As for gyroscopic stabilization and pellets, most barrels are too fast for pellets. We can get away with it due to the hi drag. Over stabilized pellet flight get mitigated by the drag. So nose up attitude tends to get replaced by corkscrew flight path. This is exacerbated by poor center of form or deformation. The real intent of spin stabilization for pellet came from a need to mitigate muzzle blast effects on the pellet. The pellet is basically a lead shuttlecock. If you've ever played bad miten on a windy day, recall what happens in a hard tail wind. Muzzle blast is a hard tail wind, effectively. Spin helps mitigate that. Slug, different story. I thing a .177 thru .25 would be better suited with a twist rat of 1:19 - 1:22 with max velocity of Mach .75 or under but no mor than Mach .85. Slugs on the other hand, 1:18 probably a bit slow.
I make quite a few TJ's barrel for folks. I sure you all have heard enough about them over the years. They're used by just about any air gunner that has endeavored to rebarrel an air gun. I prefer them over LW because I've found them to be more consistant from one blank to the next. Not downing them just sharing my experience. Most recently, I did a barrel for a gentleman down in South America. It fit a Impact M3 in .30 cal. It is a .294/.300, 1:26. Chocked. He reported shooting .5" groups at 100 with a 44.7gr at 930. I can recall if it was pellet or JSB slug, I think they weigh really close to the same IIRC. It is carbon fiber sleeved. Epoxy not tensioned. Last year, did another for Preist II in .30. He claimed he was getting 1.125" at 190yrds(175m). That guy is a little gregarious and braggadosious so, there's that. I take them at their word. Many other .25s and .22s.
My methods are a bit different than most. A lot of time and effort goes into the setup. I tend to run things with a tighter tolerance than most manufacturers, with regard to throat and lead length. I focus very intently on parallelism, on 2 planes of reference, and concentricity to the bore. Same with the crown and muzzle threads if those are desired. Everything is true to the bore. I've had DIYers come to me with a barrel that wouldn't perform. I have them set it upon my machine to show me how they went about it. Many will simply chuck one in a 3 jaw and go to work. Crow isn't concentric nor perpendicular. I can't even get into the issues with the other end. I could geek on this for quite a while. Last thing. Someone mentioned going through dozens of barrels to find the perfect one. That probably being nice. Perfect is a tall order to fill in a field that can be very subjective. F class shooters who build their own guns often shoot an entire lifetime and find 3 or 4 barrels that they wish would never wear out. That's after nearly a hundred barrels or more.
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