Which part to modify? AR mag issue

[Rittmeister]

I've got two ARs. One is an older DD "Mil-Spec+" upper on an Aero lower. Iron sights, lightweight and fun. It has basically given me zero problems.

The other is a BCM upper on an Anderson lower that has a red-dot, light, etc etc. I have had consistent problems with the bolt not locking open on an open mag when shooting. It always does when I just use the charging handle on an empty, the problem is only while shooting. All of my mags are Magpul Gen 2 30-rd Pmags, except for a few 10, 20, and one 40 rounder I have that are Gen 3. Only the 30s have the problem. It did this with the original mil-spec catch, with a BAD lever on the catch, and more recently with a PDQ lever I installed.

After doing some googling and some detective work on my mags, as well as carefully looking things over with the upper off the gun, I'm seeing that the front of the bolt catch is rubbing on the back of the mag some of the time - particularly if I grab the mag and rock the bottom forward, which levers the top of the mag towards the back and thus towards the bolt catch. It's easy to imagine that this could happen with the bolt "dragging" the top of the mag backwards a bit on the last couple rounds during firing, but not while things move relatively slowly by using the charging handle.

Side note - I didn't whip out the calipers but I suspect the channel for the bolt catch in my Aero lower is slightly further back from the magwell than the one on the Anderson lower. I'll have to measure someday...

Anyway, having identified the problem, I need to remove material somewhere. So, at long last here's the question: I could either remove a few thousandths from the front of the bolt catch (actually the PDQ lever), or I could remove some material from the back of a whole bunch of Pmags. What would y'all do?

Thanks!

 

[Charlie98]

Try the bolt catch first... that way if you bugger it up, it's only a part, and an easily replaceable part at that. You bugger up the magazines... well...

 

[Carl N. Brown]

I can't visualize the problem. The only point of contact between the bolt catch and the magazine should be the empty magazine follower pushing the bolt catch.

 

[TonyAngel]

I would modify the bolt catch and see if you can get it to work reliably. Modifying the magazines may affect the way they work in other rifles. Besides, you really don't want an AR that will work properly only with certain magazines.

I really don't want to start a debate over the quality of ultra low cost receivers, but what you are experiencing is the main reason that I stopped using Anderson lowers... parts incompatibility.

You might get lucky if you just try a different bolt catch.

 

[entropy]

Get rid of the BAD and the PDQ; they act on the bolt catch front to back, and can cause what you are experiencing, and give very little advantage in return, even if they work as they should.
My Dad gave me a BAD he took off his AR after trying it, so I popped it on, tried it next outing, and took it off right at the range.
If you're a vet, it screws with the manual of arms that was drilled into your core. If you aren't, it's extra stuff you don't need.

 

[edwardware]

Which part to modify?

The cheap one (the one with lower future costs to fix). In your case, the bolt catch is cheaper than a pile of mags.

 

[Rittmeister]

I can't visual the problem. The only point of contact between the bolt catch and the magazine should be the empty magazine follower pushing the bolt catch.

OK. Lemme 'splain.

1. Here's the lower with a Pmag in it. On the Gen 2 Pmags, the spine at the back of the mag comes up higher and has a U-shaped relief so the tongue of the bolt catch can engage the follower. In this pic I'm holding the bolt catch down and you can see the material I'm talking about.




2. In this pic, I've released the bolt catch but I'm holding the mag forward from underneath. The material in question is rubbing the front of the bolt catch and preventing it from moving upwards under tension from the follower.



3. Finally, here's a pic of two Pmags. Gen 2 on the right (which give me problems) and Gen 3 on the left. Note that on the Gen 3, the spine comes up and ends flush with the top of the lower receiver. I have no trouble with these. The profile seems to match the GI-pattern mag in the pics above. The Gen 2 spine comes up higher and is causing the issue.



I hope that illustrates the issue.

I would modify the bolt catch and see if you can get it to work reliably. Modifying the magazines may affect the way they work in other rifles. Besides, you really don't want an AR that will work properly only with certain magazines.

I really don't want to start a debate over the quality of ultra low cost receivers, but what you are experiencing is the main reason that I stopped using Anderson lowers... parts incompatibility.

You might get lucky if you just try a different bolt catch.

I totally get this. I got the Anderson lower for a screaming deal with a nice stock on it and a VERY nice trigger in it. Obvs I'm still using it, and the low cost of entry also factored into my decision to hack it up myself and put the PDQ in it. I really like the PDQ so far.

