Hey folks,
Like Snapping Twig, I use a gas check on top of the powder, then fill the case with #9 shot, and finish it off by topping the shot with a second gas check slightly crimped. A side view of the placement of the gas checks in the case would look like this: powder(shot-------------shot) with the "(" and the ")" indicating the directional placement of the gas checks in the case.
When first working up a snakeshot load, I would start with a fast powder charge like Bullseye and put about four grains of it in a case that was full length sized - but it did not have the spent primer removed. Then I would seat a gas check right on top of the powder with the open sides of the gas check up. Next I would fill the case with #9 shot, and then I would dump the shot in the scale pan and weigh the load of shot. If the shot weighed 150 grains or whatever, I would look in my loading manuals for a loading for that weight or something a little more than that weight. Then I would check to see what a safe charge weight of my chosen powder would be. If it would turn out that four grains of XYZ powder was a reasonable load for the weight of my shot load plus two gas checks, then I would make shot loads with those weights of powder and shot. If the powder and shot would have to be adjusted, I would use the inertia bullet puller to empty my test shell of the gas check and powder and start again. After one or two tries, you should be able to get a safe combination of powder and shot.
I liked Bullseye powder because it took less space in the case and left more room for shot. At fifteen feet, I could not see the doughnut type of dispersion in the shot that many folks report. My loads seemed to be pretty well dispersed. As far as leading the barrel, I never shot enough rounds to find any leading problem. I doubt if I ever shot more than ten or twelve rounds at one time for testing purposes, and routine cleaning left a clean barrel.
With the plastic cups, you can get a little more shot in them, but I also found they sometimes moved in the case. The twin gas check method works pretty well in .357 and .44 Mag.
Best wishes,
Dave Wile