If it doesn't shoot 69, 75, or 77gr match bullets tight with the 1:8 twist, I would try the 52gr match bullet.
If it doesn't shoot that bullet tight then you really need to check your optics.
I don't know how much material was removed off the top of your A2 style upper to mount that rail. But I would expect aluminum to break before bending much, so as long as it hasn't broken, it leads me to suspect that your scope or mount is at fault, if your rifle won't print 52gr pills reliably.
If you are worried about the upper receiver, get a flat top slabside, that is as rigid as you are going to get in aluminum.
Machine guns are awesome until you have to carry one.