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How To: Convert M&P40 Mags To Feed 9mm


StealthyBlagga

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With M&P9 magazines being made from Unobtanium these days, I picked up some inexpensive used M&P40 mags from CDNN in the hope of being able to run them in my M&P9 5" Pro. My goal is to add an extension to make a 140mm magazine that holds an extra round or two (say 23 easily reloadable, or even 24 rounds) over the equivalent M&P9 magazine (which, with an Arredondo base, holds 22 rounds and is barely reloadable).

The magazines that I received were in excellent condition. Ex-police I think, they are very little used - just light surface finish wear. However, when I tried loading them with 9mm ammo, it was clear that the feed lips are too widely spaced; they barely held the top round in place. When I tried shooting the gun this way, I had a couple of feeding issues. Not good or acceptable.

I decided to try tweaking the feed lips to get better retention. Unfortunately, the first one I tried gave me an excellent illustration of how hard the M&P magazine tube is - the lips cracked after barely moving at all. Scratch one M&P40 magazine.

I'm not very bright, so I decided to keep going. This time, I annealed the feed lips before bending them. The photos below show the process step-by-step:

1) Disassemble the M&P40 magazine, then measure the feed lip gap at the forward-most release point. On my M&P40 mags this was around 0.385", whereas the M&P9 mags are only around 0.345" - this means each side has to be tweaked inwards by about 0.02".

2) Gently secure the tube in a clamp or vise. Be careful not to crush or mar the tube.

MampP40-9MagModification01_zps3c3a50ef.j

3) Use a torch (mine is propane) to heat the feed lips to a dull red glow before attempting to bend them.

MampP40-9MagModification02_zps09cc7cce.j

4) I suggest making a simple tool for bending the feel lips. I took an old piece of steel bar stock and cut a slot about 1" long by about 1/8" deep using a Dremel cut off wheel. It basically needs to fit over the entire length of the feed lip, and down as far as the first horizontal bend. Such a tool will be easier to use, give more leverage and be less likely to damage the lips than a pair of pliers.

MampP40-9MagModification03_zpsdb841a0b.j

MampP40-9MagModification04_zpsc3ac0cb7.j

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5) Bend one side first, measuring the feed lip gap-at-release-point periodically. You want that lip to move in about 0.02". After one side is adjusted, heat the other side and adjust it by the same amount. By doing one side first, then the other, we are trying to keep the lips symmetrical and thus maintain a central feed orientation.

MampP40-9MagModification05_zps94739748.j

6) When both feed lips are adjusted to your liking, reheat both lips to red hot, then quickly quench them in motor oil or other hardening fluid. Do this outside in case the oil flares up.

MampP40-9MagModification09_zps01c3ccae.j

MampP40-9MagModification10_zps87c8e509.j

7) Lastly, use a sharp blade (e.g. box cutter) to shave some plastic off the upper-left side of the .40 follower to ensure it rises enough to actuate the slide lock.

MampP40-9MagModification11_zpsa78a3481.j

Edited by StealthyBlagga
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The photos below show the finished magazine, along with stock 9mm and .40 magazines (all loaded with 9mm ammo):

LEFT: Stock .40 mag. MIDDLE: Stock 9mm mag. RIGHT: Modified .40-9mm mag.

MampP40-9MagModification08_zpsd1aadf08.j

MampP40-9MagModification07_zps2e62639e.j

TOP: Stock .40 mag. MIDDLE: Stock 9mm mag. BOTTOM: Modified .40-9mm mag.

MampP40-9MagModification06_zpsd9afbba1.j

Edited by StealthyBlagga
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Last night I gave this magazine a baptism of fire - I ran it untested in our local steel match. After all, we all know that our equipment runs 100% until the beep, then the wheels fall off, so what better way to be sure the mag is OK? I used it as the first magazine in each of 4 stages, so it got fully loaded, shot near empty, then dropped on the ground for reloads multiple times. It ran flawlessly loaded to the max with 23 rounds:

I measured the feed lip spacing before and after the match - there was no change, so the lip hardening seems adequate. Overall I'm pretty happy so far. I'll post updates as this mag gets more use, and if I encounter any problems.

As for the first mag with the cracked feed lips... well, it has FrankenMag written all over it :devil:

Edited by StealthyBlagga
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Sweet tutorial. Thanks for the post. Now the real question is what will it take to get you to "tune" 10 of my CDNN mags? ;) I can sweeten the deal with a crisp dollar bill stuffed into each mag.

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Sweet tutorial. Thanks for the post. Now the real question is what will it take to get you to "tune" 10 of my CDNN mags? ;) I can sweeten the deal with a crisp dollar bill stuffed into each mag.

Make it a crisp Benjamin and I might think about it :devil:

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  • 2 weeks later...
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Wanted to say thanks for this as well. I went a slightly different route due to tool available, and I ended up purchasing 9mm followers instead of modifying the existing ones, but the end result was the same.

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  • 3 weeks later...

I picked up few 40 mags from CDNN and following your method i was able to "adjust" the feed lips. I put a few hundred rounds down range through 4 separate "tuned" mags and did not have any problems. I opted to use new 9mm followers since they were available. Thanks for posting this up.

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