Magpul P-Mag Modifications For Large Bore Rifles


One of the top selling points of the .458 SOCOM and 12.7x42mm rifles are that they share so many components with their parent platform the 5.56mm. Right out of the box most magazine will work with no issues. That does not mean that there is not room for improvement. I wanted to have a few magazine to work on, but was not ordering from my usual suppliers any time soon. For that reason I picked up these from Amazon to take advantage of free shipping here: Magpul PMAG Magazines.

Magpul Pmag modification for .458socom, 50 beowulf
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From the start I noticed a few things. The much larger case of the big bore rounds prevented them from having an ideal feeding angle. While not an issue with pointed and FMJ rounds, this was an issue as usually is on the open hollow points. The first few go arounds involved jigs and power tools. Only later did I realize this process is faster by hand.

Pmag magpul taken apart
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Start by removing the baseplate, spring, and follower. Now tightly wrap a small strip of 200 grit sandpaper around the shank of a 1/2" drill bit. This gives you a sanding mandrel that is almost the exact size of the rounds we are developing for here. This process goes quickly so use care not to remove too much material. If you are applying even pressure to both sides the width of the sanding marks will be uniform. From what I have seen the best angle to sand at is around 30 degrees in relation to the magazine.

Feed lips opened up
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The other that needs addressed is the front of the magazine. In the un-modified magazine below, you can see the sharp point at he front which causes hollow point to hang up. This can be radius off, which solves the problem. Remember the goal here is not to remove the ridge, and stop, but to slightly change its profile. This process may take some trial and error on the first few magazine to get everything "just right". Once you are happy with the function, take a cotton swap and lightly wipe down the area you sanded with MEK. This will melt the plastic and remove any sign that the magazine was ever modified.

Follower modded for extra round
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This step has nothing to do with reliability, but will allow you to load one additional round into the magazine, and still be able to use it even with the bolt forward. The goal here is to sand the follower flat. You will also need to radius the rear of the follow to allow it to come all the way up to the top of the magazine body.

New angle
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In profile you can clearly see that the follower sits a few hundredths higher now. If you correctly radius the rear of the follower everything should work 100% reliably with an extra round. If the bolt catch does not work after this modification, you need to radius the rear of the follower more.

Here is a short movie to help you with the first few steps: