doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Dating changeover from 12b to 12a shells

I have an early breechloading hammergun with Birmingham proofs and marked "13". In spite of the "13" marking, the bores measure .729 and .722 and the chambers fit standard 12 gauge RST shells. I am guessing that the gun may have originally been chambered for 12b shells with smaller bores to match and then re-chambered for 12a shells, when they became standard, with a corresponding enlargement of the bores. Can the transition from 12b to 12a shells/chambers be dated? When were the first and last 12b breechloaders made?

"13" is marked on the underside of both barrels, next to the proof marks.

The 13 was the plug gauge that would fit the bore. The 13-1 wouldn't go in. I think I'm remembering right that 13 is .719. The .722 is still in proof , but the .729 must have been honed/reamed cleaned up and is now slightly out of proof. With adequate wall thickness, not an issue. There was so much variation in tooling at that period the variations of dimensions was really large.

Courtesy of the PGCA Rich
http://www.parkerguns.org/pages/faq/12B.htm

It appears that the 'a' and 'b' were chamber, rather than bore size indicators.

Thanks, Drew and Mark. As I understand the PGCA write-up, with the 12B shell size (i.e. chamber size), the interior diameter of the shell was meant to be the same as the bore and the exterior diameter was 12 gauge diameter, about .729. Since .729 exterior diameter of the original 12B shell, because of the thickness of the brass, would have been greater than the interior diameter, in order to match up the interior diameter to the bore, barrels with a smaller bore size, e.g. 13 gauge, might be used. Once the 12A became standard, i.e. a shell with an outer diameter larger than twelve gauge, let's say 11 gauge, then the interior diameter of the 12 gauge shell would match up better with a 12 gauge bore. If this theory is correct, then it stands to reason that when 12A (i.e. today's shell) became the standard 12 gauge shell, many of the guns originally chambered for the smaller 12B shells may have been re-chambered for the larger available 12A shells. In that case, if 13 gauge barrels had been used in the original configuration of the gun in order to match up with the inside diameter of the 12B shell, the owner's gunsmith may have honed out the 13 gauge barrels to .729 or so, in order to have the bores match up with the interior diameter of the "new" 12A shell. Since my gun, a P. Mullin 12 gauge with serial number 3, clearly has the modern 12 gauge chamber size (12A) and bores of .729 and .722, I am theorizing that my 13 gauge barrels with a bore size of .700 or so were honed to .729 and .722 in order to match the new shells. I am the world's most ignorant human when it comes to mechanical design issues, so I may be totally off base about my theory, but I can't think of any other reason why my old hammergun with 12 gauge bores and a modern (12A) 12 gauge chamber started out as a set of 13 gauge barrels. Discussion, comment and reasoned debunking invited. Many thanks.

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.

Thanks very much, Miller. I appreciate this info.