I got three new USGI-type 20-round NHMTG magazines today (manufacture date of 3-09 for all of them), and two of them won't drop free from the magazine well. There is almost immediate resistance when you start to insert the magazine. They will drop a little bit when you press the magazine release button, but then stop because of the tight fit. The front-to-back width is the issue.

The only other magazines I have (which are also the only other magazines I've ever used in this rifle) are Colt USGI. One of them is a well-used 20 round magazine from the early '70s with an aluminum follower, and the other is a like-new 30-round magazine from the late '70s (or maybe early '80s) with a black plastic follower. Both of them slide into the magazine well and drop free with zero resistance, just as they should. The one NHMTG magazine that does drop free is pretty close to the upper size limit of what would drop free, closer than the two Colt magazines are.

So does NHMTG really make Colt magazines? If so, has the dimensions specification changed since the '70s, or do my two Colt magazines just happen to be particularly undersized?

Who made your lower receiver?

Nodak Spud LLC NDS-16A1

Yes NHMTG makes mags for Colt. Colt, NHMTG and Okay are all made by the same company..

Actually Okay makes Colt mags and NHMTG mags.

This.

I don't see how stamped sheet metal magazines made on the same equipment in the same month could have a significant amount of size variance. It is not like a machining operation, it is just a big die that stamps out sheet metal to a particular shape. As long as the die doesn't change, and the sheet metal thickness doesn't change, how does a significant size variance get introduced? It would make more sense if all three of the new NHMTG magazines didn't drop free, but with one of them dropping free, along with the two old Colt magazines also dropping free, somehow those two magazines have a significant size variance compared to the others.

Stamping dies over time will wear out and then the tolerances change. Also the dies could be new manufactured dies.

I had an NDS-A1 lower that would scrape the
shit out of magazines, tight as hell.

Call Mike or Harlan [NDS], tell them what you're
experiencing, and follow their instructions.

Okay makes Colt & NHMTG mags.  The only difference is the the logo that is stamped on the box & base plate.  Also, the feed lips are inspected 100% on every mag in a fixture that simulates the mag well.  More often than not, the tolerance variation on lowers is the culprit for mags not droping out.  Another common cause is that a mag was dropped & being aluminum, the mag outer box dimensions will tend to go out of spec.