I have found the .22 ones useless except for rats and wasps (don't ask).
The .38 variation, as said above, works well at 10', max. I find it extremely uncomfortable to be within 10 feet of a big poisonous snake and wont shoot at that distance except in a dire emergency. So .38 ones are also kind of useless for me.
.44 shells work very
well. Sadly, I don't have a .44 revolver right now.
So I am all for a .22 solid to the head at 15, or better yet, 18 feet. There are several "brands" of nasty snakes that will "align" themselves with a long barrel, greatly aiding aiming!
And yes, we're not supposed to kill snakes, no matter how poisonous, yadda yadda. Sorry, I have three children and I don't want them (the poisonous snakes) close to my house.