Frank,

I believe the weapon shown in this famous photograph is neither a standard Enfield rifle musket nor a small bore Enfield.

It appears to be a standard U.S. Model 1861 Springfield rifle musket of .58 caliber.

I believe I remember reading in years past that this rifle musket appears in many Brady et al. photographs of the period and likely belonged to him or one of his entourage, and was often used as a prop to enhance the image being taken.

Mike