Okay, first, I know that it is highly desirable to mount any optic to a flattop receiver.
That said, I have an BM M4 Type with the fixed carry handle. I have a KAC RAS with an Aimpoint Comp M2 mounted to the RAS just forward of the reciever. Per the RAS instructions I have properly torqued the RAS to the barrel nut (20 in/pounds) and I not noticed any significant (if at all) movement of zero with this set-up while shooting.
I should have purchased a flattop but that's water under the bridge. My fixed carry handle upper even has had a KKF A3 TacBrake installed. I have been tossing around the idea of buying an A3 upper from BM but the money I spend on a new upper could buy many other things (ammo, mags,etc, etc...).
I would like opinions from all of you out there on whether you think it would be worth it? Thanks.