"Even a 12-gauge shotgun is unreliable on large rattlers. I was dove hunting near Albuquerque many years ago, and we encountered a diamondback, later measured out to 6 feet plus. I backed away a few yards and let him have it in the head several times with a 12-gauge 870, using dove loads (7 1/2 shot). The darn thing just writhed a lot."

RIO brand made a run of 12ga & 410s with # 12 shot - appears to work good. The 12ga has 2000+ pellets so you get plenty of chances!!