DIY: Add Five Rounds to your Sig MPX Magazine

Manufactured by Lancer Systems for Sig Sauer according to strict specifications, these mags are valued as reliable OEM magazines. If you’re wanting to increase the magazine capacity of your Sig MPX, here’s a simple how-to guide.

Before we start, though, take a look at the mag. Inside the magazine is a base plate that the spring pushes against and locks the magazine floor plate in position. The base plate has a long protrusion that hits the magazine follower too soon.

Sig MPX magazine, compressed spring.
If you look into the magazine you can see there is still space for the spring to be compressed.

The first step is to use a punch or similar tool to push the base plate up and slide the floor plate off. Be sure to use your left thumb to keep the base plate and spring from flying out, otherwise, you might be dealing with some kind of flesh would or spending time looking for them.

After you slide the floor plate off, remove the the rest of the magazine contents.

Sig MPX magazine baseplate, spring, floorplate.
Sig MPX magazine shell, baseplate, spring, and floorplate.

Remove the black base plate from the bottom of the mag spring. You might need a flat head screwdriver to pry the spring out of the little hole in the base plate.

Sig MPX magazine baseplate with spring removed.
The baseplate with spring removed.

Next pick a spot and cut the protrusion down. Then reinstall the spring.

Sig MPX magazine baseplate, modified to add capacity.
I was a little conservative and cut it in the middle.

Now you’re ready to reassemble the magazine and load the ammo. On the first try, I was only able to fit 34 rounds of 9mm in the magazine. You could chop the baseplate all the way to the hole for the mag spring but make sure to leave the hole as it holds the spring in place.

Now you have 35 rounds of 9mm in your Sig MPX magazine. Might be a good idea to get a mag loader and save your thumbs, just sayin’!

Nicholas Chen is a firearm aficionado. Growing up in California, He learned about firearms and hunted with his father growing up in Southern California. Once he moved out to Pennsylvania and then spent a few years in New York State, he learned to truly appreciate firearms and the second amendment. It was in New York State where he became a USPSA competitive shooter and learned about 3Gun. He is an avid fan of running and gunning. Nicholas is a fan of pretty much anything that shoots although the slicked-up race guns are what interests him the most. Having spent time with FFLs and other friends with NFA items he has begun his collection of NFA items and has learned that everything should be suppressed and full auto where possible and legal.

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