I have a CMMG MK9-PDW that I'm getting some odd fliers with.
In its original form, I had no such issues. But since swapping to a 4.5" Faxon/Kaw Valley barrel, I'm all of a sudden seeing some fliers. I'm sighting in at 15 yards rested, and it'll shoot one ragged hole for the most part, with the exception of the occasional (every 3rd round or so) flier that is low & right by a couple inches.

I shoot 100% suppressed with an AAC EVO-9. I've tried direct threading it, and recently swapped to a 3-lug mount - same fliers in both cases. There are no obvious baffle strikes. The same silencer, ammo and shooter shoots straight with a CZ Scorpion (and w/ the CMMG's 8" barrel). I've triple-checked the barrel nut, the red dot mount, etc.

I suppose it could be the shooter! But i'm shooting supported, seated at a table, and I don't see a similar pattern with my other guns.

Have you tried it without the can?

Nope, not really an option (on my property). I've pretty much never shot most of my guns unsuppressed. :)
Understood that this might be able to tell me if the barrel itself is jacked, or if just the threads aren't concentric.... Either way it's a new barrel though.

For flyers like that 1st: optic, check, check and recheck. 2nd: ammo, try different brand, proj. weight etc. If you are using heavy subs try lighter 115gr just to eliminate stability issues as not all twist rates are the same. Also a heavy buffer offers more consistent port opening timing and theoretically better accuracy but this may be just trivial.

You don't happen to have problems racking a round out of the chamber do you? In my Kaw Valley there was almost no leade in front of the chamber resulting in pulled bullets when un-chambering some brands of ammunition. The reason I ask is that once I got that fixed my accuracy improved quite a bit.

Would of been nice if Kaw Valley had the same attitude. Nothing money couldn't fix. Outside of that I really like their upper and that linear comp is the cats meow. Your barrel is sweet. Very light.

Racking a live round out of the chamber? Done it a few times in my last range session, and they come right out.

My barrel is a Kaw Valley 4.5", and as I understood it, was manufactured by Faxon - is that correct? And would that issue have been fixed on Kaw barrels? I ordered it about 10-12 months ago.

Lastly, is there any concern with them no stabilizing heavier bullets? I'm shooting Fiocchi 158gr. Twist rate is 1:10, which is the same as my other known-good barrels.

I'd double check on ejecting live rounds and check for marks on the bullets from the lands

I believe the 158's are pretty long so if the chamber is a little short those would be the bullets you'd see an issue with first