Yes I would use a modern bullet.
Finding out the caliber is no rocket science. I just melt an old candle, prepair the gun (put a bit of newspaper into the barrel an inch forward of chamber, lightly oil the chamber), pour the wax in, wait a bit, take it out with a cleaning rod and put it into fridge overnite, take the measurements next day - and publish them here smile

With kind regards,
Jani