A few things from the John Garand Match at Camp Perry.

[Kirk]

Anyone tried putting Rain X or soap on the top of the bolt to break the surface tension?

[John Kepler]

The Parkerizing/grease/oil surface of those parts already provides a supremely hydrophobic surface, and a lot of the water DOES roll off (into the interior of the rifle....but that's another issue!), but a lot isn't all, and something like Rain-X....assuming you could get to actually work on the rifle (highly doubtful without COMPLETE de-greasing of the metal....the stuff has to have a CLEAN surface to stick), surfactants (Rain-X isn't one BTW!) will allow the metal to hold MORE water, not less. At the end of the day, I doubt it would work any better than what's already there!