Al Miller wrote an article for Handloader Magazine several years ago, specifically about snake shot loads. His conclusion, which fits with my experiences, was that the 22 LR CCI shot loads performed better than anything else, due to using #12 shot, rather than the #9 shot used in every other caliber. Snakes don't need large shot & the smaller shot made better patterns (smaller holes in the patterns). He also found that shot patterns were tighter out of shorter barrels.
I have personally noted that rattlers generally were DRT with 22 loads, but still move a bit after being hit with 38, 44, or 45 shot loads.
Mark