has been fairly consistent vis a vis piston guns being different, which is reasonable given how proprietary they are. You could probably also make a semi-sane argument that side-chargers are GTG, but haven't seen that rigorously tested yet.
My concern is, since my ARs were bought as 5.56 Hbars, swapping the uppers simply make them legal. I do like the AR platform, but I was late to the game getting them. I hate the way they are trying to criminalize previously legally purchased guns. Not sure how they can do it since it was written specifically to exempt the Hbars. Now it's a big whoopsie.
Another weird option is the Troy PAR. I know not a semi, but for kicks and giggles, my buddy did a little 3 Gun shooting with his, and he did ok. He said it was different but fun, and he likes the gun. The gun has had problem with one part (can't remember what he said it was) bit a NZ company makes a beefed up part.