The problem with relying on the one patch wrapped around the ball to adequately grease the entire bore,and grooves, to prevent rust, is one of patch thickness, the amount and kind of patch lube used, and the condition of the bore when that first PRB is run down the barrel.
I prefer to deal with any possible condensation in the barrel by flushing the barrel with isopropyl alcohol. That also dissolves and evaporates and oil/grease that is in the flash channel. I follow dumping out the dirty alcohol with a drying patch or two to remove the remaining alcohol liquids, and to dry the bore.
The thicker the patch, the more lube it can carry. However, when you have a very long barrel, that lube may not be enough to grease the entire length of the bore, thoroughly. Its the skipped parts that are open to rust, then.
Using a second CLEANING PATCH greased with a non-petroleum lubricant, provides a larger Patch size, and MORE GREASE to Thoroughly lube the entire length of the barrel in front of the PRB, to protect the barrel from rust the entire day- and for days and weeks later, if you don't unload the gun after an unsuccessful hunt. That extra grease also FEEDS the smaller patch around the ball, as the ball exits the barrel, so that there is no chance that the patch will burn from having been starved of its grease while loading the gun.
I started MLing with a POS foreign-made gun that had wide grooves and thin, shallow "Lands" that were little more than burrs. The gun required very thin patches, and tight fitting RBs to shoot well. I learned the HARD WAY about how much grease a thin patch could( and cannot) carry. And I learned what happens using FFFg powder in that gun instead of FFg powder, and how important it was to find a load combination that allowed me to use a thicker patch to hold more lube.
I don't have any problem with some shooters, who live in mild weather parts of the country, and can get away with just running a lubed patch around a lead ball and not worry about rust for months when the gun is left loaded.
I am sure that is Exactly your experience. However, in other parts of the country, you would have a ruined barrel in a matter of days doing what you are doing. The added grease to a barrel cannot possibly hurt your gun in any way. It might just save it should you venture out of your usual hunting territory and wander into some other climate conditions.
For those reasons, I will stick to running that extra patch- this one a large, 3" square cotton cleaning patch with grease--- down the bore of my .50 caliber rifle, to protect all 39" of the bore from rusting during a hunt.
I have had that gun on hunts when the temperatures were in the low teens in the dark of the morning, and had risen up to the low 50's by mid afternoon, melting the ice and snow that covered the forest floor when I walked out to my stand before daylight. I have another gun with me when the hunt began with temperatures in the 50s, and dropped down into the low twenties by late afternoon. My rifle has been in the field when a storm blew up and it was rained on before we could make it back to camp. On another hunt, the stock took on so much moisture that I could not pull the Ramrod out of the stock, until the gun dried out overnite in a motel room. All this occurred in hunts in central and Southern Illinois.
In all the hunts were I was smart enough to take the precaution to grease the bore AFTER I had loaded my PRB, I have encountered NO rust in the barrel. I have been using Young Country 101 lube, a predecessor of Wonderlube, as my patch lube for this purpose for more than 20 years now. I think the wax helps to hold onto the oil, and keep it on the surface of the steel bore better, than when only an oil is applied to the bore.
Obviously, from posts above, other's opinions, and experiences vary from my own. I am happy for those shooters. :hatsoff: :hatsoff: