Living in North Australia's monsoon tropics, and hunting with vintage Brit double rifles almost exclusively, one quickly adopts a pragmatic attitude towards torrential downpours.

If the guns have been hunted in the outposts of Empire in the past, they have already seen much worse. In full agreement with 400NE above, just give them a chance to dry thoroughly after the hunt and all will be well. Boxlocks in particular are just about weather-proof and will rarely get water inside the action.

However...
I must confess to a little precautionary work when I first take delivery of a DR. The action and locks are usually completely stripped and cleaned, and any parts which show the slightest hint of roughness, tool-marks, or staining are polished to a very high finish. All parts are then smeared with my secret recipe lube (not so secret, actually... 50:50 mix of Alcom No.2 waterproof grease and high-grade german light machine oil), and reassembled. After this treatment, I can be completely confident that even long-term full submersion would not result in any damage to the metal.

The wood will need regular waxing or oiling of course, and a stock-refinish every few years if you want the old girl to look her best,

An ounce of prevention...