Stan, I wonder if a lighter, or heavier load will both keep convergence at the same speed? Is it the speed or weight of payload that causes the issue, or a combination? It might be fine with a lighter load, going fast, or a heavier load, going slower. You might have discovered slow and heavy is the same as lighter and fast. More likely they used 1/2 ounce loads to set convergence than the harder to find 3/4 ounce loads. If you would just learn how to pump one of my 42 pigeon grades convergence would not be a problem. But with doubles you do have it working, your sweet spot, stay there. Your seems to be about 1200 fps.

Yes, we both enjoy using a .410 for Dove. It is also my default Sporting Clays gun which gives me a lot more shooting chances than just Dove. For me I don’t think an extra 1/16 ounce shot will gain me more than 3-5 yards, if that. But I also am trying to stretch it out to begin with. You know I am happy using a full choked .410, out to a measured 30-33 yards for Dove. Sporting Clays you can take longer shots because there is no worry about not cleanly killing the bird. All hits, that make a chip or better are scored the same.

Knowing your exact distance is key. I’ve seen a lot of shooters, who pride themselves with their 50-60 yard shots, which measure 40 yards at most. Worse are those who measure to where the Dove is recovered, not where it was shot. The real math gets bigger the longer the shot. If I can keep leads in the 5-6’ range I can do well, stretch them out just a couple more feet and I don’t. The .410 patterns just whimpers out for hunting.

I agree this is an interesting problem. Guess first step is fire forming a bunch of shells. Be aware that if only the upper part is annealed the lower half will not fire form well. So you might end up with a bit of internal taper which will create a wad sealing issue. Might be inconsequential as the wad sealing in the taper should continue to seal as the shell and bore gets larger. It might even make all wads usable but I would stick to the larger diameter wads. You testing will tell you in that if the wads seals well, there will be little variance in speed and pressure because you have no blow-by. If you do get blow-by, a card over the powder may be needed under the wad.