I had John pin and weld an SOLGW NOX with Dead Air mount onto a 13.7" barrel with a 13.5" URX-4. The integrated barrel nut on the URX-4 required that he do the work with the rail installed. There was very, very, very little clearance and he got it done with no issue.

I do not know on shimming - I sent the upper with the muzzle device already properly shimmed and timed.