Tried searching, but couldn't find anything. The bushmaster upper receiver doesn't completely seat rearward leaving a gap between the buffer tube hole and the rear of the receiver. This keeps the locking lug holes from lining up front to back. The lower is a Compass Lake lower and is machined to minimum tolerances to snug things up. It was noted when I bought it that some fitting may be required at the rear.

Here's what's happening, The rear outside corners of the rear locking lug are backing up against the rounded well at the rear of the lower receiver.

So my question is, Should I radius the rear locking lug at the rear verticle corners, Or file/grind space for where the corners of the lug are hitting in the lower receiver. It only needs to move back about .04 inches. I would prefer to fit the upper to the lower, and not the other way around, so if I radius the edges of the lug, would this degrade the strength enough to cause worry?
Any one have any thoughts?

Called Compass Lake, and they told me to break the edge of the rear lug of the upper just a little with a file to get it to snug up. Their lowers are designed this way so you don't have to use an accuwedge or any other device to keep it from rattlin. Just thought I'd share the outcome of this.