In the scene where Corbo and Hofstetter run into the Sioux horse herd, firing their pistols into the air to scare them off, it's obvious that Corbo's pistol accidentally discharges while he's still holding it at his side, pointed toward the ground - you can see the muzzle flash but no shot is heard. A few seconds later, after he raises his pistol into the air and fires off another shot, you can see the muzzle flash AND hear the sound of the pistol going off.
Capt. Phillip Donlin: None of us are fit to ride 50 miles, let alone 250. But we're gonna do it. And we don't have time to go around obstacles. We'll go straight through them, no matter what or who they are.