Generic Windex. Not the expensive stuff, the cheap knockoffs. I pour an inch or so into a plastic tub (like a butter tub), heat for a bit in the microwave, pull the bolt from the Mosin, muzzle down in the solution and drag a patch up and down, drawing the solution through the barrel. Pour a bit down the breach, use the patch to wide down, then patches to dry and light oil.
Or pony up for non-corrosive ammo.