Hmmmmmmmm. Are you shooting a Minié bullet? Or a patched ball? Bearing in mind that a RIFLED Enfield was designed from the off as a Minié bullet shooter.
I've never tried a sub in mine, but you'd have to start by measuring the volume of the service charge of 2.5 drams - 68.5gr of FFg - and go from there.
Any standard phial from around 75mm in length will hold any charge that you are likely to be putting in your Enfield.
tac