These are the calibers I'm aware of
.177
.20
.22
.25
.30
.357
.40 (one of the newest/rarest)
.45
.50
I have a spring powered .25 that shoots a 25gr pellet at 650fps. This thing has serious power and takes squirrel and birds with authority. Headshots on squirrels, clean through, messy instant death. Heart/lungshots on squirrel are also usually clean through and cause expiration in between 5 seconds and 30 seconds.
It is a handy rifle and I am happy with it for what I paid. It has about 18-22ft/lbs of energy and I am confident it could easily take a human or dog if shot in the brain. It is said it takes 13ft lbs of energy to penetrate a human skull (dependant on sectional density and shape of impact point etc..
I need to stock up on more pellets. I would love to have a nice German rws .25 springer but they are too expensive for my blood. My rifle is Turkish made and was about $150. I use iron sights and with practice can bust a 16oz water bottle every shot at about 35yds.