I think you are confused by the method of measuring barrel length in
calibers,

Thus a 5 inch, 50 caliber gun has a barrel 5 x 50 = 250 inches long.

I think that the US Navy had a 5in 50 caliber gun in WW1.


If memory serves (and it doesn't always) the 50 caliber naval round is 5
inch
diameter.

Dan, U.S. Air Force, retired