There continues to be a myth that Superior Standard barrels are only for pellets and you need a Superior Heavy barrel to shoot slugs. From my personal experience, the Standard barrel generally shoots light to mid-weight slugs as good or better than the Heavy barrel in both .22 and .30. Also, the South African gun tubers (who developed the Patriot Javelin slugs) use the Standard Superior liner with 21gr, 23gr, and 26gr .22 slugs. Another great example of "just needing to try and see" was a recent video where Gerhard was Shooting new H&N .30cal 50gr slugs through Justin Welch's 1:40 pellet liner! That sort of thing no one expects to work.
I think one of the biggest advantages of .22 slugs right now is the selection, and the amount of testing that has been done in that caliber. You have many makes, many weights, and most manufacturers offer at least two diameters in each one (NSA offers 6 diameters). If you compare that to .25 and .30, the weight offerings by manufacturers are much fewer, and usually only a single diameter is offered. This makes it harder to find a good match for your gun. If you found a 1 MOA slug for your .25 barrel, consider yourself lucky!
I like 20-22gr .22 slugs in a 600mm barrel. I have my 600mm M3 tuned for NSA 20.2gr at 965fps (requires reg of 120bar), and it gets 100-112 shots off the 480cc bottle. The crown will be slightly less, but should still be pretty good. This slug tune offers a nice balance of pellet-like shot count with slug trajectory and wind benefits. Keep in mind you aren't really going to 'gain velocity' by going from .25 to .22. If comparing the same weight projectile, the .22 will have to work a lot harder (higher reg pressure, stronger hammer spring) to achieve the same velocity. This is due to the smaller surface area that the air has to push on. I think on a crown you are going to be limited to around 26gr .22 slugs before the gun gets maxed out.
