Hey Randy,
I make shotshells with both .357 and .44 mag cases. Instead of cardboard wads, I use gas checks in both cases. The process is the same for both calibers, but the two parts to my response may seem backwards in chronology.
Once the cases are sized and the mouths are very slightly belled, they are charged with whatever gunpowder you choose to use. Then a gas check goes in the case with the "skirt" pointing upward. Once started in the case mouth, I use a wood dowel to seat the first gas check all the way down on the powder. A predetermined amount of shot, usually #9, is added and fills the case to the mouth. A second gas check, with the "skirt" down, then goes on top of the shot, and the loaded cases are then finished by running them up in the bullet seating die to remove the belling of the mouth and to put a slight roll top crimp on the mouth. The crimp holds the 2nd gas check in place quite tightly.
Now, how does one determine the amount of shot and powder to use? It is really a trial and error method for both calibers. First you have to determine what powder you are going to use. Since the amount of the powder and the amount of shot are co-dependent upon each other, you might as well choose your powder first. Remember, since the case is completely filled with powder and shot, the more of one you put in mean the less of the other you are able to use. Since I am not looking for any high speed as you might want out of a bullet, I am more interested in delivering the maximum amount of shot possible at a short distance but with enough speed to kill little critters. Generally speaking, the faster the powder you use, the more room you have for shot.
Now to figure out a safe recipe, I go through the following trial and error process. Since I have my .357 load information handy, I will proceed as if I am working up a recipe for .357 Mag. First I would fill a case about 3/4 of the way with #9 shot and weigh that amount of shot plus two gas checks. Lets say the shot and the two gas checks weigh about 130 grains. I look in my loading manuals for .357 Mag loads with bullets at or near 130 grains, and I compare the charge data for Bullseye powder. I see in my old #8 Speer loading manual they list a 158 grain cast bullet with a Bullseye charge between 3.7 and 4.7 grains. Like I said, I really don’t need a hot load, so I put 3.7 grains of Bullseye in the .357 case, put a gas check firmly on the powder with a wood dowel, and then put my 130 grains of shot on top of the powder. I see the shot does not quite fill the case, so I pour out the shot, remove the gas check and powder charge, and then put in 3.8 grains of Bullseye. I then put my gas check and shot back in, and find that it fills the case to the rim just fine. I place the second gas check on top of the shot (skirt down), and then crimp the gas check in place.
This .357 Mag load of 130 grains of shot and gas check with 3.8 grains of Bullseye powder is actually a rather mild loading considering the manual lists a Bullseye load range of 3.7 to 4.7 grains of Bullseye under a 158 grain bullet. It does, however, work very well. I use it in an old Ruger 2 3/4 inch Security Six revolver, and when patterning the shot at 15 feet, I do not find the “doughnut hole” effect so often mentioned when using shot loads in rifled barrels. My patterns have no center hole, and no snake or rabbit would find room to get between the shot.
The practice of working out a load as I described above is somewhat misleading. When starting from scratch, it may take a half dozen or more tries to get the right combination of powder and shot/gas checks. It takes a bit of time, but it is not difficult to accomplish. The .44 Mag. holds a lot more powder and shot and will, of course, deliver a lot more shot to the target. I used the same procedure as described above to come up with a recipe for my 7.5 inch barreled Ruger Blackhawk, and the shot pattern is just the same as my .357 Mag at 15 feet. I do not have my .44 Mag recipe at hand, so I am unable to provide the details of same.
In any event, both calibers lend themselves very well to making shot loads that are quite effective at least at 15 feet.
Best wishes,
Dave Wile