I'm leaning towards trimming the PDQ/bolt catch right where it rubs the mags. Or, I suppose I could sell all the Gen 2 mags I've got and buy Gen 3... seems like more trouble to do that though.

Thanks for all input!

 

[Charlie98]

Never noticed the difference between the Gen 1/2/3 (I have them all...) Interesting.

Given that, maybe take one Gen 2 and mod to see if it actually solves the problem... and go from there.

but what you are experiencing is the main reason that I stopped using Anderson lowers

I was all set to buy 2 Andersons... someone had them for something ridiculous like $25 or something... and then I read the reviews.

 

[W.E.G.]

Trim the gimmicky latch.

Don't mess up a bunch of perfectly good magazines because you fell for a gimmick.

That said, the gimmicks have their place. They make reloads go faster in shooting games where you're on a timer. At least they do when they actually work.

We constantly hear about these magazine-latch gimmicks causing problems. If you don't mind the problems, its a free country. At least for a few more days.

 

[drobs]

Get rid of the BAD and the PDQ; they act on the bolt catch front to back, and can cause what you are experiencing, and give very little advantage in return, even if they work as they should.
My Dad gave me a BAD he took off his AR after trying it, so I popped it on, tried it next outing, and took it off right at the range.
If you're a vet, it screws with the manual of arms that was drilled into your core. If you aren't, it's extra stuff you don't need.

+1

 

[jmorris]

3. Finally, here's a pic of two Pmags. Gen 2 on the right (which give me problems) and Gen 3 on the left. Note that on the Gen 3, the spine comes up and ends flush with the top of the lower receiver. I have no trouble with these. The profile seems to match the GI-pattern mag in the pics above. The Gen 2 spine comes up higher and is causing the issue.

I am with the guys that say mod the cheap part in sprit but if the issue is with “A” specific generation of one brand of mag, as your problem appears to be, I wouldn’t have much issue with a few strokes with a round file to make them functional.

 

[Carl N. Brown]

Looking at Rittmeister's post #7, the Gen 2 magazines appear to be more non-standard than the aftermarket bolt catch.
Modifying the catch to work with Gen 3 & Gen 2 Pmags (and GI mag) might be easier (and I like easier).
If they were my magazines I'd modify the problem Gen 2 magazines to match the non-problem Gen 3 profile IF removing material doen't.compromise strength.

(I don't own an AR-15 but I have participated in AR-15 builds and beta tested a couple of my son's by using them in military rifle matches. So I am learning AR-15 the hard way.)

 

[Rittmeister]

Thanks again for all input. I've modded the PDQ bolt catch slightly and it already works the way it should with some of the mags. A little more off the front and I feel confident they'll all work as they should.

As for using non-standard/non-Milspec parts - I know that I'm a bad person and that the ghost of Eugene Stoner is likely to give me an angry visit, but I tinker with and modify just about everything in my life and my guns are no exception. I really like being able to lock the bolt back without moving a hand off the grip, just like a pistol, and the BAD levers and now the PDQ fill that bill nicely. They have given me ZERO problems of any other type, and after my detective work and all of your input it's clear the problem here is a loose Anderson lower and some tolerance stacking - and if you read my original post you'll see that this whole issue happened with a mil-spec bolt catch in the gun in the first place.

In all sincerity, I don't mean the above as a gotcha at all. Y'all have helped me solve the problem quickly and neatly. I really like this forum for its lack of flaming and for its huge knowledge base. Thanks again!

 

[TonyAngel]

I'm glad that you're on the road to working it out.

As for BAD levers and similar devices, I've been running them for a long time and have them on all of my ARs. I consider them standard equipment. Yes, reloads are faster, but the real advantage is in clearing a malfunction that requires locking the bolt back.

 

[RussellC]

I used to use Spike tactical upper receivers and their lowers as well. Last several builds have been with Aero Precision which is my new go to. I have used a few Anderson upper receivers as well as lowers with no problems at all. Maybe something changed? Possibly. The only lower that is giving me trouble was a sale priced lower from Wilson Combat. I did not buy it direct from them, but another ffl vendor who escapes me at this time.

Pressing the mag release, the msg wiggles a bit, but must be pulled out. Decided to use it on a build using 7.62x39 round. The mags I was suggested to use stick even worse! They don't even budge when release is pressed, you just have to pull it out while pressing release. I can see a distinct wear line on the mag, I was thinking of removing a bit of material from inside of receiver, or just pitching this lower. It was on sale and didn't have the best finish either. Surprised, Wilson Combat is usually top notch stuff. Even the logo on the receiver is not up to normal quality they have.

Russellc