We recently received what is probably my favorite customer question ever: "Will your Chisos stop a rattlesnake?"
The bottom line: it depends.
Rattlesnakes can grow to be pretty large, up to eight feet. Their biting pressure can exceed 150 psi (on a direct hit). Couple that with the coil spring they use to project themselves at you and you've got a formidable opponent on your hands.
Your comfortable cowboy boots will not protect you from a bite on the vamp (the top and side of the foot area), but the thick sole will protect the bottom of your feet.
Most snake bites happen in the calf area. If a rattlesnake bites straight on, attacking the shaft of your cowboy boot, it will certainly penetrate the leather. But there are a few other things that could play in your favor: Are you wearing thick jeans? Is there space between the side of the boot and your leg? Was the snake's attempt to bite you indirect, thereby limiting the depth the fangs go through all the material?
Wearing your Chisos boots are certainly going to be better than nothing—and better than most other footwear options—at protecting you against snakebites. They're not snake-proof, however.
So tread lightly my friends.
(Bonus: no matter which boots you wear, keeping your leather cleaned and conditioned will enable them to perform at their best.)