Sharps cartridges were normally loaded with heavier bullets than the Black Powder Expresses.

One friend has a .450 BPE that does best with 300 grain bullets, and would probably shoot closer with 270s if he could find some, or a mould; while the big Sharps .45s, were normally loaded with 480 grain or heavier bullets for long range. Winchester offered a .45-125-300 to emulate the Express but it never caught on, you could do nearly as well with a .45-90-300 repeater.

Another friend loaded some .500 BPEs for a client with .50 Sharps or Government bullets heavier than the double was bored and rifled for and the bullets just keyholed.

Not the same cartridges, not the same applications.