I suspect you exaggerate a bit about the effective range of that being 20 feet. I've done the exact same thing in the past with .357 Magnum and #9 as you say with the gas checks and the pattern density was about five pellets per square foot at seven yards. Not too good for killing snakes. Even with 255 grains of #9 in .45 Colt from a 7-1/2" bbl and a good, taped wad, the pattern density at seven yards is only about two dozen per square foot, that's why I go to all the trouble to make my own custom wads.
Gear