Since we may never see replacement/different 7.62 uppers from Colt for the Modular Carbine and seeing how it has so much potential as a SBR, the gears started turning (perhaps grinding).
The DPMS GII uses a shorter, for 7.62, bolt carrier than the large frame AR's. The relationship between the fire control parts and magazine well has probably maintained. Did DPMS use AR15 dimensions for the rear portion of the upper, as Colt did to facilitate the use of AR15 uppers?
I asked in the AR Variants forum for someone to try dropping an AR15 upper on a GII lower and was met with much WTF?! Evidently most readers couldn't get past that I was NOT thinking that a AR15 upper could be used on a GII lower. With a 901 but no GII the theory couldn't be tested, until yesterday.
Went to the local range yesterday and one of the regulars showed me his new Adams Arms 7.62 AR. Adams Arms uses the DPMS GII pattern receivers!
An AR15 upper will not drop on a GII pattern lower, the pivot pin lug hits the front of the magwell BUT the GII pattern upper drops on a AR15 lower! (Unfortunately didn't take the 901 to the range yesterday for more testing.) The pivot pin went in and the bolt carrier would move into the buffer tube. Have to make arrangements to meet up again with both guns present to see how the upper fits the 901 lower and if everything lines up where it needs to.
There may be a chance the GII upper could be adapted to the Colt 901, if any GII uppers are made available.
Did this make sense to anyone but me?