Howdah gun's ballistic handicap compared with longer barrels?

[Naphtali]

Since the howdah gun most of us might have seen and/or used is one of Pedersoli's, I'll ask for information about it.

Compared with standard barrel length (25-30 inch barrels) cap lock 20 gauge shotguns, what is a good rule of thumb for the amount or percentage of muzzle velocity for larger pellets and buckshot Pedersoli's 20 gauge howdah gun loses by way of its short barrels? I am anticipating propellant charges remain reasonably constant between shotgun and howdah gun.

[Daryl]

I have not chronographed a short barreled 20 bore pistol, but Taylor's .60 cal. pistol was running mid 500fps range with 25gr. 2f, I think. 

Also, a 20 bore pistol (Pedersoli's Howdah) might likely to be using 2 to 2 1/2 drams, won't it? That would be 55gr.  to 68gr. I would expect up to perhaps 700fps with a .595" to .600" ball, the appropriate projectile for shooting tigers off your elephant, from the Howdah.