That is an underbored barrel, and I would switch to using the .585 diameter balls. I don't think the .595 would give you enough room for a good thick lubed patch. I hesitate to use the .005, and .010" patches in these large bores. I think .012 to .015" patching is about as thin as one should expect to go. I have some pocket drill that compressed to .012". I have .015, .017, and .020 " pillow ticking, that seems to hold true to advertised measurements.
I recommend buying some Over powder cards. The Ox Yoke, 1/8" thick OP wads should work just fine, in the 20 ga. size, for you. ( If you don't already have them, drop me a note and I will send you what I have.) That will seal off the gases. I don't know your barrel length, but its probably longer than my 30 inch barrel on my fowler. I found that my patch did not lubricate the entire length of the barrel, no matter how much wonderlube I put on it. I would get crud dried and caked about 4-6 inches behind the muzzle. If I hit it quickly with a spit damped cleaning patch, it would break free, But it took a bit of scrubbing.
So, instead, I tried a trick learned from some chunk gun shooters, and ran a lubed cleaning patch down the barrel AFTER seating my PRB. My brother tried the same, and ran it over his chronograph. He tried the with and without testing, and found that he got a slight increase in velocity, but a substantial drop in SDV. He also found that the residue that was left in the barrel was easier( meaning faster) to clean after each shot. I found that after 3 shots, my barrel seemed to be easier to load with wads, and PRB, as some of the lube apparently worked its way into the pores of the barrel, and remained even after I had cleaned out the residue with both a spit-dampened patch, and a dry patch. That made loading subsequent balls much easier, but I saw no change in velocity, POI, group size, or in any build up of crud in the back of the barrel.
I think the added mass( weight ) of the thick OP wad helps to increase the chamber pressure and that in turn helps to burn the FFg powder granules more efficiently, and more completely. I don't find much in the way of large chunks of crud in the barrel any more, and the residue is no different than what I was used to getting using FFFg powder. Of course, that .600 ball weighs in at a hefty 325 grains more or less, and that certainly has something to do with raising chamber pressures.
You might also consider using 1/2" thick styrofoam, but you will need a punch in a drill press to cut them out. Being synthetic, the foam will not absorb lube, but it is extremely light weight, easily collapses, and fills the bore, blocking gases . The only down side is that you have to use these wads with some kind of OP wad, or OS card under them, or the powder will light the styrofoam on fire, creating a fire hazard in dry conditions. I have seen trapshooters using the styrofoam wads in their guns to very good effect.
I don't use them, because the materials are not what was available in the 19th century. Fillers, and paper wads were, and were used in combination with the PRB for the same reasons we should use them today.
