13 gauge = .710" diameter. It is likely this gun was built prior to 1887 when the 13/1 size was not used. A bore marked 13 thus could have measured anywhere from just accepting a .710 plug gauge up to just Not accepting a 12 gauge (.729") plug gauge.
I am personally of the opinion whoever wrote that up for the PGCA was mistaken. Everything I have ever seen indicates the chamber dimensions we still use today were essentially the original chamber dimensions. (4 gauge guns were originally built for brass cases & had small chambers, they still do) Tolerances weren't as close so a few thousandths variation will be found.
The B size came later for brass shells made for standard gauge size barrels.
As I understand it the 4 was originally made as a rifle thus the smaller case size. Most 4 gauge shotguns will be found with bores proofed as a 6. I don't have any info on the exact dates of either the introduction or demise of the B size cases, but they were never made in vast quantities. To date I have never seen a gun so chambered or have I seen one of the B size brass shells. I do not have a large shell collection but do have a few "Very Old" brass shells & they all fit standard chambers.
It is also well to note that many of the older Parker, Lefever & perhaps others are found with oversize bores. My understanding on this is they were bored intentionally for use with brass shells & their corresponding larger wads. Early Lefever catalogs recommending loading wads Two sizes over for use with Black Powder & one size over for Smokeless. This would indicate brass cases for thee black & paper for the smokeless. The guns could be "Ordered" bored for use with smokeless or To Gauge so to speak. It was also noted that black powder could be used in these guns bored for smokeless with complete success.
It is also noted that in later years card & felt/fiber wads were marked as to intended gauge but made that one gauge oversize. Thus standard later 12 gauge card or fiber wads will measure to about an 11 gauge size. A wad marked 11 for use with brass shells will measure close to 10 gauge size.
My personal feeling is this gun was made as a 12 & so chambered. With the 13 mark by the proof house obviously neither barrel would accept the 12 plug gauge at proof. The one which now measures .729 could very well have been near that size when built, It doesn't take a lot to remove a couple of thousandths out of a barrel over that many years of use & cleaning.
I do not personally believe the B size shells ever had much of a following. They were essentially the solution to a non existent problem. Brass shells with their oversize wads can be fired in guns with regular bore size with no harm. I have done so on a good number of occasions. Personally I would not fire them in a gun with "Stepped" chamber. No problem if the cone is at least 3/8" long. Fortunately not many of those stepped chamber guns were ever made.
PS; Note that pin fires were the first breech loading shotshells. All of these were originally paper cased & were with the exception of the rim of same dimensions as modern shells. It thus seems that paper cases of same size as present day ones were the original shotshell cases, not he B size ones.
Last edited by 2-piper; 04/26/18 07:42 PM. Reason: Added Data
