One thing I've wondered about when a cannon is set up in the field but is not in immediate use: Is the breech supposed to be left open or closed? I was thinking that if it were open for any length of time, water and other contaminants might get in, but if it were closed, the mechanism might jam if the cannon suddenly had to be put into action.

I pulled the lanyard on one of Randy's M110s at Camp Grayling in the late 80s and can attest he is right on. Everything is mission driven and no matter what the mission is if you ain't firing your cleaning. I only have a week experience with self-propelled artillery but 18 with M114s and M198s. With towed If the mission requires only one gun, the rest of the howitzers could be still hooked up to their prime movers all in convoy mode for rapid displacement.. If the entire battery is laid and are not firing as Randy says, the breeches are open because XOs are usually nervous and like to see a gun's status just by looking at it.. If other duties require you to supply men to battalion you may have a couple guns emplaced with the muzzle covers on, the breech closed and the pantel in its case on the prime mover. Battalion always seems to have some chore requiring gun bunnies.
The pantel is always covered when not in use as Gary (parascope) said.
Its been 20 years since I worked with the guns and this is how I remember. It all is mission oriented and what the battery"s SOP says.
One of my fondest memories of the 182d FA is having some sort of a meeting while we were standing next to a M88 tank retriever. It had rained a lot and the idling M88 caused the earth to vibrate. I looked down and saw the damp earth turn to mud and slowly watched as the mud began to cover my boots. It is amazing how little room there is in such a big vehicle.