Above, are the standard military 20 round and 30 round magazines.
Match rules require that a magazine of the same dimensions as one of these be used.
Nearly ALL of us use the 20 round for across-the-course service rifle shooting. That 20 rounder is very easy to grab and insert in the rifle. 20's are hard to find and expensive, if you can get decent ones for under $25 your doing good.
Our team is using the 30 round magazines for Infantry Trophy aka Rattle-Battle matches.
30's are slightly easier to find and less expensive. I bought several "new" a few years ago, but they all jammed. Usually, they would not feed the last 2 rounds.
I put the stronger springs and green followers in my 30's. I still had trouble jamming. Now they would jam at around 15 rounds in the magazine.
I took them apart. I lightly emery'd the inside of the lips, moving the paper forward and back (not up and down). I emory'd the green follower edges up and down.
I put NO oil or grease inside.
I inserted the follower and taped a stick to the tab on the bottom.
I sprayed Rem Dry-Lube (teflon powder floating in a volatile solvent) all around the inside of the box. I ran the follower up and down over and over again while the solvent evaporated, it slides VERY smoothly now.
I reconnected the spring and reassembled the magazine.
Using ammo loaded on NEW brass those magazines functioned PERFECTLY
in today's (16-jun-01) IT practice.
Isn't that a cool paint job on that Junior team rifle?
There were a run of (post-ban) (dumb new laws) 30's cut down to the
size of 20's but plugged for 10rds, they had 2 or 3 rivets thru the side
that earned them the nick-name "Franken-mags". The ones I got real cheap
seem to work OK.
Bushmaster is now making a 10-rounders that have a slight bend, they are about the size of 20's but are likely also cut down 30's. They look pretty good.
Some folks have good luck with Canadian or Isreali "Thermold" 30 rounders. Others have had problems.