I have 12 alloy mags for my PTR-91 and after some testing, I have found that 4 of these magazines rarely feed rounds into the chamber after pulling the cocking lever, 2 feed rounds 50% of the time, and 6 seem to work flawlessly.  I don't see any obvious difference between the mags and it does not seem related to the date of manufacture either.  Does anybody have any suggestions on how to troubleshoot/fix these magazines?  Is this performance typical?  Am I correct to assume that this is a magazine issue and not an issue with the rifle?

Throw them out.  They're all of $5 to replace.

Quoted: Throw them out.  They're all of $5 to replace.

Agreed...they are not that expensive.  I guess I am also trying to figure out if there is a way to keep from buying more mags that don't work down the road.

Stretch the springs, and or, check to make sure the feed lips are parallel with one another.

Take a look at your alloy mags .

If you take a good close look at the feed lips,each side has
2 layers of aluminum pinched tightly together.

At that seem,if you see a riple,seperation,or something tiny stuck there,
either force the layers back together evenly ,or just toss the mags.

I had one with a 50% first round feed rate.
Originally ,I thought it was the rifle.
After I got the first round loaded into the chamber,the rest ran perfectly.

I looked for a while ,and found a tiny seperation that caused the round to sit
just barely at an angle unless I was careful that I didn't catch it when loading
it.

If I was really carefull to put the round in even,it worked every time.

I pinched it in a vice and smoothed it out with a nail file to see if I
was right.
Works perfectly now.

Quoted: Take a look at your alloy mags . If you take a good close look at the feed lips,each side has 2 layers of aluminum pinched tightly together. At that seem,if you see a riple,seperation,or something tiny stuck there, either force the layers back together evenly ,or just toss the mags. I had one with a 50% first round feed rate. Originally ,I thought it was the rifle. After I got the first round loaded into the chamber,the rest ran perfectly. I looked for a while ,and found a tiny seperation that caused the round to sit just barely at an angle unless I was careful that I didn't catch it when loading it. If I was really carefull to put the round in even,it worked every time. I pinched it in a vice and smoothed it out with a nail file to see if I was right. Works perfectly now.

If you do toss our crappy mags, be sure and keep the followers floor plates and springs for use in your other mags.  It can't hurt to have too many spares. On a side note, I have seen personally mags that worked in one gun and not at all in another, the mag catch was off.  (I attributed it to the stacking of tolerances, as both receivers were clones one an aluminum and the other century stamped steel).  But do find mags that work in your gun, as cheap as the aluminum ones were you could have bought a couple hundred and thrown out half and not "lost" any money on the deal.

Thanks for all of the great advice!