There are some naysayers screaming caliber doesn’t matter. I’ve shot a lot of squirrels with .177 (12 FPE) head shots and they bounce around kicking & flailing. It takes some patience for them to stop moving around enough to put a hole in the skull. I tried a .177 chest shot and it ran away to die somewhere else out of visual range before I could reload. A couple times, .22 polymag flying out the muzzle around 750fps and impacting a bushy-tailed rodent head 15 meters away popped an eyeball hanging out of the socket but the dang thing would crawl around trying to hide. There have been several super-squirrels I’ve shot in the head and still have deliberate movement, then coninue to survive multiple follow-up shots in the head and chest. I know they weren’t misses because the body part being shot would get knocked, then I would see all the holes after recovering the pest. Most .22 polymag shots to the chest would cause blood oozing out as they cling to the tree for a few seconds and drop without all the flailing. BUT, a .25, a polymag chest shot (typically 15 meters) would drop them immediately with a startling “p0p”. Sometimes it was a gory scene with a lot of blood falling from the tree all at once instead of pouring out like the .22. I forget what velocity my .25’s are, but not very fast because I cranked the hammer tension lower until the shots were quiet with pesting accuracy at close range. Sure, it might be dead the millisecond it caught .177 lead in the brain, but if you don’t want all that flailing around, going with a bigger caliber high in the chest might be the ticket.
The good old saying is there is not replacement for displacement. Of course .25 is better, heck, go straight to .30! LOL
BUT it does depend on pellets too, like you said polymag and hades do a number on critters so you don't really need massive pellet to drop them dead. I've done vital shot with .177 hades and they don't run more than a 10 feet. Even the lowly 7.4 crosman pointed pellet can drop a squirrel right there if the placement is right and I've done it many times, I prefer the challenge of a good head shot vs brut force of .25 which is feels more like extermination than hunting. However I'm shooting urban squirrel which most of the time would sit in my tree and star at me while a pellet is moving towards it's forehead.
Only way to drop them DRT is an accurate shot to the neck and sever the spine but that's a harder shot than the brain shot. However shoot whatever floats your boat, no point arguing about what you want to shoot on the internet. As long as the squirrel is dead then it's dead!
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