and the other doesn't.
For a couple years my brother used .177 CP heavy pellets for his squirrel hunting because he could drive that pellet 8" long ways through a squirrel at 30 yards so he would take quartering shots from the rear if he could put the pellet through the vitals at the angle he was shooting. For the last few years though he has been using the .177 CP lights for his squirrel hunting because the flatter trajectory made it easier to put the pellet through the vitals past 30 yards. He has to be more selective with shots now due to less penetration but he's found it an advantage to sacrifice penetration for more precision in pellet placement due to a flatter trajectory. For a while he used a .22 cal barrel on his R9 (a .22 R1 barrel that was chopped and choked) and out to 30 yards it worked well, however he had better success with .177 cal past this distance.
Another consideration is the size of the game you're going after. Both my brother and I have taken a few raccoons with our .177 R9s and for this application the deep penetrating CP heavy really outperforms the CP light. Of course the raccons are taken at a closer range and have a larger killzone (head shots only for raccoons and ground hogs)than a squirrel so pellet trajectory is less of an issue.
I posted a reply above that I really don't think the .22 is any more effective on squirrels than .177 as long as the pellet penetrates through the vitals. My experience is that pellet shape has more bearing on effectiveness than caliber (only considering .177 & .22 cal, not the fat .25 cal). For a while both my brother and I used the rather heavy 9.5 grain .177 RWS SuperMag wad cutter pellet for squirrels and they actually worked better on squirrels than domes as long as the vitals were hit. The fly in the ointment is that the accuracy of anything except domes in our R9s fell apart past 30 yards (20 yards for the Crow Mag pellet) so the success rate (percentage of hit to retrieved squirrels)dropped considerably past this range in spite of the more effective configuration.
Anywhoo.......for squirrels, IMHO, it doesn't make a nickels difference what cal or pellet you use as long as the pellet penetrates the vitals.
