I don't think there are any spoilers in this but here's a warning regardless. I want to quote the larger passage for context as well as to point out several of the other men use their ramrods as rests to aim rifles. I believe (and this is just a best guess) that the rifles in question are the Colt First Model Ring lever rifles. This guess is based on the fact that they are revolving rifles in 1849 and that seems to have been the only one on the market.
"[Glanton] reholstered the gun and stood his empty rifle upright against the saddle and held it with his knee while he measured powder down the barrels.....He was watching a rise to the north where a band of mounted Apaches were grouped against the sky.
...Glanton brought the rifle to the crook of his arm and capped one drum and rotated the barrels and capped the other. He did not take his eyes from the Apaches. Webster stepped from his horse and drew his rifle and slid the ramrod from the thimbles and went to one knee, the ramrod upright in the sand, resting the rifle's forestock upon the fist with which he held it. The rifle had set triggers and he cocked the rear one and laid his face against the cheekpiece.
Glanton turned in the saddle without taking his eyes from the indians and held out his rifle to the nearest man. This man was Sam Tate and he took the rifle and reined his horse so short he nearly threw it. Glanton and three rode on and Tate drew the ramrod for a rest and crouched and fired. The horse that carried the wounded chief faltered, ran on. He swiveled the barrels and fired the second charge and it plowed to the ground.....
Glanton had drawn his pistol and he gestured with it to the men behind and one pulled up his horse and leaped to the ground and went flat on his belly and drew and cocked his own pistol and pulled down the loading lever and stuck it in the sand and holding the gun in both hands with his chin buried in the ground he sighted along the barrel. The horses were two hundred yards out and moving fast. With the second shot the pony that bore the leader bucked and a rider alongside reached and took the reins.
I think you are mistaken in your conclusion that the rifle was a Colt revolving weapon. Given the descriptions the only thing that makes sense is an over-and-under swivel barreled double rifle in which the second barrel is rotated up to fire the second shot. They aren't exactly common but they are period correct. He loaded the "barrels" rather than chambers of a cylinder, and he capped two "drums." He drew the ramrod when the Colt only has a cleaning rod carried separately from the rifle.
Using the ramrod as a rest is possible, although it isn't something our army trained to do. Civilian use - who knows. Austrian Jägers were trained to use their sword bayonet scabbards as a rest in the the kneeling and sitting positions.
The section on the pistol is still bullsh*t. With the loading lever dropped he couldn't have fired the second shot. Plus, getting enough elevation for a 200 yard shot from the described position is implausible.