A quick update. I made the muzzle setting device pictured below and after a total of 4 resolderings I'm happy with the regulation.

In addition I tried few other things including Darryl's muzzle shape (as pictured above). Thicker patches again, different dry lube system.

I also made hand reamers with pilots that fit into the bores with half a thou of clearance to square muzzles to the bores (90 degree reamers) and to cut new 45 degree crowns. Here is one 45 degree reamer.

So after all this I have the following results. If I load both barrels with the current best load of 80 grains of 3F with .575 round ball, a 230gram per sq meter cotton fabric patch (it squeezes to slightly under 10 thou - unsqueezed it measures 18 thou if I remember correctly). I can be pretty sure of getting holes touching at 50m for first two shots. At 100 m my first two shots are an inch and a half apart. When I continue shooting the group grows a bit to 3 inches at 50m,but as I'm shooting standing resting the back of my hand on shooting sticks then the gun on that I suspect it is me l. The gun seems to be perfectly capable of putting all shots into one big hole at 50m. It is shooting very consistently with as mentioned before first shots (when I'm well rested) always going to the same poi.

This is a typical 8 shot group(both barrels - 4 each) at 50m.

Compared with my single barrel and smaller caliber guns that is nothing to write home about, but I'm pretty happy with the consistence of the results and I read in "Shooting the british double rifle" by Graeme Wright 3 inches at 50m fot a round ball shooting double is average performance. So that's fine.

I set the regulation for 80 grains (the most accurate load) to shoot slightly converging to cross at around 75m. It is possible to load 110 grains to shoot parallel with maybe 20% less accuracy than the best load. I also tried a huge 640 grain Pedersoli 580 minie bullet. It goes wide even with 80 grains of 3f. 80 grains of 2f sends them almost parallel, but the fps is pretty low at around 1000. So I decided to stay with patched round balls.

Everything is soldered back together now including ramrod holders etc. I just have to put some solder in the front of the muzzle to make it neat and rust blue it. However, I'm going to leave it be for a while to make sure I don't come up with any other ideas to modify it before rebluing. I'll post reblued pictures when it is done.