An old thread, but the concept is still viable.
I started making .44 shotshells about 40 years ago when I got my first Ruger Super Blackhawk .44 magnum. I even bought a RCBS .44 Shotshell size and de-prime die to reload them.
Like Ross posted, I make my cases from .303 British or .30-40 Krag cases that I cut to just under the length of the cylinder.
The diameter and thickness of the rim of the rifle cases must be filed down to the dimensions of the .44 pistol cases.
Instead of gas checks, I use .410 shotshell wads that I cut about 1/4" shorter. This reduces the shot load but it keeps the shot from touching the rifling.
I used to make top wads from thick paper using a .45 acp case with a sharpened mouth, but I recently found that the large holes punched out of paper for a 3 ring binder work well. After loading the shot, I press a punched out paper hole that just fits in the case mouth and I seal it with a drop of white glue.
I use #8 shot over 6 to 7.5 grains of Unique or Clays Universal powder.
Before firing, the cases made from rifle brass have tapered straight sides, then after firing they have straight sides and are bottleneck with a slight shoulder. A case fired in a S&W cylinder will have the shoulder further ahead than the shoulder from a Ruger cylinder. The RCBS .44 Shotshell die re-sizes the cases so they will easily fit in either cylinder (I have both S&W and Ruger SBH pistols).
Because of the rifling in the barrel, the shot comes out spinning instead of coming out straight as it would from a smooth barrel. This quickly spreads out the shot pattern. I get about a 20" shot pattern at 10 yards with plenty of space between the shot holes, but at snake distances or a 5 yard shot on a rabbit or grouse, it would be lethal. It is also fun to break station 8 Skeet targets with them.
