Sig Sauer MPX CO2 Air Rifle - Air side part 2 

The "Chatter" is the hammer not resetting on the sear. This is a "blow back" system. Its the same principle used in low quality paintball markers. They scavenge some air from the air that fires the projectile to also reset the hammer on the sear. Some areas to check are the point of interaction between the hammer and sear. Look for excessive wear where the sear touches the hammer. If worn, the hammer may slip off the sear several times before catching. These blow back operating systems are also sometimes dependent on back pressure. The pressure used to reset the hammer is higher when there is a pellet being fired. The force required to push the pellet causes the "back pressure" used to also push the hammer back to the sear. If therer is no pellet, there is no resistance on the air flow resulting in less pressure to reset the hammer. This causes the hammer to repeatedly strike the firing pin because its not bing reset, its free floating back and fourth. Also, Co2 losses air air pressure if fired repeatedly. The faster you release Co2 gass, the lower the pressure in the tank becomes untill it warms back up in a few moments. This loss of pressure can cause the hammer to not reset also. However, using HPA will alleviate that issue. Back in the hay day of paintball, I worked as a gun technician from 2000 to 2015. I've repaired 1000's of paintball markers and the functionality of this gun is no different. If your gun is firing more than once on its own, the hammer is striking the valve more than once because its not catching the sear. The cause could be one of a few issues. I dont believe its possible for the valve pin to "chatter" on its own. I hope this helps.

There is something strange about the way of thinking with Sig and their experiments with mcx and mpx that give bad results. I guess it has something to do with the production costs. After 5 years they have finally solved the problem with the firing pin on the mcx and mpx by serrating it, but they have replaced the brass sleeve that holds the nylon gasket in place at the back of the valve body with a sleeve made of the same crap material as the rest of the innards, so now it is this sleeve which falls out instead. This brass sleeve on the older versions was as tight as if screwed in. The older versions also had a better construction of the air supply with screw connections at the brass tube that pressed against the o'rings. It would have been better if another airgun manufacturer that takes pride in making good airguns had taken over the production rights and made the necessary improvements, then these airguns could have been really good.

I've had my MCX sitting around for a year now due to a leak, and this video gave me the courage to open it up and get to the bottom of it! Thank you for both parts, good to have a no bullsh*t guide to follow. Saying that I couldn't find any obvious issue, just gave everything a clean and applied some Pellgunoil to the seals and it seems to be working great (only fired 1 magazine so fingers crossed). Edit for anyone wondering, the innards of the MCX appear to be near identical to the MPX.

The internal metal looks like some kind of zamec type alloy, really weak, shame, if they used 7075 aluminium alloy with better seals, gun in theory would be much more reliable and last much longer.

The firing pin popped outta my old MCX once. Once. I put instead an M4 cylindrical hex head steel screw into it and thru it. Took off the small knob from the rear of the hammer/bolt and attached a round 6mm thick nut to lock the new firing pin and to keep the main spring front centered in place behind the bolt. That all threadlocked and it ain't coming out again. The firing pin issue, along with a new "nipple" moving lug system was developed from the .22 Virtus PCP version and forward. I'd recommend you buy next the Sig Rattler Canebrake or the now outcoming MPX gen2. I definitely now want a gen2 MCX. My Virtus has been awesome - but I'd like a new MCX in .177 with the newest internals. That old thin co2 transfer line just does not work as well with air. My old MCX I had shooting way hard once - but the co2 cooled down so damn fast that it was not fun for that long.. I really woulda wanted the .22 Sig Virtus PCP in .177 also - just because the .177 ammo is cheaper to plink with.

My mind keeps going to the idea that maybe replacing some parts with a more robust metal but say im right and we replace aluminium mechanical parts with steel parts how will that affect the momentum and timing of the function

hopefully it is fixed

Thanks so much for your 2 videos. great help to me with trouble shooting my problem. Your camera placement was excellent. Your commentary was great as well.

I just did all this and the thing just spit out a whole new 88g canister. It started with a bit of a sputtering of co2 after a couple shots and a slow leak, after taking it apart and putting it together it just spit it all out without stopping flow. This thing was such a waste of money

hi! Can I turn this gun into a fully automatic PCP? My fingers hurt and I'm tired.

my black shroud has disintegrated suffice to say I am getting loads of air from the magazine and about 1fpe!! any ideas where I can get one?

Thank you for your video and part 1 also. It helped me fix mine. The pin had fad fallen of, and the plastic spring thing was stuck in the same time.

I'm wanting to paint my mpx to a coyote brown. I figured I would take it apart and paint what I wanted with some rustoleum. While I had it opened I was gonna clean and oil some parts. Should I just use pellgun oil? Is there any pitfalls I need to be aware of before opening it?

This MPX is configured the same as my MCX but my issue is a bit different. With about 300-400 rounds through the MCX v.1 CO2 air gun, it out of nowhere, stopped firing. Unlike this and other similar reviews I have read, mine is not an issue with pressure leaks or the hammer popping out of the bolt. It took a long while to determine but it appears that the hammer, when pressed in at the factory, is pressed to a very specific depth (from the front of the bolt). Thus, the hammer had not been seated against the bolt, leaving an additional amount of space before the hammer would actually seat against the bolt. My suspicion was that during the initial 300-400 rounds, the impact of the hammer against the firing pin, finally pushed the hammer deeper into the bolt. Now, having increased the distance between the hammer and the firing pin, it prevented the bolt/hammer mechanism from actually contacting the firing pin. I popped the hammer out of the bolt and added a washer of 1.4mm thickness to the shaft of the hammer then reinstalled it back into the bolt. The MCX is now back to firing. Extremely poor engineering. Shame on you Sig.

Pot metal and dreams hold these together. I am surprised they shoot at all.

Thanks so much for the video. I had taken mine apart and some of the pieces fell out and didn't know how they went in there. You are right about the gun. It is a nice looking and had potential to be a great pellet gun but very poor as soon as you open it up. I am tired of going thru 10$ cylinders for only about 5 shots . I got an adapter that I havnt tried yet because I didn't know how to put the thing back together but do now thanks to you.

Be careful not to turn on the air while apart! I just about lost an eye! lol

Great video, very very helpful, I just wished I discovered it sooner 😅

Mines leaking from the big white o ring where the pin punctures the co2.. it sprays out right by the slide hammer

is there any way i can make it full auto?

I put mine back together n the trigger won't pull why is that?

Great video very helpful thank you