Dear All: New, old guy here. Couldn't resist commenting on this thread so I signed up.
My answer, no matter how many mags you have, is one (1). That magazine is the first one you grab, then carefully inspect (new or old) for overall condition. Are you ready to bet your life on it?
I look at the feed lips first just based on experience as I find many are beat up, bent, rough or too sharp on the edges. Take a special look at the corners of the feed lips for cracks. Then look at the mag body for dings and dents. If I find any problems, I'm done - that mag goes into the 'fix it or trash it' pile.
If it passes those checks, I take it apart and detail inspect the follower, spring(s) and floorplate - note: You need to know what is supposed to be in the mag - anything extra is a 'Fail'. Back to the guts, look at all the bits when dirty for clues that something is about to go bad. Cracks tend to start at the corners of parts whether they are metal or plastic - sometimes dirt (any sort) will collect in cracks and make them easier to spot. Buy yourself a small magnifying glass if you are old like me.
Followers are the most critical part in my mind (but the spring and floorplate are close 'runners up'). Look for any signs of wear that could indicate a contact or binding point in the mag body - if you see anything odd (not 'normal' wear) figure out what's causing it - then fix or Fail. Springs are easily checked - are they clean and show no rusted or pitted areas? If you can't wipe off any surface rust with an oily rag, the spring is suspect and could make for a 'bad day'. You need to know what the 'free length' of your springs should be - that is, how long should they be when uncompressed (outside of the mag). Ask your armorer - the FM probably won't tell you. Check that length - compress the spring and check it again.
Floorplates are either straight and flat or not, tabs should hold them in place securely. If they fail you will see how fast a rifle can unload itself.
Clean everything thoroughly and inspect it all again. This includes looking at the inside walls of the mag body - point it at a bright light and look for dents inside that you may not have noticed on the outside inspection - use your eyes and fingers, sometimes you can feel problems that you can't see. If it is a new mag, look for overall quality - are the walls of the body smooth?, is the body warped or twisted? If it is a metal mag, are there broken/separated/cracked spot welds on the back spine or catch pad? If it is a used mag all the above goes double.
Look for any rust (steel - reddish) or corrosion (aluminum - whitish) - this is a judgment call after you have done your best to clean it out - do you think it's OK?
Put the mag back together. Check that all the bits seem to be happy. Fully load the magazine (Dummies if you've got them) - note the 'feel' of each cartridge as it goes in - anything unusual is cause to start over. Mag loaders and stripper clips will often hide problems. Note, if these are mags you've used and will load out with, load with your issue ammo and inspect each round as you load it - bent, twisted, corroded, too long, too short, bad vibes, whatever - trash it. Make sure the mag is correctly marked with whatever is in it.
By my definition, new or previously unused mags are not serviceable until 'proofed' - that is function tested. If you are headed to the range anyway, great, have fun. Pay attention to what happens - any failure that occurs should tell you to stop and figure out what happened. Since this is my opinion on magazines, I'll limit the comment to magazine problems. We all got into the habit of doing 'immediate action' whenever something didn't work right. Try to resist the impulse. Just stop. Keep the weapon safe and down range and look at it. BTW, stop any bleeding that may be going on. Presuming that the shooter and any bystanders are OK - proceed, carefully.
What happened? Failure to feed?, double feed, smokestack, something never heard of before? Now, just because I've limited this comment to magazine problems doesn't mean that whatever happened was caused by the magazine. But if the mag was involved the problem may be obvious.
FTF - Failure to Feed: may be caused by the mag not pushing a round up into place where the bolt could capture it and push it into the chamber - what could cause that? Several possibilities: weak spring (especially on burst or auto fire - usually the bolt closes on an empty chamber or 'overrides' the next round and hangs up), sprung feed lips (too wide and then a double feed), bent/distorted feed lips (probably too narrow - misfeed (angle) or trapped round), bad mag fit in the mag well - too high/ too low (stopped bolt or bullet tip jammed into chamber face) - hence a possible mag catch slot on the magazine that is worn/deformed or possibly a mag catch that is worn, bent or broken.
Or, if it's an DI AR platform it could just be dirty - chamber, lugs, bore, gas block, gas tube, carrier key (partially blocked), bolt carrier itself, bolt, misaligned gas rings, bad ammo, bad karma etc.
Disclaimer: I went through Basic and AIT with the M-14 and learned to love that rifle more than my first .22. When I went though RVN orientation they gave me the 'New M-16!!' I was not a happy camper. I eventually put about a million miles on the Ultimate weapon - the M-60 - just sayin'. Since then, older and smarter, I've come to respect the AR and appreciate its virtues. It requires all the care that I gladly lavished on my M-14. I now have two M1As and two homebuilt ARs. Good stuff.
I digress.
FTE - Failure to Extract: Fired case gets stopped by next round in the magazine - may be caused by mag feed lips allowing that next round to rise up into the extraction path of the fired case. This may just cause general confusion in the action or the classic stovepipe jam.
FTE - Failure to Eject: Related to the above - too many pieces of brass in the same space.
Troubleshooting any weapon problem gets complicated - fast. Fortunately mag problems are pretty easy to identify and cheap to fix. 1) try another mag. If that doesn't work, 2) check out the weapon; inspect, clean, inspect again, test with a known good mag. 3) Fix or replace whatever isn't working - chances are very good that it was the mag all along.
If all this was successful, you now have at least one good magazine. Grab another magazine, inspect, clean, inspect and test - now you have two. Keep going .............
Detachable magazines have many virtues - generally they are inexpensive, simple, easy to maintain and have even been considered to be disposable as well as collectable. They are of course also ‘evil’ if they have too much capacity - again I digress. They are critical to the reliable functioning of our battle rifles and carbines and MSRs and even common hunting rifles and of course handguns. Further, they are also cantankerous, trouble prone, dirt trapping, fragile, frustrating, annoying ……………
Any mag that has not been inspected and tested is a crap shoot at best. Brand new mags, from the best of manufactures have been known to function poorly or fail outright. If that mag is still in its original wrapper it is ‘user beware’.
You really want to figure out how many good magazines you need/want. Getting them will take a bit of time and effort.
IMHO,
BH