Option #1 I can tighten up the fitting of the upper/lower AR-15 receivers by installing a set screw that engages against the upper receivers retaining pin boss. The set screw can be accessed by removing the pistol grip. To adjust put a drop of Locktite on the threads, turn the set screw until the upper receiver movement is minimized. Check to see if the lower receiver pin can still be able to move with finger pressure. If necessary back the set screw off slightly until the pin can freely travel. This procedure can eliminate up to 95% of upper/lower movement. Price runs $25 with trigger work or $30 without.
Option #2 Another tightening option is adding 2 set screws to the upper receivers pivot pin boss and 1 set screw in the rear take down pin boss. The pictures below are of a DPMS LR-308 upper assembly, depending how tight one adjusts the set screw, virtually all the movement can be eliminated. When set this tight, a punch and a hammer is necessary to tap out the pin.
These modifications are not visible when the rifle is buttoned up. Price is $29 with trigger work or $39 with out.
Return shipping price depends on where you live and if its the stipped or barreled upper assembly.
Any questions, please EMAIL me.