I use a slavia 630, with the old canada spring, it wasn't always a clean kill.
With the help of a member here, I got a new spring and seals, and I'm estimating velocity is over 500fps, maybe near 700fps
It kills clean with head shots(red squirrel pests)
Now the important part. What pellets? Unless you got a pellet gun over 1000fps, it makes no difference on shape. They will not expand. Penetration will be the same.
So get whatever is accurate in your airgun.
Red squirrels are cute rats. They've done hundreds of dollars worth of damage to our property, along with minor damage that require a large amount of time for clean up.
Disgusting.