I suspect the #9 shot is the problem, not the .410. I use #4 shot for most pest control, and killed a snake that was messing with my dog at ~30 feet, a shot which resulted in complete separation of head from body (snake, not dog, who was behind me at the moment).
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Didn’t I see a video showing a snake head can remain viable and dangerous for hours?
If the head is still intact it can move and attempt to bite; not sure if it can actually secrete venom. Body moves for quite a while though. I literally JUST shot a copperhead outside on the side of the springhouse; 20 gauge over/under with #4 shot. I saw a big one this weekend, and decided to check hot spots, and sure enough, hah! Headshot, and there wasn’t enough left to bite. The body was moving along quite nicely though, even after ten minutes so I nailed it to a post.