20rd GI Mags not Reliably Locking Bolt Open

[AR-Tenner]

Hi folks,

I have a good collection of early 20-round AR-15 magazines 14; 2 loaded bandoliers-worth) to complement my obscenely large stockpile of 30s. I test all my magazines and I’ve noticed that about half of the 20-rounders fail to lock the bolt back on empty between 1/3 and 2/3 of the time. They are all in excellent condition and have great followers. 12 have metal followers and 2 have plastic, and interestingly the plastic-follower ones never have a problem locking back.

I tried this across a wide range of ARs (my 2 20” rifle-length and 16” midlength, all with fixed stocks and rifle buffers, and several 16” and 14.5” carbines belonging to friends), and the problematic magazines consistently did this in all the ARs in which I tried them. The ARs in my stable have tens of thousands of rounds through them, and not only have none ever jammed for any reason with any magazine (that includes field use in subzero conditions and after having been dropped in the mud, matches where their internals were getting deluged by blowing dust, and thousands of rounds between cleaning and lubrication), but using well over 100 30-round magazines from every era and manufacturer and even limp-wristed in extreme cold with weak PMC Bronze ammo, they have never failed to lock back.

I figured it had to be weak springs, even though not one of my vintage, well-used black-follower mags (I of course changed the followers out for Magpul anti-tilt) has ever failed to lock back, even though I intentionally test this with every mag I own by firing one round of PMC bronze that I kept in the freezer from the mag through a very dirty rifle while holding it at my hip with as light a grip as possible, and repeat 5 times.

I therefore changed out all the 20-rounders’ springs for new GI-spec ones. However, the problematic magazines confined to fail to lock back as often as they did before, and the good ones didn’t become bad.

I am at a loss, and am wondering whether y’all might know what I’m missing.

Thanks and happy New Years!

[bamashooter]

I would take one of the ARs and remove the lower. Insert a "bad" empty magazine and observe the relationship of follower to bolt catch. Take pics if necessary. Do the same with a known "good" magazine. Compare the two. It could be followers, depth of magazine insertion, magazine catch, etc. Found this older thread regarding similar issues but with PMAGS. Possibly helpful but has some good pics regarding follower/catch contact. https://www.m4carbine.net/showthread...igured-out-why

[rebelram]

Replace the springs, especially on the metal follower mags. If those mags have the original springs they are most like weak enough to cause failure to lock the bolt back and you will probably also start to see feed failures as well.

Don't use anti-tilt followers in 20 Rd mags, they aren't needed. Some of the best 20rd mags ever made were the metal follower ones. If you want new 20rd usgi, then nhmtg and okay/surefeed are your best option. Both have black plastic followers.

[AR-Tenner]

Replace the springs, especially on the metal follower mags. If those mags have the original springs they are most like weak enough to cause failure to lock the bolt back and you will probably also start to see feed failures as well.

Don't use anti-tilt followers in 20 Rd mags, they aren't needed. Some of the best 20rd mags ever made were the metal follower ones. If you want new 20rd usgi, then nhmtg and okay/surefeed are your best option. Both have black plastic followers.

Yeah, the springs was the first thing I tried, but as I said, that didn't fix or even change anything in any of the mags. The followers are all original, and I now not to use anti-tilt in those 20-rounders; they actually need to tilt due to the straight body.

Given that the new springs didn't do a thing and the magazines feed perfectly, I figure it must just be that some of the metal followers don't ride high enough to engage the bolt catch.

[_Stormin_]

Given that the new springs didn't do a thing and the magazines feed perfectly, I figure it must just be that some of the metal followers don't ride high enough to engage the bolt catch.

That’s why you would want to take a separated lower and observe the follower to bolt catch contact.