I try to make it a habit to run at least one full magazines worth of ammo through each mag that I plan on stockpiling, but it's hard to keep track, and it's not always financially feasible to run 3000 rounds through 100 mags.

I'm curious what your experience is with otherwise good quality mags (Pmags, Lancers, good brand aluminum GI, etc) and finding defects or lemons right out of the box. I've personally never seen it, but I don't shoot rifles nearly as much as many do, so I'm curious to hear other's experiences are.

For me, GI issue or Pmags.

There are some lemon 30 GIs out there from the ban years, but not so many.

A mag is essentially disposable, so if it fails you, mark it and put it aside or toss it. Most of them work just fine.

I've tossed a dozen mags or so in the past 20 years that just didn't work, and only felt bad about it during the AW Ban years. Otherwise it's just taking out the trash.

I've got hundreds of mags these days - for AR types mostly GI 30s with green followers, and some with the newer tan followers. Also about 20 vietnam-era straight 20rd mags mostly OK or UI, and some Colt, that work just fine and fill out an ammo can reasonably well when you're storing loaded mags away in a 50-cal ammo can.

I have some C-Mags, and they work, but are inconvenient to store or carry or even transport, IMO.

I had some Okay Surefeeds a few years ago that fed fine but would puke rounds if you reloaded with any force at all.

Returned the case and haven't had any problems since.

Never had an issue with quality mags.

I have broken multiple pmags, mainly feedlips and cracked spines. Any mag poly or metal will break if subjected to enough impact and abuse. I switched mainly to lancers, haven't broke any of them yet.

I've only experienced two issues that I can recall with new magazines right out of the package. The first involved a Tango Down Mk2 ARC magazine (the newer version), one of seven that I own (five 30-rounders and two 20-rounders). One of my 30-rounders would not lock the bolt back after the last round in my primary HD rifle. I ran about 4 or 5 full loads through it, and then it started to periodically lock back (maybe 50% of the time), like it was breaking in. I sent Tango Down a question about it and asked if there was anything I should look for (e.g. plastic flash on the follower or inside the mag). Since they aren't really designed to be disassembled by the user, it's difficult to troubleshoot them. They asked me to send it back for testing and reported back that they couldn't replicate the issue in any of their test rifles. They sent me a replacement mag, and I experienced the same issue in the same rifle with the new mag. Very strange, since I never had a problem with any of my other ARC mags. I was out the shipping for sending the first one back to them, and the whole thing kind of caused me to question the design, so I don't plan to buy any more. I marked the one mag with a square of blue painter's tape on each side and will continue to use it as a range mag to see if the issue goes away.

The second issue I've experienced was with one of the much-discussed Okay Surefeed E2 mags. One of the three that I've tested jammed three times out of 30 rounds in my Bushmaster carbine with rifle feed ramps. I don't get to shoot often enough to thoroughly test the nine E2s that I have, so I'll probably keep them as range mags and not buy any more. I don't think there's really a big issue with them, if any, but I know that standard Okays work great, so I'll just stick with the regular ones.

I don't worry about testing brand new PMAGs, Lancers, or Okay Surefeed mags - I'm comfortable with storing them until I need them. Any used mags that I pick up get cleaned and then added to a bag that I keep separate for range use. Like you, I put a full mag of ammo through each of those as I get to them, and then I store most of them. Any that seem questionable become range / training mags or get discarded, if they really have problems.

One C Products SS 30 rounder gave me a double feed in the middle of timed/scored drill in a class: 0 points, pissed. Nothing drives home clearance drills like a real-world experience on the clock. I don't trust those mags to this day....

Gen 2 20 round PMags, had several FTL's, thankfully, on a hobby range. Tossed `em. Gen 3's have been fine.

Nothing else has barfed on me: Gen 2 PMags, Gen 3 PMags, Lancers, D&H GI mags. Had some ASC 6.5G mags that wouldn't load a full 15 rounds; they sent me some different springs, and they've been GTG.

YMMV

I stick with GI mags....you know the names. With every mag that I receive, new or used, I use a MagLuLa to load/reload 3 or 4 times as a function test. I have never had an issue with any new mag. Some of the used ones would bind up a bit so I rebuilt them with a new spring and follower. Every range trip gets a new set of mags for testing. I have yet to have one fail.

When I was in the Army, I occasionally ran into mag issues at the range. Typically feedlip related. I would swap them out and watch as the armorer "removed them from service".

Only bad-out-of-the-box mags I've had were a ProMag for a 1911, a Kimber 1911 mag and a couple of not-ramline-but-still-plastic Ruger Mk II mags.  Don't feel so bad since the ProMag and Ruger mags were given to me and I traded them for two of the Kimber mags.  One worked one didn't, sent them both down the road at the next show...

From a Glock perspective, I’ve found the ETS mags will bind if you don’t load them carefully, causing the rounds not to stagger correctly. In spite of that, no problems running them.

Also, Hotlips 10/22 mags don’t feed consistently (only examples I have are the metal lip ones). Definitely stick with factory.

During the AW Ban years, I got burned by a few of the USA Brand(?) mags. They were junk. Nothing but feed (FTF) problems. During that time, I also bought used USGI 30 rounders that'd occasionally give feed issues - probably bad feed lips.
In the Army, I very rarely encountered bad mags. Actually, the only time I ever had an issue was when shooting blanks.
After the ban sunsetted, I stocked up on a bunch of D&H and Colt marked mags. Have since bought more D&H, Okay, and PMags, and have experienced hardly any issues. (If I suspect a mag might be damaged, I toss it.)

USA brand =ProMag=consistent junk IME.

I've really never had any mag problems. Even back when mags were hard to come by, those nasty surplus Israeli Orlites always worked until the feed lips cracked and fell off. Up till then they always worked, even on FA.

The only other problem I had was with old GI 20's with weak springs.

Never tried those.

Don't plan to either....

I've had no problems with Lancer or the good G.I. mag manufacturers, which is the main things I stock.  I also have a lot of others, Pmag, HexMag, ETS, Amend2, have all been fairly trouble free.  I've had some G.I. mags made by some no name companies that I've had spring issues with, but often times putting a better spring in them & they are fine.  There are few others I've tried that were problematic & I don't use them anymore, don't even remember the name of 1/2 of them.  I now try to concentrate on getting Lancers & good G.I. mags.  If someone has a good sale on what I consider my secondary choices, I'll snap them up with no hesitation.

I have d&h, Okay, Brownells, some old Bushmaster marked ones, a few Pmags, and I had 10 C products.

4 of the 10 C products were useless right out of the packaging. The mag bodies were mis shapen, the rounds would nosedive into the feed ramps, jamming up the works. You'd have to drop the mag to clear the malfunction.

All the other brands listed I've had good luck with.

During the ban, I bought quite a few mags marked "Chartered Industries Singapore". I had to fiddle with some on them to get them to work. I use them for range mags. I buy Pmags and Okay Surefeeds.

Having used AR mags since the 1980s, the only out of the bag mags I "personally" ever had issues with were Magpul Gen II Pmags and Black Dog Machine .22LR mags. Both broken feed lips. I shy away from polymer mags now. I am sure they have made great improvements in recent years (companies mentioned above) but with quality GI mags available, I have no "personal" reason to switch.

This is only my "personal" experience and not meant to praise or condemn any specific mag.

There's a really good use for untrustworthy standard capacity mags.
You can mail them to totalitarian POS during mag bans.
Congratulations! You're a Felon in possession of a magazine!
And you're a hypocrite, because you voted to make yourself a Felon but you won't turn yourself in!