Got my SIG Mod X-9 out of jail on Saturday and got it to the range this for about 125 rounds this evening. Host was an 8" MPX sbr. Here are some initial impressions.

The fact that SIG includes a 13.5x1LH adaptor along with the 1/2x28 is great, so I had no problems running it on the MPX. Just pulled out the 1/2x28 it shipped with and swapped in the metric piston and fixed barrel spacer.

Suppressor is LIGHT, even in the full baffle configuration. For comparison, it's noticeably lighter than my Nomad Ti, which is 1.5" shorter. Can is also pretty slim like an Obsidian or Erector. Very easily sleeves under the MPX handguards.

Not only is the whole suppressor light in its full configuration, a single baffle unit is shockingly light.

Did most of my shooting in the full baffle configuration, though I also tried 5 and 4 baffles. Shooting was done indoors, so I was still wearing ear pro.

Shot a few rounds of 115gr supers. As expected, dropped the sound to .22 levels.

Shot mostly 150gr Federal Syntec and some of my 124gr loads over 4.2gr WSF, which are clearly subsonic.

Can was stupid quiet with the subs. Like BB gun quiet. There was first round pop with the subs, but after that, it was great.

With subs, I couldn't detect much difference between the 7 baffle and 5 baffle configurations. At 4 baffles, I started to notice a difference in sound and I was having fire coming out the ejection port, lol.

Pros: super lightweight and very good suppression. Love the modularity and it's easy to tinker with.

The only con is the surprising amount of gas. Like, damn, it was a gas chamber.

I was really surprised, given the fact that the MPX is a piston gun. Shot plenty of suppressed DI ARs and AR10s or suppressed pistons like the MCX and they are nowhere near as gassy as the MPX/Mod-X combo.

Gas was clearly coming out the ejection port and not through the charging handle, like you'd get on an AR. I wouldn't say there was a noticeable increase in backpressure. Gun handled the same as always, just with a receiver full of gas. Strangely enough, the brass was still pretty clean looking, especially compared to brass coming from a suppressed DI gun.

I did notice less gas with 5 baffles than the full stack of 7.

Maybe it's not so noticeable outdoors, but indoors, 10 or 15 round strings were harsh.

Would be curious to try it on a blowback rifle or something roller locked to see how gassy those are.

I don't have any pistols with threaded barrels, so no input on how it fares with a handgun. I bought it specifically to live on the MPX, though. I've got more experimentation with baffle configurations and defensive ammo, to include chronographing, planned for the future.

Very cool. I appreciate the write up. Although I don’t have an MPX, I have been trying to get to the range to test my Scorpion, AP5, and APC9 against each other with my Obsidian 9. I am a bit surprised the MPX seems gassy. Reinforces my decision to hunt down a Dead Air Wolfman.

With my MPX (4.5" barrel) and the older Sig SRD9 silencer there has always been some gas coming out of the ejection port. Like the OP stated, stupid quiet with SUBS. I dont think anyone knows I am shooting a firearm outside when I use 147 grain through the suppressed MPX.

Gas to the face has been manageable with the exception of the recent use of Sig Sauer 124GR Elite Ball FMJ. Holy cow, that stuff had my right eye tearing up the entire time. I wont use it again it was that bad.
I agree with the OP, certain strings of live fire can be harsh whether its indoors or outdoors. Suppressed MPX is still fun as hell.

I shot an MPX with the Modx-9 at the Sig experience center this summer. It did seem to throw a good bit of gas out of the port and I had to pull it away after a 10 round string. I shot 100 rounds through it though and it was a barrel of fun. I had been thinking I would get one for my MPX but of I am considering a Silencerco Octane 9 2.0. I already have a Silencerco Hybrid 46 and all of the accessories so it would fit right in. Plus the cost with the stamp is less than the Modx by itself.