Last month Bushmaster was awarded a patent (#7810271) for an AR-15 (or AR-10) compatible modular rifle system. The patent proposes using a mag well that can be removed from the lower receiver. The mag well could be any conceivable length in order to accommodate special upper receivers. In theory, cartridges as big as the a .338 Lapua could be used with this system.
While this patent is interesting, what makes it even more interesting is that back in 2006 Colt Defense was awarded a patent for a similar concept.
Solider System dug up the Colt patent when we were all trying to work out what Colt’s upcoming modular rifle would look like. The Colt Modular Carbine CM901 has since been unveiled and we know that it does not make use of a detachable magwell.
It will be interesting to see if Bushmaster run with this concept.