Hot water and soap solution-at least in a rifle range scenario. What works best is a tight fitting patch with the muzzle immersed in water. Then pump the rod back and forth from the breech end with the patch acting as a piston.
In Brit/Cdn service there was a special funnel issued which could be inserted in the chamber to allow a hot water rinse down the bore.
In WW2 the US Army issued small cans of a foul smelling water based concoction which could be carried in a troopies ammo pouch.