Hi Y'all,

I have been reading for years about how many 30rd AR-15 magazines don't like to lock into the magazine well when loaded to capacity (30, not 31 for the ones into which you can squeeze that extra one), and I am on a quest to discover the actual physical causes behind this happening. While I had read and heard about it, I dismissed it as either a problem with old mags or simply some folks not being strong enough. I am a big guy who lifts weights daily, and I had never found a magazine that would not lock in when full on a closed bolt. However, a few months ago, when changing out my "on deck" magazines for my ArmaLite AR-10 (Gen II, B-Pattern mags), I found that one, when loaded to 20, simply would not lock in when closed, unless I gave it a hard slap, which I really don't like doing. When going through my 40+/- other ArmaLite mags I discovered that one other had this issue. I had almost never inserted a full mag on a closed bolt before, so in the years I'd had my AR-10, I had never noticed this.


I immediately started on a research project to determine the cause and correct it. The first thing I did was try downloading, and what I found with those was very surprising. With 20 rounds the mag would not lock without a hard slap, and with 19 it went in almost like it was empty. The surprise came when I went to 18, which was every bit as hard to seat as 20. This continued all the way down, with literally 2 rounds in the magazine (identical issue with both problem mags) being almost as difficult to seat as 20, with only about 10% less force required. This even-numbered issue was constant, too, with no matter how I pushed the rounds down by the nose or base, or situated them forward or back (the ArmaLite mags are superb and give you a very long potential OAL). I took over a dozen measurements of the problem mags and some control ones, and quickly realized that having an even number of rounds was causing the magazine to tilt to the left, with the top round being on the left (ArmaLite's followers are like the USGI tan ones, and have the raised portion on the left) pressing on the sloped left side of the bolt and pulling the whole assembly that way. When I emptied and disassembled the magazine, I could recreate this issue with just the empty shell of the magazine by tilting it left when inserting. This showed me that it was either the side of the mag on the left or the right feed lip that extended a bit too far. Use of a sharpie to color the contact surfaces showed me it was the right feed lip, which I shortened with a file by about half a millimeter, keeping the same contours of the original. This solved the problem perfectly, and now these two magazines insert flawlessly when full on a closed bolt, and I tested each by putting 100 rounds through them (40 clean and dry, 60 with a generous helping of dust and dirt dribbled into them), and they fed and functioned without issue.


I want to reiterate that I have never encountered a magazine variety that is as high quality as ArmaLite's Gen II B-pattern mag, and that in the years and tens of thousands of rounds I have put through mine, I have never had a malfunction of any kind; not even a failure for the bolt to hold open on the last shot! This is more than I can say for any Kalashnikov, and although my AR-15s have never jammed, I haven't run them nearly as hard or as long, and some of the .223-marked retro Colt 20-round magazines don't lock the bolt back reliably.


Well, this discovery and subsequent hard-earned remedy inspired me to test my AR-15 magazines for the issue. I have had a huge collection of Lancer L5 AWM (my absolute favorite, go-to duty mag) for many years, and they all performed perfectly in this respect. The same was true for my couple dozen modern-production Okay Industries SureFeed mags that come with Magpul followers, as well as my goodly collection of retro 20-rounders. However, I have been bitten by the pre-ban bug in the last year, and of my three dozen, two experienced a similar issue. I know liking pre-ban 30rd mags is strange for a Virginian, but I like the history behind them, and being able to get them for a negotiated $36 for a dozen at gun shows makes it pretty enticing. When I'm rooting through surplus bins at gun shows I bring one of my Colt lowers along to make sure they drop free empty and engage the bolt catch, and when I get them home I disassemble, check the spring length, clean, and replace the black followers with Magpul yellow. Any that don't drop free I don't buy, and any that have a shorter spring I replace with new mil spec one. I discovered last night that 2 of the 36 that had passed the drop-free and spring-length evaluations and are in rotation for me had a lot of trouble locking with 30 rounds.


These mags would go in easier with 29 and were harder with 28 than 29, though not quite as bad as 30. These mags did not show the same clear cause that the AR-10 mags did, as they usually would lock in fine, and only one in about every 5 insertion attempts would they "hit a wall." It did not seem to matter in which direction I attempted to insert the mags, whether the two or three top rounds were back or forward, or whether I pushed the rounds down in the mag after loading it but before insertion.


I am hoping to hear from someone who has diagnosed what causes this type of issue in the GI aluminum AR-15 mags.


Thanks in advance!

Only thing I can figure is the extra pressure of that top round on the bottom of the bolt in a fully loaded mag is preventing proper seating.

I bet any fully loaded mag will seat just fine if the bolt is retracted.

That, or the mag (or mag catch) is out of spec.

Good thoughts. It almost certainly is something about the top round or two interacting with the bottom of the closed carrier, because they all insert great on an open bolt. I am quite sure the mag catch isn't the problem, as you can't get more "in-spec" than an original Colt lower with all original parts (except for the ArmaLite 2-Stage National Match trigger and BCM receiver extension.

I have found the follower to be the main culprit.

There is a very fine line between "easy closed bolt insertion" and to much "slack" in the follower.

If you have to much "slack" from a shortened depth follower, a dropped ( on the baseplate ) fully loaded mag can eject the first few rounds easily.

What I have done is shorten the followers legs about 1/8 inch.

Closed bolt insertion is much easier... but it does increase the chance of ammo falling out from what I described.

I prefer not to have to struggle at all, in the heat of the moment, during a mag change.

Break them down and trim the legs on the bottom of the follower.

Did this to mine and it makes it much easier to lock in.


If you don't want to do this, or unsure.... try it on one.

From archived thread from Lancer:


LancerSystems [Industry Partner]
5/5/2011 4:02:09 PM EDT
The follow stop just sets the maximum capacity of the magazine.

Cutting it down will not effect the performance of the magazines and will not void the warranty as long as you don't load more than 30 rounds.

BEWARE - CAUTION - if you cut down the follower stop we recommend you still only load 30 rounds in the magazine, loading 31 without enough free space will result in a stoppage or cause damage the magazine.

I have a number of ARs and quite a few GI mags both 20 and 30 round.  I have never had the problem you discuss with any of my GI mags.  However, I decided to try some Magpul mags of the 20 rd variety.  I have experienced this problem with them in my M-4gery with a RRA lower and Doublestar upper and BCG.  No issue inserting a mag with the bolt open, but when I try and insert with a closed bolt, it does not lock.  I will try the even/odd test you mentioned and see if that chages things as I have only tried it with 10 or 20 rounds, not 9, 11, or 19.  Will be interesting to see if there is any difference.

This is a great post - and not the discovery I was expecting.

Thanks for the help! It turns out that the problems these AR-15 magazines were having was very different than the ones I encountered and fixed so readily with the AR-10. For almost all, it was simply that the follower was too tall and shaving off a bit at the bottoms of the legs made them lock in just fine every time. I did encounter one in which this did not help much, and it just turned out that the top of the front magazine wall was a bit high compared to the others, and filing dimples down into them so that the bullet tip would not jam between them and the carrier/receiver fixed it perfectly.

There's something you have to consider when forcing loaded mags to lock against a closed bolt; it's possible to push the base plate out of the mag which pretty much takes you out of action at least until you can get another mag out and try it again. All this to prove a point. If you have time to fight with a mag, you have the time to lock the bolt back. Many auto-loaders of all types with removable box mags have problems locking the mag against a closed action - the AR15 is not alone in this.