MR-10 Mag Mods

Trapr Swonson

Magpul PMAGs have been a remarkable product since their inception. In particular, they helped solve many of the issues faced by AR-10 shooters having to deal with original metal mags. They have proven consistently reliable in that platform, and many would now consider them the “default” AR magazine.

Given their massive success, it’s no surprise that Magpul has broadened their PMAG lineup to AKs, Glocks, etc. So, because they have such goodwill from me, I decided to give their bolt action magazines a try in my JP MR-10.

In short, the PMAGs run great in the MR-10, and I strongly recommend them. The caveat is, they need a little modification to function reliably. But, it’s a quick and easy job, and I’m going to show you how to do it here.

These magazines come in two varieties: 10-round (MAG579) and 5-round (MAG549). Both models are designed to function with short action AICS-compatible platform such as the MR-10. I’d been seeing some spotty reliability in my metal mags mostly due to dust and grit gumming up the function. So, I ordered up one of their 5-round 7.62 AC mags to test.

When I first slipped the new mag into the rifle chassis, it fit fine. Unfortunately, it would not lock in place or allow rounds to run through the gun. So, I measured my existing metal mags and determined that with a few modifications, I could get them up and running.

Fortunately, the modifications were simple and required just a little trial and error. Step one was to narrow the feed lips on the Magpuls a bit to fix the feeding issue. Step two was shortening the locking tab to ensure that the mag catch would retain it reliably.

After carefully proceeding with the first mag, I got it to functional reliably enough for me to run it in the 2016 Competition Dynamics Steel Safari. It ran without issue and has been doing so ever since. I recently ordered two more mags and documented the process so I could share it with the benefits with you.

In addition to your MR-10 and the new PMAGs, here are the tool you’ll need to get them friendly with each other:

  • files: one round 1/4” to 5/16” in diameter, one triangular
  • dial caliper
  • dummy rounds
  • vise
  • something to stuff into the empty mag body to help hold its shape in the vise

Having the rifle handy is a necessity since you should try the magazines regularly in the gun as you go along. You can only go wrong if you file too much between checks in the rifle.

I timed myself on the second modified mag and total time with filing and fitting was only ten minutes. Totally doable even for those who aren’t too handy. Here are the steps:

  1. Disassemble the magazine by using a dummy round or some other tool to press in the colored spot on the floor plate of the magazine. At the same time, slide the floor plate towards the front of the magazine. The floor plate retainer, spring and follower will come out easily once the floor plate is removed. Check YouTube for video demos of this if you’re having trouble.

  2. Place your magazine filler inside the empty mag body. Then, secure the magazine, feed lips up, in the vise careful to use only enough pressure to hold the mag still and not crush or deform it.

  3. With your round file, thin the base of the factory feed lip as shown. You are removing material only at the base of the feed lip just above the mag body itself. File parallel to the direction the bullets slide into and out of the magazine. You can see here the before and after of what you’re striving for.

  4. Once you’ve removed enough material to create a groove along the entire base of the feed lip, test the mag in your rifle. Insert the mag somewhat forcibly as though you were performing a reload under a time constraint. Remove the mag and look for a small rub line on the mag body. This line is produced by the feed rails of the rifle making contact with the mag body and is a sign you need to remove more material.

    If a significant amount of material still needs to be removed, it will be scraped up by the sharp edge of the rifle feed rails. Continue filing carefully at that location until only a line is produced by the feed rail and no material is scraped up.

    In the above picture below, you can see a small amount of scraped-up material at the back of the mag and at the very front of the mag’s feed lips. This area was then filed a bit more so that the scraping stopped and only an imprint of the rifle’s feed rail remained.

  5. Re-secure the mag body in the vise with the locking tab facing upwards.

  6. Using your triangular fine file, remove a small amount of material from the bottom of the locking tab. For your reference, the factory tab measured 1.377” from the top of the mag body to the base of the feed lips. The modified version measured 1.352” when I was finished with it.

    Ensure that you maintain a flat filing stroke across the bottom of the locking tab. It doesn’t take many strokes to get it right, and only a few more to screw it up. Again, test frequently in your rifle.

  7. Once the mag locks in without scraping material from the feed lips, load your dummy rounds in the mag and check feeding. If you followed the steps carefully and tested as you went along, the feeding and function should be reliable.

That’s all there is to it. I hope you found some value in this and more value in your MR-10 as well. It’s a great rifle platform, and I’m glad I can get even more use out it.


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