Last week I was in Wales shooting driven birds. Our group of 8 guns had various old and modern guns with damascus and steel barrels and shot 65 and 67 mm cartridges interchangeably because CIP says you can - read the last line at the bottom of page 1 of the linked document.

http://www.cip-bobp.org/homologation/en/tdcc_public?page=1&cartridge_type_id=7

65 and 67 mm British #6's of 7/8 ounce in 20 bore and 1 ounce in 12 bore are very effective on pheasants, mallards and red-leg partridges.

A side note - a Purdey representative loaned me a Beesley action Purdey demonstrator for the last drive of the trip. I killed 15 high pheasants on the drive and enjoyed shooting a VERY nice gun.