22cal - 25cal squirrel hunting question

There's another issue or two.

Internal ballistics are a well-known science, as are external ballistics. The real iffy part is impact ballistics. While studies have been wide-spread, no one can realistically determine what a projectile will hit. Oh sure, a head shot should be fatal if there is enough energy expended to penetrate deep enough to do the job. But think about this.

We've all shot prey inside even 10 yards, and gotten a solid hit, yet have the prey fly or run off. And most have gotten lucky hitting prey at over 100 yards, and have them drop in their tracks. As alluded to above, we won't know why unless we do a necropsy of the prey, And then we might not know! So as NM Shooter said, the question is moot!
 
Any caliber will work with a head shot, the issue I have run into is the body hits. That's where bigger has been better for me anyway. I own .177 .22 and .25 guns, all have killed more critters than cancer. That said I do see more performance out of the .25 especially shooting past 75yards. I won't be going to .30 cal because everything I hunt is easily killable with the .25 and the lack of barrel jump is nice too. If I can have a chance to use a range finder on the game before I shoot it's dead. But like I started this reply with, it's all about placement. You can carry a 44 Mag snubby for protection but if you can't hit a barn door with it what good is it!
 
I've been wanting to use my Hatsan Striker 1000x .22 for squirrels and small game in general. But I broke the scope stop off because the mount keeps walking backwards. So that one'll have to wait. I'll use my Winchester 1400cs .177 magnum instead. 4-12x40AO Hammers scope and H&N Baracuda Hunter 10.34gr give low 977/21.92, high 987/22.37. Should be plenty! 
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I personally try to limit my squirrel shots to 50yds. Beyond that range finding and wind doping becomes very important. For normal field conditions I feel there is too high of a chance to botch the shot and wound a squirrel. Depends on how you feel about not recovering the animal... makes me feel terrible so I don't push it. If a squirrel sat still long enough for me to take out my range finder, get rested, and line up on a shot, I might push it to 70yds with an air rifle that can shoot close to an MOA. I also shoot at the vitals(heart/lung). In my experience it's a much larger kill zone and I have had excellent recovery rates. Almost always within 10yds. I used to go for head shots till I watched a squirrel writhe around for a while, then get up after a 15yd face shot with a .22.
 
"Grandfather"Good stuff guys. Trying to decide on 22 or 25 cal and your personal experience is helpful. Thanks again, Tom
If you are looking to have just one Airgun my vote is for a .22. I have many and reach for my .25 Streamline shooting at 47fpe when I go squirrel hunting. A 26-30fpe .22 is shorter, quieter, lighter, and is plenty to drop squirrels. My point blank range and terminal performance is a little better with the .25, but at the expense of noise, weight, and shot count. Either are great for squirrels. I don't think my harvest numbers would change between the two calibers. I would base my decision on what is available in the platform I'm interested in. For example, the MROD .22 barrels are questionable, so I'd go for the .25 if I wanted an MROD hunter(unless I were planning on an aftermarket barrel). A .22 Daystate Regal is a fine hunting gun, but I wouldn't buy that rifle in .25, due to the small cylinder and poor shot count. An FX Streamline has good performance and shot count in either caliber. The .25 Streamline is pushing my tolerance for length, so I'd go with a .22 if I wanted a quiet moderated gun.