The item you have was without a doubt used to fire salutes on holidays. In the US these were usually mounted in a heavy dug-out wooden block, so the gun barrel was horizontal and half-above the top of the wood block. The rear end of the gun barrel butted up against wood and the front end (muzzle) would stick out in front of the wood block at least a few inches. The barrel was held down with two "U" bolts, you can see the outlines or shadows where these contacted the barrel.
This is the U.S. equivalent of the European "Thunder Mug" which was fired vertically.
I wrote an article on thunder mugs which was published in "The Artilleryman" back in the mid-1980's, but I can't find it right now to give you the exact citation.
You could fire yours vertically if you want to shoot it, and don't want to waste time mounting it unnecessarily. Be sure and get the advice of an experienced black-powder cannon shooter before you do it as many people have lost some or all of their eyesight wnen the gun fires unexpectedly due to procedural errors.
I've heard that these items were often made out of discarded railroad car axles.
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