Been playing with .177 cal slugs for few months now .... crazy accurate and flat shooting. ( have .177 .22 .25 & .30 cal slugs and guns that can shoot em !! )
Easiest slug caliber I've found is .22
Now as too the .30 .... have some NSA 43's that shoot only @ 20 fps slower than 44.75 JSB's and while they have better BC and don't drop as much, there still a short and fat projectile and really IMO no where as good a long range slug as a longer and lessor frontal area .25 or .22 and no where near as slippery as a .177
Retained energy nearly always goes to heavier weights no matter caliber .. so pending what your trying to do, game size etc .... Sometimes less caliber is more.
It's just fun playing with differing calibers just like it is with powder .... Same applies, sometimes you need more gun, sometimes less is good enough.