Pellet spiral

Have you tried shooting without the moderator?

I have a rifle with tight chocke and have also many lead dust in the shroud

1 Check your barrel crown maybe it harms the skirt 

2 can you size down your pellets?

IF you haven’t already done so, I would first check the speed to get it in the 880 to 900 FPS range at the muzzle, then problem solve from there (over Chrono, no guessing the speed). I have gen 2 Vulcan in 25 and it shoots JSB 25.4 and AA 24.5 grain pellets very accurately with Donny FL Sumo. FYI, the Vulcan sat idle for 6 months, pulled it out this past weekend .....and still shooting at 900 FPS with 1 STD over 40 shots. I use the Prochrono to keep track of FPS.

My guns do that when the barrel is dirty. I know you stated you cleaned it. But i would do a thorough cleaning and check your fps over a chrony. Good luck. Let us know what happens.

I had a problem with my Royale 500 and Kings at 880 FPS a few years ago. It was more pronounced because I was shooting 250 yards on a 3x4 piece of steel painted black.

Common wisdom at the time was that the Kings did spiral, my Royale 400 at the same time never spiraled, so it made sense.



Roachcreek

I had spiraling with .25 kings too out of my Veteran before also. Chocked it up to bad pellets as polymags were a flippin laser out of the gun