If my memory serves, DPMS was marketing a design that was developed with CZ a couple of decades ago. The Nordic RB upper can be had for less than $450. I did a quick search for the chamber dimensions on it, but got nothing. I like the Nordic upper, but had misgivings about using a stainless barrel.
The S&W 15-22 seems to have reliability and low cost as its twin virtues. You can find them with a magazine and flip up sights for around $300. I am a proponent of polymer in the right place, but the problem with a polymer upper receiver is that flex makes the POI move considerably.
IMO, the big drawback of the CMMG is the sloppy sporter chamber; my guess is that it is used to allow reliable feeding from the Black Dog style magazines.
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