The "Modern Shot Gun," Vol;3,by Burrard, pages 202-3 details the standard chamber lengths and corresponding loads for 10G black powder cartridges.I would recommend inspection of your gun by a competent gunsmith and his/her confimation that the barrels measure,"In Proof," and that the barrel wall are of a satisfactory thickness,prior to any use.Provided the gun is approved by such an inspection, you may wish to consider using the loading recommended by Burrard for 2 5/8 chambers even if the chamber measures longer than 2 5/8[ Play it safe!].
AS A PRECAUTION ,PLEASE DOUBLE CHECK BURRARDS POWDER CHARGE RECOMMENDATION, HE STATES DRACHMS THIS COULD BE A SERIOUS ERROR AND SHOULD READ DRAMS![SEE GOUGH THOMAS, "GUN BOOK," PAGE 266].
BLACK POWDER USERS, PLEASE COMMENT ON THIS POTENTIAL SAFETY ISSUE?