So, I picked up a 3.5" Copperhead barrel off the EE with the intent of having it threaded and having a nice tuck setup.  When I got the barrel, I put it on the gun took it to the range and tested it at 25 yards.  Golf ball sized groups!  Perfect!  When I got it back from threading, it shot equally as well without a suppressor.  But when I put my Tirant 9M on, it turned into a shotgun.  Litterally... groups sometimes up to 10ish", but strangely still produced perfectly round holes.

Tried different bullet weights (115-147) and different powders (Titegroup to Longshot) to no avail.  Have an TBAC Fly 45 that I'm waiting on a stamp for and luckily the FFL has a range on site.  Well, everything shoots as well through the Fly 45 as it does without a suppressor (and sometimes better).  I think the back pressure in the Tirant 9M is causing the bolt to unlock and unseal the case too early, and that this is causing erratic velocities.  Haven't put it over a chrono yet, but that's my hypothesis.  

The Tirant 9M has always lived on the MPX, so I'd kind of like to get it running in this barrel if I can.  The gas port is already only ~0.055" (judging with gauge pins on angle).  The barrel is only 3.34", so I'm not sure if there's enough room for the long ILWT adjustable gas plus that goes down to 0.040".  The short version isn't compatible with this barrel.  Not sure the fixed 0.0469" plug is going to do much over the existing 0.055" hole.  Wondering if anyone else makes a fixed restricting gas plug that isn't long and fits a Gen 2?

Did you put an alignment rod down the barrel?
Have you inspected the suppressor to ensure the bullet isn’t striking the baffles or end cap?

It's aligned and there are no sights of baffle strikes.  Intact carbon on all of the baffle holes.  I'm direct threading it, no 3 lug.

It's also a lot gassier with the 9mm can...

https://geissele.com/mpx-recoil-spring-kit.html

It’s worth a shot giving these a try as a secondary solution, but I would look for an adjustable gas block or plug that works first

Awesome!  Thanks!  That should help some amount!

Got the Giessele springs and an ILWT adjustable gas plus (it JUST fit!) and did some more testing.  The short barrel won't shoot with the AAC Tirant 9M no matter what I do.  Not even with the gas turned off completely, running it as a bolt gun.  Still patterns like a shotgun...  Strange because the Tirant works fine on the 8" barrel and on other guns.  No idea why, but I did find a lot of threads online with people saying the same was happening with their Tirant suppressors on MPX-K 4.5" guns.

Tried my DA Wolfman on the gun and that it shoots just fine.  No accuracy issues with the Wolfman or the Fly 45.  So, I guess I'll just get a Plan B FH for the MPX and alternate the Wolfman between this gun and my 9mm AR.

Didn't end up using the Giessele springs, but thanks for the suggestion.

I had a similar issue with a .22 hand gun and a can. An o-ring solved the problem, night and day difference. Not sure if yours has enough shoulder for an o-ring.

Where was the O-ring placed?

Between the suppressor and barrel. This was for direct thread.

I have a .22 suppressor that has reversible baffles.
Orient them one way and accuracy is just fine.
Turn them around and staying on paper at 21' is a challenge.

Will that work for you?
Don't know... maybe try it if your suppressor will let you.

Not possible, they are cut to only go in one way.

I have more or less given up on the 4.5" barrel length.  I first utilized the Sig 4.5 barrel and then also tried the ILWT 4.5 barrel.  I tried ILWT adjustable gas plug, , Geissele recoil springs, different suppressors, different suppressor mounting options (direct thread and tri lug).  In the end I couldn't get satisfactory groups with any setup using the 4.5" barrel.  I found a 6.5" Sig barrel on closeout and grabbed it.  I'm finally able to get acceptable groups with this barrel and direct thread with my cans.

I assume you're talking about a rubber o-ring?  Other than space the silencer out another 1/16 inch, how exactly does this affect any change?

I'm guessing that's what he means.  I'm not sure how that would help.

An o-ring would force the silencer against the threads and allow some play for the silencer to align with the bore differently.  Tightening the silencer against the shoulder would force the silencer to align with the shoulder, not the threads.  If the threads are not perpendicular with the shoulder there could be a big difference in alignment between the silencer and the bore.  

Another possibility is that there is no thread relief on the barrel and the o-ring spaces the silencer out.  If the threads bind without a relief then it would cause the silencer to poorly align with the bore.


On topic: I have a sig barrel cut down to 4 inches and a factory 8 inch barrel.  Neither has accuracy changes with or without multiple different silencers.