LOL, I've tried to reload a modern CARTRIDGE revolver (Python) on a galloping horse, LOL good luck!!!! It was down the fenceline of my own pasture, so the speedloaders were easy to find, didn't loose any.. But it was close to impossible, very impractical... Wouldn't bother trying it with a cap n ball Remington, and forget it with an open top Colt... That would be humorous to watch someone try tho... Even tried it with a Schofield Smith, and it was still almost impossible with loose cartridges....
I own horses, tried these very things in reality... Stand pumpkins on the fence posts as targets for Indian bow and lance, swords, machety, pistols, shotguns, and carbines... Ride down the fenceline at full gallop, take a single shot at every other target... Believe me, Its not an easy thing to do, even with practice... It is actually much easier to connect with a Sabre.... And no, my regular hunting animals aren't gunshy anymore... This is how I trained them NOT to be!!! On a GUNSHY horse??? LOL, Have fun, LOL!!!!
Never mind hitting something, you'd be lucky to stay on the horse at all, LOL!!!
Thats why Cavalrymen or Bushwhackers would carry multiple pistols and a sword... In addition to the ones on their person, you could count on a couple more hanging on their saddle... ALL of them, secured by lanyards.. And horseback tactics would involve a series of charges/recalls, so troopers could reload while waiting to reform the charge...
RC