At the Workbench: SCAR16 MOE PMAG Modification

I have a lot of magazines. It comes with the territory.

I buy half a dozen here and there and have done so for years. This has resulted in quite an inventory that has come and gone over the last decade.

Within the pile sat eight old original Magpul FDE PMAGs. Back in the “dark ages” of AR15 magazines where the black and green follower GI’s were standard and issued magazines were a crap shoot, I purchased these eight.

They worked flawlessly in my M16A4 and served me for several years. I was forced to retire them from military use with the rise of the M27 since the original design was incompatible with the magazine well of the new rifle (now the USMC uses PMAGs as standard, victory is mine!).

The current generation M3 magazines are stronger, lighter, more compatible, and more capable than the legacy magazine. Due to their flawless service to me I have not retired the eight and I do not plan to.

A situation has arisen that required addressing though. The PMAG was originally designed to address the flaws of the M16’s magazine. Not the STANAG standard as a whole but the M16A4 and M4 specifically.

Concurrently with PMAG development other rifle systems were being developed and deployed. They were using the STANAG as the magazine standard, not the emerging PMAG. There is a substantial amount of engineering lenience that can keep a rifle able to feed STANAG magazines but the magazine well will not be the same as the M16. The HK416, FN FS2000, ARX100, and the FN SCAR16 were all not compatible. Either the magazine or the rifle required modification.

I’ve acquired a FN SCAR16s for a long term project. The rifle is arguably one of the best available and I was, to my surprise, lacking a standard 5.56x45mm carbine. My eight legacy magazines were not ready for use in the SCAR16s.

There’s a simple modification that corrects that.

When you look at the back of a standard aluminum STANAG magazine you can see the squared up portion of the body where the magazine follower interacts with the bolt catch.

On the PMAG you can see I’ve ground away a small amount of material on the left side making the profile more consistent with the STANAG.

It’s necessary to remove a small amount of material from the magazine body due to the way the original magazine bodies were formed. The polymer in the rounded cut out area where the follower will interact with the bolt catch would lift up the catch on its own. This causes the catch to constantly ride against the bottom of the bolt carrier during cycling resulting in premature and unnecessary wear.

After the modification the function of the PMAG will be unaffected but it will no longer partially lift the bolt catch.

Bolt catch remains flush with its housing in the receiver while the magazine body is inserted. It will only lift with the follower as intended now.

If you have MOE or older generation PMAGs and want to modify them for proper function in the SCAR16 the steps are simple.

  1. Insure the magazines are empty and your work place is prepared. Using a rotary grinding tool is recommended but a hand file will work. Use of safety glasses with the rotary tool is recommended.
  2. Disassmble the magazine by pushing up into the magazine follower spring base on the bottom and then sliding the magazine base plate partially forward.
  3. Support the magazine follower spring base while you slide the magazine base plate the rest of the way off since it is under spring tension.
  4. Slowly decompress the spring and then pull the spring, follower, and spring base out of the magazine body.
  5. Support the empty magazine body (a table or bench is recommended) and use your rotary tool or hand file to slowly remove material from the left side of magazine body in the follower slot. From its equilateral original form you should make the left side into a more L like shape. Similar to a STANAG magazine.
  6. A perfectly squared profile is not necessary. Only enough material needs to be removed to prevent it interacting with the bolt stop. Insert the empty magazine body into the SCAR16 lower (separated from the rifle) and observe whether or not bolt stop raises above flush. 
    Note that the MOE PMAG Bodies are thicker than Gen3’s. They will generally fit tightly and not drop free.
  7. If you see the bolt catch sitting above flush remove the magazine and go back to Step 5. Remove more material incrementally until the bolt catch sits flush when you insert the magazine body.
  8. Reassemble the magazine.

My full compliment of matching PMAGs are now SCAR compatible, I have plenty of M3’s and they’ll get run most often in the rifle but… those eight magazines mean a little more to me than just feeding 30 rounds, and now they can continue to serve.

Keith Finch
Keith is the former Editor-in-Chief of GAT Marketing Agency, Inc. He got told there was a mountain of other things that needed doing, so he does those now and writes here when he can. A USMC Infantry Veteran and Small Arms and Artillery Technician, Keith covers the evolving training and technology from across the shooting industry. Teaching since 2009, he covers local concealed carry courses, intermediate and advanced rifle courses, handgun, red dot handgun, bullpups, AKs, and home defense courses for civilians, military client requests, and law enforcement client requests.