jvp05:
Welcome to the forum, mate!
You will need to offer a little more information than this to get a decent answer to your query. We'll assume its a rifle, since this is a rifle forum, and that there is no recommended load amongst the proof-marks. So...

Is it original or a modern reproduction? Maker and country? Barrel wall thickness and overall gun weight? ...and twist-rate of the rifling?

Also, anything special like sight-ladder markings, 2-groove rifling, etc would help. Sorry to be so onerous, but these details are needed to determine whether the rifle was intended for round ball or conical, and how 'heavy' a charge/load is likely to be appropriate.

Also, where are you from? You can update your profile to indicate your home country, this may help with any recommendations on prices, availability, gunsmiths, etc in the future.

Well, there's your simple question answered with twenty more! There will be someone here who can help I'm sure, given the extra info.