I was fortunate enough to pick up a 395 mm .25 for my dreamline lite, from Utah air a couple years ago, much hander than a 600 mm barrel. I don’t think they offer the 395 mm anymore. I haven’t seen it offered since I bought it.
I hope they work as well for you as they did for me, the accuracy I got with them was one of the reasons I went with the regulator and bottle, I wanted ALOT more shots with that kind of accuracy!
Did you try the H&N Field Target Trophy in 5.53 - 5.54. The 5.53 made all the difference with my p rod ( same set up as yours done by Airgun Revisions )
Utah airguns has carbon fiber tubing for the FX barrels, if you don’t mind dealing with them again. There are other vendors who have them also, but I got mine at Utah air.
My Prod bottle conversion from Airgun Revisions ( thanks Rich). I think my Prod is the new picture on his website! (one of the first ones completed with the new bottles, and the bolt being on the left side) Will be reinstalling the stock and the scope, and shooting her over the crony tomorrow...