One thing about the USGI 20-rounder that I want to bring up. If you own more than just Mattel Toys or Mattel Toys-looking guns (i.e. AR15 piston guns), make sure that 20-rounder works in your STANAG mag well.
It's weird but I've found out the Galil ACE 5.56mm does not work with the USGI 20-rounders at all. It wouldn't even lock the mag in place. I tried it with my old Colt 20-rounders and new Okay 20-rounders. Didn't work. I thought that it was my gun, but another guy I know recently bought a Galil Ace 5.56mm and I asked him to try. The same thing happened with his Ace. So that's only two out of tens of thousands sold, but still more than one is a pattern.
Strangely enough the various brands of plastic 20-rounders worked just fine in the ACE.
My BREN 2MS 5.56 also does not like the USGI 20-rounders either. The mags will lock in and feed BUT it's a very tight fit and I would have to drag and yank the mag out.
Once again, all plastic 20-rounders and USGI 30-rounders worked fine in the BREN 2.
On the other hand, the previous generation BREN 805 has zero problem with the USGI 20-rounders.
So, just a headsup for those who own STANAG mag well guns - try out the 20-rounders first before making the assumption that just because it was USGI, it would work.
For the life of me, comparing between the 20-rounds and 30-rounds aluminum mags, I can't figure out why one wouldn't lock in place or fits very tight while the other slips right in and out.