well actually for airguns best caliber is in .22 range more likely 257 or 223 but only Airforce airguns offer .257 barrel as a caliber and this ammo caliber is not so common but the idea is to have thin long and heavy slug. Thin for better aerodynamic/ BC , Long to carry heavier load in the thinner profile and heavy to be stable in wind at long range and to have a harder hit if you want to puncture targets or hunting but because cannot maintain high speed at distance twist rate need to be fast like 1:8 or 1:9 in order to continue spin the projectile even when speed is low over 400 yards to a mile and proof is the old world record with Altaros m24 pcp Airgun shooting .223 with 1:9 twist rate i have a friend who mounted firearm 1:8 twist rate .223 and makes his own boat tail pointy slugs at 4.5 grams/ 71 grain and successfully shoot at 1200 meters steel gong on multiple accessions even got some videos in FB groups. I guess those are not in mass production intentionally as well as pointy slugs. Otherwise for rest of us regulars shop buying users i would prefer .30 because of heavier weight slugs over 50 grain yes it is not so accurate as .25 and do not fly so far as .25 but currently only 60 grains .25 slugs I know on sale are Altarus slugs and they are hard to get maybe will switch back to .25 in future if more heavy and pointy boat tail slugs enter the market for now just hollo points all over.