Optimum calibre ?
Depends on your tgt and your own physical build.
2500gn projectile with 24 drams of propellant up it's arse will sort out most problems that breathe within fifty metres, but you'll be unlikely to get much practice.
Nor will it be offered in a lightweight version.
But it is pretty.
"Nobody who had fired this weapon once, wished to fire it again..." may apply.