If an animal takes a deep penetrating brain shot, does it immediately die
and fall over limp and not move or, can it's muscles start to spasm after death?

It can go either way,most times their will be some movment but it doesn't last long.
gary

Probably over half the squirrels I have taken with head shots do this commonly known stretching thing, where they stretch their legs for a few secs and then no movement. They 're really dead, it's just a nervous system reaction because the nerves and muscles are not getting any instructions from the brain on what to do. I have also experienced this with groundhogs. When shot in the head they stretch their back legs causing them to jump forward on pellet impact. At first sometimes I thought I missed or I hit them somewhere else and they leaped forward to run away, but immediately after, you know the pellet hit its intended target. But it could definitely go either way where you have no movement at all after the impact from my experience with rabbits, squirrels and groundhogs (birds too). Overall I 'd say 2/3 of the time you will get some movement after a successful head shot but it doesn't last long.

That oftens happen with squirrels,birds not so much.Then again Ive taken down a fox with a headshot (177 cal) 1100fps. And it didn't move a inch.

While deer hunting one day, my friend shot a very large doe in or near the heart. We could not field dress the animal as it was a special hunt and the biologist wanted it field dressed at the station. Well we carried that deer for about a mile. We decided to sit down and take a rest. Five minutes into the rest the deer pickup up her head and then jumped up to run away. She only made it about 30 yards before collapsing again. But it was really weird.

When I was out hunting we did the same thing. It appears dead but then jumps up and runs away then falls again.
Could It be nerves? Or just not a take down shot??

The animal was not fatally shot so it's still alive but unconcious, like most things or anyone would be, after being shot in the vitals by a high powered rifle. Sometimes the deer will recover or wake up from the initial shock. Huge amounts of adrenaline then get secreted from the sight of predators (humans) so close to it, that it gives its heart added energy, temporarily enabling it to get up and run.

When I used to go big game hunting, a buddy of mine always brought a stethoscope with him! The same thing had happened to him when he got his first deer and scared the crap out of him. It wasn't a whole hour like Dave, but about 10min. later the buck got up and tried to run away while he was posing for a picture next to it. Well he happened to get another deer one time him and I went out (in '89), but he was afraid to get up close with the stethoscope and listen to its chest. I was cracking up. He thought the deer might jump up again and injur him. He got scratched a little in the arm the last time when the buck got up because he was holding its antlers posing for a shot with 1 hand and the rifle in the other. He got knocked back to the ground and also dropped his rifle when the deer got up. The stethoscope's tube is only about 16" long and he didn't want to risk it again, so he kicked the animal for like 5min. then waited another 1/2hr before he started field dressing it. It was hilarious. He had deergetupophobia ever since