I've heard some discution about loose fitting upers/lowers in just about every post I seen about AR platforms. I don't really know if this is a "perscribed method" to do this or not, but it worked for me. I had an old well used Colt at one time and the upper was very loose to the lower, I did a bit of measuring with some feeler gauges and found that the copper foil tape they use for stained glass window leading ,was just the right size. Just put a line down on the top of the lower where the flats are that contact the upper and trimmed it with an xacto knife. Very soft metal, stayed in place(tape) and really tightened things up nice. Had it that way for several years and kept an eye out for any signs of excessive wear on the front pin, but never found any problem. I don't know why I haden't posted this before, brain fart I guess!! It's a reall simple "thightener upper" for the slop in the well used AR's, and the rubber wedge thingies work pretty good too. I don't remember how thick the foil tape was but it was around a couple thousands or so and the adhesive on it seems to hold it in place quite well as long as youre carefull about wiping down the lower when it's separated from the upper. The Colt also had some oblong-ing of the uppers retaining pin holes, some butcher musta had it before me Mr. Hammerman I think!! I fixed that with a larger pin and stainless bushing pressed in the upper. Was as tight as a bolt gun when it was done, but that was more of an exstream case and the perminent fix for it. Lucky it was the old small pin one. The foil works great just to stiffen up things though. Yankee injunuity, afro-engenering, n*g*er rigging, or what ever you want to call it, at it's best!!!!!