While camping around or boating, I used a tinfoil funnel that layed out in the cleaning kit very well. Boiling water poured from my Thermos or campfire to dissolve the salts from the corrosive priming, let the bore dry (its pretty quick), dip my brush into my gas tank for a good solvent scrub and then a bit of lube oil to keep 'er in shape, out in the Ocean, or from condensation in the deep cold. A Mosin tool kit with patches and a brush, .32 cal and threaded to the Finn M-39 rod, or a boresnake to boot or a German pull through chain, came and went over the years, and the cleaning rod with brush has kept the chambers 'sticky' free, as well as the rod itself poke out the snow that finds its way into the barrel while riding the snowgo, or a boat.
Syenthetic snowmachine lube for 2 strokes dosent thickenin -50, it has "TCW-3" and you usually have a quart or two of lube when you ride a snowgo, well, as a lube at very cold temps, that works on a gun.
At home I had a dedicated funnel and cleaning kit from hell, Hoppes #9 and Rem oil. Marvels Mystrie oils on the bore have worked out well, to prevent rust in storage.
I also use high temp grease on the rifles action and barrel, beneath the wood, and the water that did managed to get spilled didnt rust the barrel, nor did salt air or spray or condensation.