All you really need is water and a buit of rag, spit too and polish. You can clean and oil a Mosin, or any other rifle while walking, wrap it for protection, carry and hold it all the doo daa day, or clean and oil it and leave it ona rack.....
The obvious is that every man , regardless who or where needed and carried water.
The water dissolves the salts and the scrubbing takes off the crap, a light oil, of any type, to cover the clean bare steel.
I agree whole heartedly with Ash, U.S. infantry up untill Viet Nam used corrosive ammo too. It was the M1carbine's gas piston and vent that was rusting out bacause it was closed, so the US developed a non corrosive priming.
Most Junky bored rifle I have come across were imported here, most likely in good/issuable condition and some idiot bought one and a box of surplus.....and just didnt clean it again after shooting.....
From the US's inception with Black powder till the 60's, the US was as busy washing and oiling their rifles like all other rifle owners.
Back then, a rifle was a Mans tool and a symbol as well as a weapon.
They knew what would happen in they rusted, which at least, it wouldnt work or work long........ at worst the Commisar would make you kneel in front of the assembled Company and put one from his Nagant pistol to your neck for neglecting your soldierly duty and destroying military equipment of the USSR.......a well known motivator during WW!!.