I tried No. 9 shot with Wonder Wads in my 1858 Remington a couple of summers ago.
Not a success.
As stated, the shot pattern is doughnut shaped. Patterns are atrocious. I tried it against a 4X4 foot piece of plywood, beginning at 25 yards.
The shot was literally all over the board.
Then I moved closer, about 15 yards. Shot pattern was likely three feet in diameter; it was hard to tell with the previous shot in it.
One thing was certain: penetration was poor. Many of the pellets were stuck only halfway in the plywood. The sheet looked like it had pimples.
So I moved to within 7 yards.
BAD MISTAKE!
Some of the shot bounced back and peppered me.
END OF EXPERIMENT.
I wasn't hurt by the shot, just startled. One of the shot ticked my eyeglass (good reason to wear shooting glasses whenever you shoot ANY firearm).
I decided that loading any cap and ball revolver with shot was nearly useless unless you wanted to kill mice at very close range, without regard to damage.
I've also tried shot loads in my 1851 Navy in .36 caliber. Such a small amount of shot could be fitted in the cylinder that it was nearly useless.
Smith & Wesson recently introduced the .50 caliber revolver.
Now, if someone introduced a .58-caliber cap and ball revolver, with a cylinder about 4 inches long, THEN you could probably get enough powder and shot into the thing to make it feasible.
But frankly, if you want to shoot shot in a black powder pistol, I'd suggest a single shot. Revolvers, by nature of their design, limit the amount of shot and powder that could be placed in them.
I should think that a reproduction Deringer, such as the one used to kill President Lincoln, might be useful at very close range against snakes, mice, rats and such.
I'd rather leave the snakes alone, they keep the mice and rats down for me much better than a load of shot.