Bill,
I would suggest two things that helped with patterns in my 1938 Husqvarna M15. First, use only one 1/2 inch filler wad, second, use 1,0 oz of shot. I would also second the suggestion to use FFFg burn rate powder. I always used the lubed 1/2 inch filler wads sold by Ox-Yoke.
I found that if I used too little shot (7/8 oz) or too much wad, that I got patterns with weak centers, as if the wad was bumping into the shot just after it left the barrel.
In all the old doubles I have shot, ones that still had the original chambers, forcing cones, bores, etc., a load of 1,0 oz of shot and the same volume of FFFg burn rate powder (Pyrodex-P, Goex FFFg), separated by a nitro card wad and a 1/2 inch filler wad, patterned quite nicely, usually to the marked chokes. In my M15 these loads patterned good enough to do 25/25 at 16-18 yard trap. This is basically a 2,5 dram load.
Niklas
