I've probably killed 1100-1200 squirrels myself, most of them with my RWS45M air rifle.
My pellet of choice was either the English made Sabo sub caliber
(it's a little copper 12cal boat tail in a sabot) or the Beeman Beeman Silver Arrow pellets.
Expansion is not what you need to reliably kill squirrels with an air rifle, you want and NEED penetration, the beeman pellets would penetrae a squirrel diagonally (and shatter major bone groups in passing) out to about 50yards.
But would kill them dead well beyond that.
the problem is getting the pellet through their flexible hide.
I've killed some with "blunt" pellets that only cause massive internal bruising
I had an unfair advantage, I knew EXACTLY where the squirrels would be as they liked running along utility lines and pausing on top of the utility poles...
Jumping to specific tree branches and they'd habitually "pause" at
the same point... If you KNEW they were going to paust you could
have the pellet on the way before they got there...
In a relatively narrow field of fire in my back yard.
I knew the exact range to the fifty-ish spots a squirrel would be
to within an inch or so...
Always using the same rifle and the same pellets from the same firing point
I could hit them even when they didn't pause.
The principle is similar to what artillerymen call "pre registered coordinates"
If you've already dialed in your weapon to hit a specific spot on call all you
gotta do is wait for your enemy to blunder into your kill zone then let them
have it.
I was in essence "shooting over bait", my "shooting gallery"
had several varieties or fruit and nut tree that the squirrels "raided"
for a food supply. (No, squirrels don't eat apples or pears but they
will chew a hole in the side to get at the seeds.)
they were also fond of mulberries.
It was 81yards to the mulberry tree from my kitchen door.
if you know the range and know your trajectory...
I lived there for decades, so anything bigger than a golfball within
that so-very-well-known field of fire was dead if I decided to shoot it...
Now a days I leave food out for the squirrels, no not bid seed, but food
they'll eat that I won't... cornmeal with bugs in it or oatmeal that's been
around a little too long... Feeding it to the squirrels is better than sending
it to the rats in the local landfill.
And the squirrels should be grateful I'm not hungry...
AD