The ATF has classified the BRP upper as a title I firearm. So their reasoning is that if a non-C2 takes the BRP upper and makes it full-auto (even by using a transferable M16), then they've created a post-86 machine gun. In comparison, the Fightlite MCR/Shrike is not classified as a firearm, so anyone can attach it to a transferable M16 and it's no different legally than attaching any other upper.
I believe the reason why the ATF classified the BRP upper as a title I firearm is because the BRP's recoil system is part of the upper, and the lower is just a glorified trigger pack. The MCR/Shrike uses the regular receiver extension and buffer just like any other normal upper.
Nothing so needs reforming as other people's habits - Mark Twain
Tact is the knack of making a point without making an enemy / Where is the wisdom we have lost in knowledge?
Is it possible that you saw Robinson Arms announcement that they are re-introducing their Stoner 63 clone? (Because it will be different this time.)
That's a complete rifle, and not an upper, but it was announced early last year they were making another go at it.
I was into 10mm Auto before it sold out and went mainstream, but these days I'm here for the revolver and epidemiology information.