“I found that Speer has released a 38/357 birdshot round. It has a 5 - 10 yd range.”
You will be hard pressed to kill a snake much past 5 feet with those rounds. 15 to 30 feet is wishful thinking.
I meant 5 10 feet not yards. In a 6” revolver it makes an 8” tight group of 20 #4 pellets that can bulge 1/2 plywood on the backside. You can’t always take a snakecharmer which is really illegal anyway and only 1 shot.
I have a nice winchester 410 backpacker, great for rattle snakes up to 150 foot with 4 buck..great for the wife, makes adequate hole.
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Always give the snake time to line his eye with the bore of the gun before pulling the trigger.
Snakes are always curious about gun bores.