Empty brass sounds that way. I was wondering if this was the first movie to include the sound of ejected rounds?
If you want to get to niggling movie factoids, three million dollars' of post Civil War gold would run about 3 tons. Doubtful that little coach could bear the load.
What I had in mind was boxing the compass.

In the film, the sound of the ejecting 'case' from the Rifle, sounded like a plastic shell casing.
Can you fly this plane?
Surely you can't be serious.
I am serious,and don't call me Shirley

Lighten up, wildbillharding!
There were revolvers and Winchester rifles in 1866. Most people can't tell the difference between a Model 1866 Winchester and a Model 1892 (seen in a movie) nor between a "peacemaker" and a Colt Navy Model. Nor do they care.
Give this 1954 movie a break. They didn't have all the reproductions available that are now. The .44 Henry Rimfire ammunition for the appropriate rifles would not have been available. You couldn't expect actors to use actual cap and ball revolvers. Because of chain fire and flying pieces of exploded caps, they are dangerous to the eyesight and limbs of the shooters and the bystanders.
Nobody cares about this except a few of us gun nuts. I doesn't bother me in a western, though it would in a historical drama about the Civil War.
He maketh His sun to rise on the evil and on the good St. Matthew 5:45