In the scene where Lt. Haywood is fighting with the Indian, it is easy to see they're using rubber knives.
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.
On the trailer that's on the DVD, King Donovan is identified as Wally Cassell.
When Sheb Wooley asks Wally Cassell to shoot him, you see the smoke from the pistol as Wooley slumps, but there is no sound.