As a matter of topic discussion, I too prefer a good, tight/precision fit between the upper and lower. Most of my Palmetto State Armory forged uppers and lowers fit fairly well, for the most part. There is a lot of info on the net about this topic (sloppy/loose fit between upper and lower) and how to fix it, or not.
However, I had a S&W M&P Sport II AR that had a fairly loose fit between upper and lower; at least it was loose enough to irritate me. I was tempted to send it back to S&W but wasn't in the mood to send it back. I read about the accu-wedges, larger take-down pins, shim washers, etc... along with a bunch of home-made options, to help tighten up the fit between upper and lower. I decided to try the accu-shim (not accu-wedge). It was about $10 with shipping, but now my S&W lower/upper fit is excellent, and I can still remove the rear take-down pin with just my fingers. The U-shaped metal shim fits snugly in the U-shaped channel in the bottom of the upper at the rear of most forged uppers; this channel cut is there (in most uppers) to help accommodate the full-auto FCG, as I understand it. The kit comes with instructions on how to properly size and fit the shim.
Anyway, since this thread was talking about fit between upper and lower, I thought I'd mention the accu-shim. It worked for me.
For those how build their on ARs from scratch, buying a matched set upper and lower is the best route to go to get a good fit, in my view. Everything else is hit-and-miss...