Is it really wise to do so? No, I don't think so.
As Darkroommike mentions, black powder contains potassium nitrate, which is hygroscopic.
It also contains sulphur, which when in contact with water forms either sulphuric or sulphurous (can't remember which) acid.
The moisture is bad enough, but the acidic component is just insidious as well.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.