I’d recommend .177 cal, because the projectile is more accurate with increased fps.
I have two that are break barrel. Break the barrel, insert pellet, close barrel, click safety, rdy to fire.
As far as scopes don’t know, but make sure it’s an air gun scope, because they have considerable spring recoil that goes in the opposite direction of normal recoil.
Another thing that works well for squirrels are the shotgun type pellets that shoot about 5 projectiles from a smooth bore. Either way though, having had to shoot a couple pesky ones myself, they don’t die right away with any caliber or shot type. They are fiesty, and they go dying crying. Was a bit troubling.
Yup.
.177
I have a Crossman pump and that puppy shoots straight as an arrow. And near total silence, just a sort of whooshing sound like the air coming out of a balloon.