I got an Adventureline USGI 20 rounder that fits tight in the mag well. Won't drop free even with a loaded mag. I got it used, but there is no damage I can see on it. Granted this magazine is older than I am. Functions great, just don't like the fact it doesn't drop free.

Anything I can do about this? My 4 other Adventureline mags fit just fine.

Not a problem. This happens with older 20s from time to time & even older 30s. Sounds like the mag's warped due to long term use. There are a few methods that worked well for me in the past. The best method is to put the mag in between 2 blocks of wood and press it in a vice. you turn the mag snug then do some gradual 1/4 turns. stopping, releasing, and checking progress. The low teck method is to put the mag on the floor and rest the balls of your stocking feet on the mag. Shift your body weight to the mag and back off a few times. if not enough shift then gently & carefully stand on it. If you need to jiggle up & down try that next. Just don't hop. I used both methods with perfect results on old 20s and have never damaged a mag.

If you have a vice you might want to give a quick clench to the narrow side of the mags too to ensure the magazine is a rectangle [] instead of a parallelogram <>. sometimes the mag incurs a "rhombusing" and you need to box it back to 90 degree angles, but I am confident the pressing/stomping alone should cure what ails you.

Adventureline 20s had a rather thick coat of hard moly lube.  (This was a flat gray colored moly whereas the early contract Colt & Universal/Simmons mag finish had more of a thinner; silver colored moly) The thicker moly coating may add to your problem...

Sly

Quoted: Adventureline 20s had a rather thick coat of hard moly lube.  (This was a flat gray colored moly whereas the early contract Colt & Universal/Simmons mag finish had more of a thinner; silver colored moly) The thicker moly coating may add to your problem... Sly

Unless they were forgotten in a box for a few decades I would guess the mag has little to no moly left on it as it is.

Theres some gold spots showing through the finish on this mag, but the gray right next to it does feel noticeably thicker. It's not a big issue since it feeds just fine, just will make sure to mark this mag as a no no for self defence.