Don't be too hard on yourself. Siscakidd. I can't find it now, but if you find a squirrel anatomy image that shows where the brain is, it goes up to the eyes, maybe a little further. We refer to them as zombie squirrels here; the ones that should have been dead but weren't.
I think you DID hit brain with that shot, but you hit the "newer" part of the brain. (speaking in an evolutionary sense) This part of the brain is teachable and is responsible for critical thinking, but not necessarily life support. I've hit squirrels like that before, and also one or two where the pellet cut a groove across the top of the skull. In those kinds of cases, they go down, eyes open, kind of twitching and bleeding. internal bleeding creating pressure on the brain, that kind of thing.
This is why I stopped going exclusively for head shots on nutters. You've got to get the right part of the head and be confident you can get it there. I figure on maybe a 3/4" kill zone. From the side, go for the ear hole, that's about the center of the main part of the brain. Or as Beachgunner said, even a little lower is OK, as long as it's behind the jaw. From the back, just about anywhere will do. From the front, if you have a top-down shot into the brain, that's good. From the front, I usually don't take them unless I know I have PLENTY of power to spare. I prefer a heart/lung shot instead, in those cases. Heart/lung are 100% fatal and your kill zone is at least doubled. It's just that they can run a ways when lung shot, so the recovery ratio is not as good. (esp. if you're pesting them out of your back yard, and can't go tracking them all over the place)
With expanding pellets, you have to know you've got enough power to penetrate through one side of the animal at least. If you're shooting domes into the head and they're not going through, I don't think I'd go to expanding pellets yet. Usually domes will sail right through the head. You've got to have enough power to drive them deep enough first. That has to happen without losing accuracy too. With brain shots, expanding ammo is not really a big help. It's more of a help with body shots; tears them up inside for faster internal bleeding. But you're not really depending on internal bleeding for brain shots, just brain damage.
How many FPE is that gun? What caliber?
Beachgunner is also right on about power. They say that only 3-6 FPE is needed to kill a nutter, but that's with perfect shot placement. Out in the real world, More power gives more room for error. I find that 20 FPE in .22 or 15 FPE in .177 gets the job done reliably. It's enough power to expand hollow points, enough power to send most shots right through the boiler room, and enough power to cause collateral damage where less power might not do it. I hit one yesterday morning (#1) Take a look at that one. It went in just behind the jaw pivot and rather lower than I would normally prefer. But... it was a .22 and moving out at 20 FPE. That is a big impact for a little squirrel noggin. If that shot were made with my HW30s (6 FPE, .177) it may not have been fatal.
