Cow's Knee and change your priming powder about every20-30 minutes or so. Priming powder is not coated to prevent moisture absorbtion and thus does aborb moisture quite rapidly. If your lock does not get when, you can have a significant hang-fire or mis fire from moisture contaminated powder in the pan.

If it's really raining, I "cloak" the lock with my jacket and change the priming powder under my make shift "tent".

This Leatherman Cows Knee is really nice. Just oil it with beez wax or mink oil to make it soft and waterproof.
http://www.blackpowderbags.com/cgi-bin/miva?Merchant2/merchant.mv+Screen=PROD&Store_Code=TLM&Pro duct_Code=LC-900&Category_Code=AC

Tahquamenon.