While there are exceptions to all rules, it has been my observation that many guns built prior to the introduction of smokeless powder had thinner walls over the chamber area. While the thickness at end of chamber itself may not have been that different many of these early guns have less taper to the OD & thus are of a smaller dia at breech face. Max pressures have traditionally been taken at 1" from breech which is said to be far enough to catch max with any normal shotshell load. Most early smokeless powders, both dense & bulk, were faster burning than BP, thus if loaded to same ballistics they gave a higher breech pressure than did black. With the introduction of smokeless the breech area of the bbls began to be beefed up, not 12-14"s down the bbls in spite of many old wife tales of higher pressures there. While I shoot most of twist & damascus guns with either black or moderate pressure smokeless loads, these early guns having very light breeches I personally reserve strictly for black.
