Black powder, boiling water and then Ed's Red.
Shotguns get the bore wiped down after every session. For duck hunting I always pack WD-40, particularly if I am hunting salt water. If I use the WD-40, I clean the shotgun with brake cleaner and re-oil with Break Free CLP. I have a big jug of the old stinky stuff from before they changed the formula. Any shotgun that gets wet gets a complete strip down and dry before cleaning and reassembly.
Rifles hunt with a small party balloon over the muzzle and likewise get a full strip down if they get wet. I like to shoot from a clean bore, so they get bore snaked, or bronze brushed and patched after every session.
My handguns are stainless, or stainless and plastic, so get cleaned whenever the spirit moves me, which is probably more often than it needs to.
Even guns that are rarely fired don't get to dry out. If I haven't shot a gun in six months, I wipe it down and lightly oil the action. I have guns well over 100 years old without a spec of rust on them.
3-in-1 has anti-rust stickups, and I keep a couple of them in the gun safe, and a 5 lb. bag of activated silica gel in the bottom of the safe. You can reactivate silica gel by putting it in a 225 degree oven for 4 hours to drive the water out.