Has anybody taken apart the gas vent valve on the MPX?

I did and will be testing my modifications today.

To disassemble remove the roll pin that is holding the plug in place. Use a spanner wrench to unscrew plug. Inside there is a plunger with a Belleville washer stack.

The stack is arranged as follows: (())(())(())(())(())  the installed height of the stack is approx .480" with 20 washers

I removed 4 of the washers and reassembled as follows: ()()()()()()()() with an installed height of approx .440 with 16 washers

I was able to reassemble and replace the roll pin afterwards.

Doing this increases the travel of the vent plunger and makes it a softer spring rate. Hopefully this will reduce the force on the action and I will be looking at the spent brass ejection pattern to tell if there was a difference. My MPX has the factory 8" barrel and an AAC Ti-rant 9, and I am using a hiperfire trigger.

Take video!

Ok, I ran 3 different types of ammo.

Mag tech 115gn
Aguila 124gn
American eagle 124gn subsonic

Unsuppressed the mag tech cleared the ejection port about 2 feet

Aguila cleared the ejection port by about 3 feet

Subsonic American eagle cleared the ejection port about 3 inches and also had stovepipe stoppages

Suppressed the mag tech cleared a little further, and the aguila further also but the subsonic still had stove pipes.


My conclusion to this particular arrangement of belleville washers is that you will run supersonic ammo fine, with greatly reduced recoil. Subsonic use may be more reliable if using 147gn ammo but I didn't have any with me at the time. I will try adding 2 washers back in to make the 124gn subsonic work.  

Before this mod, my mpx was kicking brass 15 feet with lots of recoil for a 9mm.

Good info. Thanks for the write up.

So after this past week I've been having issues with my mpx. I had never taken apart the gas valve... So behind the plunger there was a 3/16th ring of carbon fouling.... Cleaning it now but that might be why I was having issues (trigger not fully resetting ). From the build up I would say most might want to clean that ever 3-4k rounds.

That ring prevents a full stroke of the piston.