When something very hot contacts something very cold [water] it tends to crack or shatter.I wonder if perhaps a frangible round could break up?
PhilB, I think what you are witnessing is a very lightly constructed bullet intended for Hornet velocities coming apart due to centrifugal force. Driving that fragile bullet at 22-250 velocity and spin rates almost guarantees the bullet will come apart in flight. That's why those bullets are designated for the Hornet. I've seen it happen with hot-loaded light HPs shot from a fast twist .223. Rain has nothing to do with that!
Relatively speaking, the more popular .204 bullets (32-40 gr) are light and fast but not so fragile as to break up on a rain drop. I agree with Indy_Guy_77. If it was that fragile, it probably wouldn't live through the launch and certainly would break up upon hitting an animal!
Any bullet that will "disintegrate" in the rain wouldn't hold up to initial firing. Might not even hold up to loading/crimping in the case.
And any bullet will be affected by rain drops hitting it; just like they are when wind hits them.
The real question is will any rain drops hit a bullet?
-J-