shouldn't be of any major concern, barrel exchange boltguns are perfectly legal, The Blaser is really popular
but is the upper any different on the calibres that fits the ar-platform?
(the biggest being 458socom IIRC)
couldn't it be done with "just" screwing in a different barrel with a different chamber?
and swapping the bolt?
a complete upper with gassystem and so on is out of the question