It seems that my RRA upper's rail is slightly out of spec. I have on hand an LMT fixed BUIS I purchased off the EE and I've attempted to install it on my RRA postban M4gery. In the rearmost portion where a BUIS would normally reside, I cannot hand tighten the BUIS to the point where the BUIS will "bite." If I take a nickel and pair of pliers and torque it beyond what I consider reasonable, I can get it nice and snug while buggering the knob's slot slightly.

The reason why I think the problem is my upper and not the BUIS is when I mount the BUIS as far forward as the receiver allows, I am able to handtighten until the BUIS bites the rail and stops moving. I know that handtight is not enough torque to secure it but the amount of leeway is considerably smaller at the front of the receiver and much less so (none) at the rear. I've mangled one nickel figuring it out so far and I'm afraid I'm damaging my upper or BUIS unnecessarily.

Now my question is for those who have experienced problems similar to this. It may not be RRA/LMT combo but with any manufacturer. I'd be interested in hearing how you solved the problem if you did. I'm thinking about calling RRA and getting their opinion on the matter. With my detachable carry handle, I had no issue and I didn't suspect there to be a problem. It is my one and only experience with BUIS that the RRA gives me pause.

ETA: Both the BUIS and the upper receiver assembly were purchased off the EE. Would LMT and/or RRA honor their warranties?

I have a BM upper with LMT BUIS.  Same problem.  I just torque it down as hard as I can.  Cant even use a quarter or nickel anymore.  They bend.  I use the chisel end of my armorer wrench.  Works great for me.

Its common with the LMT BUIS.  Once you get it tightened down though, that bad boy isnt going anywhere

Thanks, all. You've put my worries to rest. Good thing I've got a good supply of nickels on hand. This sure is an expensive hobby when I gotta pay 5 cents everytime I put it on/off.