The choke in your pics is a light Mod and it looks like it might be a Briley. Another good choke supplier is Carlson which is a bit cheaper than Briley. I don't shoot three gun but I do a lot of clay target shooting and I think you should find a range that has a pattern board and find out where the gun shoots relative to where you are looking. Then the stock adjusting shims come into play. If the shims won't get the gun to shoot where you are looking a good stock fitter might be your next move. It doesn't matter much how well the gun functions if it doesn't shoot where you are looking.
Charlie