"Would leaving it loaded for long periods of time cause corrosion in the breech where the powder sits? Seems like it would make it rough, or worse, rusty."
You can leave black powder loaded in a gun for years with no ill effect. I have left a cap and ball pistol loaded for two years, and all five cylinders fired fine, and there was no corrosion in the cylinders. As to bp subs, I don't know.
I have left a TC Hawken loaded for a month with bp, during deer season, and it fired fine. No corrosion in the bore. I just remove the percussion cap when I am done hunting, before I put the gun in the truck. Let the hammer down, the nipple is sealed.
I left a Savage loaded for over a year, and it fired fine after that year. No corrosion in the bore. Of course, that was smokeless powder.
I don't see a problem with leaving a gun loaded for a while, unless you have been hunting in the rain. Moisture laden air cannot get past the ball or sabot. If you put the hammer down, or close the bolt, no air can come in from the breech. I can't see what harm it would do to the powder, or to the gun.