This is a very interesting subject to me. We have rattlers in places, nothing the size of those though! I'd want buckshot lol!
My old hunting buddies favorite october pastime was rattler hunting. He took me on a day that was too windy for calling and we got into a few. He had them mostly thinned out from several years of working that spot. His tools of choice were a machete and a five gallon bucket. I should post a video, he'd hold the bucket close and get them to strike it then whack them before they coiled back up. Always cut the head off and made a hole in the ground with the end of the machete and stuffed the head in that hole, then picked up the body and put it in the bucket for disposal so he didn't have dead snakes all over to notice as he kept hunting. He said the head could still bite for awhile after being dead.
Now on to the shotshells. The cci 45's also function my 3" kimber, as do my homemade copies. I used 308 brass cut the correct length, resize with a 45 die then a 40 cal die to make the neck then use cardboard wads to separate the powder and shot and a plastic over shot wad and crimp with a 38 die somehow iirc. It's been awhile but it was a fun project. At the end of the day with all that work the cci's are probably worth what they get for them. Only shot a couple snakes with them but they did the trick. More fun was aerial shooting with it, set an empty water bottle on top of a full bottle. Load your mag with a hp on top of a shotshell, shoot the bottom bottle to launch the empty bottle then shoot that one in the air.