When Hickock is shooting at the tree, he empties his gun, removes the barrel to change the cylinder, you can see an empty 6 shot cylinder, puts a loaded 6 shot cylinder in, replaces the barrel, puts the gun back in his holster, then draws and fires 8 times from a 6 shot gun.
When Jimmy shoots the tree after loading his six-shooter (with six bullets clearly visible), there are eight shots instead of six.