Pmags sticking in Mega lowers

[slimedog]

Hi friends,

A couple of months ago, I noticed that one of my Pmags really hung up in a Mega lower that I had recently assembled into a rifle (yes, the mag release has plenty of clearance). This started a long process where I tested about 50 different mags in two Mega lowers (one of them was stripped), two Colt lowers, a Spike's lower and a Charles Daly Defense lower. I measured magwells and magazines and talked with both Magupl and ZEV/Mega. The Mega lowers were purchased about 7 years ago. I've been collecting Pmags for about the same amount of time and mine are mostly M2s, but I do have a few M3s. Here's the summary:

(1) All of my aluminium GI mags (Universal, Adventure Line, Okay) function and drop free in all 6 of my lowers.

(2) All of my Pmags, Troy Battlemags, and Hexmags function and drop free in my four non-Mega lowers.

(3) About half of my Pmags will not drop free from the Mega lowers. Five of them are so tight that they are difficult to insert/remove from the Mega lowers.

(4) There is a surprising amount of variation in the dimensions of magazines (e.g. in my sample of 30 Pmags, the front-back width ranges from 64.1 to 64.7mm), but only the Mega lowers seem to struggle with that. The degree of "stickiness" seems to be correlated to that dimension - every Pmag that is wider than 64.4mm gives me trouble in the Mega lowers. Every Pmag that is less than 64.4mm drops free.

(5) Mega and Magpul both responded to my questions by e-mail and phone in a friendly and professional manner.

(6) Neither Mega nor Magpul would tell me what their actual dimensional specifications were - I guess that isn't too surprising.

(7) Magpul offered to exchange any magazine that I was dissatisfied with - no questions asked. They did stress that there was no guarantee the replacement magazines would work in the troublesome lowers.

(8) Mega's final response was, "I did receive your findings and my gunsmith did state that the polymer mags have been known to be problematic with our sets. I asked what he recommended and he states running the steel GI mags."

I assume they meant "aluminum" GI mags. My conclusion is that the Mega magazine wells are tighter than average. I don't know if that means they are not "in spec" but they certainly behave differently than my other lowers.

At any rate, while I appreciate Mega's honest response, I'm not thrilled with having to use specific magazines in certain lowers (particularly since I have so many Pmags and they are so bloody common). Does anyone have suggestions on how to make my Mega lowers work with Pmags? I have considered getting some fine lapping compound and smearing it in the wells, then using a sacrificial magazine body to rub down the high spots.

Or should they just be relegated to "range toy" status?

Thanks!

[26 Inf]

According to page two of this blueprint: http://www.firearmfiles.com/files/AR...0us%20army.pdf

The overall magwell length is 2.398 0r 60.9092mm if you use +/- .015 it equals 2.413 or 61.2902mm on the large end and 2.383/60.5282 on the small end.

The body of one of my Okay mags measures 2.379/60.4266 which, as you can see, is small than the (-) dimension.

If I include the rib on the back, which I suspect you did, it measures 2.5275/64.1985.

The blue print calls for that measurement to be 2.533/64.3382 +/- .015 and you get 63.9572 on the low side and 64.7192 on the high side.

I'd guess your mag-well is within specs, just not the high-side specs, which apparently, some of your PMAGS run to.

What about width?

Myself, it seems to me that your problem is the PMAGs, not the receiver.

I'd like to say this issue is why I use aluminum mags, but it wasn't, up to now.

[slimedog]

Thanks for the reply - the drawing is great! I think, however, you misread the number on the print. Including the rib, that dimension appears to be 2.553 inches or 64.85 mm. I don't see the 0.015" tolerance you're using and it sure seems big to me. On the other hand, if you use 0.015", the min should be 64.85-0.381=64.47mm, which is pretty close to where I started seeing problems.

I did measure widths, but didn't mention them because there didn't appear to be a correlation and they didn't vary nearly as much. Also, all the sticky mags could be wiggled slightly left-to-right, but not back-to-front.

Does anyone know of an official STANAG magazine drawing? I haven't been able to hunt one down.

[26 Inf]

You are right, I did misread it as 2.533.

I don't think STANAG 4179 ever went beyond the draft stage.

Here is a link to an M16 magazine blueprint. Double check me, but it looks like 64.4 is the max.

http://www.firearmfiles.com/files/AR...-Blueprint.pdf

[SomeOtherGuy]

I have a MEGA forged lower from around the same time +/- and it is tight with all magazines. Pmags can be inserted, but don't drop free. I have several other MEGA lowers, both billet (c. 2012) and newer forged ones (c. 2015) that work fine with all magazines tested and Pmags drop free. I think they changed their magwell dimension at some point.

Your best option is to assign this lower as a range toy and get something else. Alternatively you can sell the Pmags and use GI, but I too like Pmags. I'm sure it's technically possible to enlarge the magwell, but it doesn't seem like a good idea.

[slimedog]

Thanks for the drawing and replies. I'm leaning towards SomeOtherGuy's view that it's probably not smart to sand on the magwells. However, I do have a good purpose for at least one of these lowers - I'll slap a White Oak upper on it give service rifle a try. I think those guys pretty much stick to the aluminum 20 rounders anyway. Finally, I also want to say that Mega and Magpul were both on the up and up here. It's probably a combination of the Pmags being on the high side and the Megas on the low side of the tolerances.

The 64.4mm max dimension from the Imbel drawing is kind of cool. That's exactly where I started to see issues.

I wonder if there's a temperature thing, too. Aluminum mags will have the same coefficient of thermal expansion as the receiver. I imagine that plastic has a lower coefficient. Maybe the Pmags will work better when it's 85F instead of 40F.

[mack7.62]

I have seen sticky PMAG problem before with different mag/lower combinations, two things to note, PMAG's have a lot number on package and mould/date numbers on mag body. Generally all or most of a lot will stick or drop free in a lower so you can track that way. Point two, with no way of knowing for sure it seems to me that the issue MCT window mags are being held to a tighter standard, ie a lower that has issues with other PMAG's sticking drops free with several different lots of the issue MCT's. Point three, there are no guarantees, even with the aluminum GI mags I have seen NIW mags stick in some lowers, but the OKAY/COLT mags I can't recall ever having a problem but YMMV.

[tommyrott]

I had a mega lower that 3rd gen pmags wouldn't drop free, took an Emery nailboard to the latch block on the magazine all it took was one stroke and it was dropping freely

[Beat Trash]

I had a mega lower that 3rd gen pmags wouldn't drop free, took an Emery nailboard to the latch block on the magazine all it took was one stroke and it was dropping freely

Thanks for this...

I had about 15 Gen3 PMAGs that would stick in a Daniel Defense lower, but dropped free from a Colt lower. These magazines were purchased in early 2016 in case the election went different. I just got done hitting the latch block on all of them with an emery board. It only took 2-3 light strokes with the emery board and problem solved...!

Thanks for the tip.

[slimedog]

Interesting - thanks for the information. In this case, my Pmags are clearly binding on the front/rear surfaces of the magazine wells. You can see wear marks on both the magazines and the wells. Maybe after a bunch of use, they'll mutually wear themselves into compliance...