The Army marksmanship team used to use epoxy to "true" up the fit between uppers and lowers... and stopped doing it as they found it made little or no difference in the accuracy of the rifle. Also, as mentioned above, it can preclude your ability to switch uppers from weapon to weapon.
Simplest technique: Open lower. Run a strip of scotch tape along the edge of the upper on the outside edge to form a "dam". Coat lower with release agent. Run a thin bead of epoxy along the edge of the upper against the dam. Gently close and secure with the takedown pin. Let set. File the excess off along the inside edge...... then enjoy your tight fit, but don't complain about how it leaked out along the tape and spoiled the perfect look.... THEN, remember that after all that, the Marksmanship team gave it all up because they found it didn't effect anything of significance.
If the rattle bothers you, use an accuwedge to stop the rattle (I can understand though if you're into spec's and feeler guages and what's standard it WOULD bother you (not a dig I work with alot of machinists that would stroke out shooting some of the rattle traps I've seen)).
Some people do so far as to use a rear take-down pin that can take up the slack. check them out here:
jprifles click on "rifle parts" and look for "tension pin".
They work, but makes takedown a b*tch though.
I know it seems counter-intuitive, but it really doesn't matter.... it won't effect accuracy, it just bothers people.
ETA: you can't BELIEVE how hard it is to resist comment about Colt QC...... shoot, I said it anyway, sorry......