A pawn shop over here has around 30 AR mags.

When I look through them, I see FN, COLT, PARSONS (I think) and a few others.  None of them are Promag, which is good, cuz I know they suck.

What do you look for to tell a bad magazine from a good one?  
Some of them have aluminum followers, some black plastic.

On some, the metal piece on the bottom has some play, on others its tight.

Other than that, I cant see any outward difference.  Pawn owner says they are all Mil-Spec.

Thanks

Hi, here is a copy of an email I sent to a buddy to help him with checking out some AR mags a while back.  This may help you:

1. GI 20 round mags had aluminum followers and in my opinion are still the best mags ever made for the M16.  Colt later cheapened the mags for their civilian production by using black plastic followers.  The only 20's I ever had malfunctions with (besides one GI that was totally worn out) were Colts' commercial 20's with plastic followers.  Green followers should not be added to 20 round mags, they were designed to fix a problem with the 30's.

2.  Exterior finish is totally unimportant, IMHO.  Mags that have been handled a lot or cleaned with solvent repeatedly can lose their powder coat finish but still be perfect mechanically.  Here's the stuff to check when evaluating mags:
A.  no dents in body.  
B.  No wear to the lip of the mag retaining catch notch on the side.  Look carefully at the bottom edge of the notch and ensure it is square, not rounded off badly due to wear.
C.  No bends in feed lips, including pliers marks from re-bending them back into shape.  Should be straight when viewed from top and side.
D.  No actual WEAR to the feed lips.  When mags are used extensively, the aluminum on the feed lips wears from the under-side up, tapering from front (more worn) to back (less worn.)  Easiest way to check this is to depress the follower a bit with your finger, and turn the mag to the side, and look at the edge of the feed lip. It should be the same thickness from front to back.  Check both sides.

Some folks make a fetish out of cleaning, lubing, etc their mags and replacing follower springs with Wolf extra power springs.  This is totally unnecessary in my opinion. I have used dozens and dozens of GI 20's over the years and have had no problems except the one mag that was totally worn out (worn white, feed lips worn to knife edge, even the front bottom corner of the mag where it met the floor plate was worn smooth and rounded).  I have one that a friend's brother shipped back from Viet Nam in c. 1969 or 70, and left loaded with 18 rounds from then until it was given to me in 1993.  I took it out, fired the ammo, it worked fine, and then I marked it as one of my 8-round mags for NRA Highpower competition.  Used it in competition for years, then took it to Thunder Ranch Urban Rifle 1 and 2 schools, each of which called for 1200 rounds per week, using 5 or 6 mags through the week.  It STILL hasn't malfunctioned.    So, get some good GI 20's and you will be good to go.

Thanks,  thats exactly what I needed!