I have the latest gen MPX with an 8" barrel and 8" MLOK handguards without the Timney trigger. I was wanting to chop it to 4.5" so I can use a Witt Machine Sound Mitigation Device and have it recessed under the handguard and then use Fort Scott 125 grain subsonic TUI rounds in it.

Does anybody know if the barrel profile of this gen pistol is thick enough for 5/8x24? Witt does have an option for 1/2x28, but I like have the extra material and I can use it on my .300 blk if I wanted.

Does anybody think there is a need to open the gas port any while using subs?

I appreciate any help. I called SIG with the same question and he stated he couldn't advise me on this type of question and pointed me here.

The barrel diameter you need will vary with exactly how much shoulder you need.
Whoever is cutting your threads and the way the device endcap is designed determines that(I don't know anything about Witt Machine).
For a quick guess (I don't have a late gen MPX) you likely need a minimum of .685 and if your device is counterbored at all probably as much as in the .745 range.
But in any case this is a question for your machinist not Sig.
Lastly there are any number of 1/2x28 to 5/8x24 adapters that don't add much length as a possible alternative solution.

And frankly why not just buy the already threaded 4.5 factory barrel and get an adapter to whatever threads you need as those are also readily available. I converted mine to 1/2x28 but there are many other choices. I doubt you will be ahead on chopping yours versus buying and selling.

I appreciate the responses. I ended up just chopping and threading it for 1/2x28 since a local guy was able to do that for me. I would have like the extra material but if 1/2x28 is good enough for most major manufacturers, it's good enough for me. Thanks again.

4.5" would have been too short for the muzzle device I wanted. The end result came out to a 5.25"barrel. I considered it, but my guy was able to do it over the weekend.