doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Overbore dimensions?

I always understood that overboring was the term for "overbored" guns from the factory such as the Miroku Brownings. Backbored was taking a standard bore & honing it out to a larger bore as an aftermarket modification.

If you pick up a gun which has been chamber sleeved from 16-bore to 20-bore or from 20-boe to 28-bore you get to see what very wide bores do. They perform very well!

I have also had 12-bore guns originally 'chamberless' and with bores of .775! They work very nicely.

If a 12 bore gun is nominally .729 what does the proof house mark on the barrels for guns manufactured with overbore/back bored larger than the proof tolerances.

Bob

I may well be wrong here but did not know there was a "Proof Tolerance" Per Se. I was of the opinion the proof house simply measured the bore, stamped its gauge according to that size & proofed it by the charges for "That" gauge. The chamber marking then tells you what shells to put in it.
If you look at the proof charts a grouping of in-between gauges both larger & smaller will normally have identical proof charges.
.729"-.739" will be stamped 12; .740"-.750" 12/1; .751"-.762" 11; .763"-.774" 11/1 & finally .775 as 10.
Th actual proof charge will change a bit at the larger end, but any of these size bores "Could" carry a 12ga chamber.

Did you ever wonder if some of these over bored Lefever 12's were stock barrels that could have been intended to be used for a regular 10 or a light 10? Mine 12 is .750 bore while a 10 would have been .775. Did they take a set of tubes that could have been intended for a 10 bore and just finished bore it at a 12 bore, .750 and then strike the outside extra? Or were these tubes extra heavy for just such a .750 bored 12 waterfowl.

The reason I doubt that is that Lefever we have been lead to believe was always be short on cash and could not have kept that much slow moving stock on hand. How much demand was there for a special heavy larger than normal bore 12? If they only kept written records, which I have come to doubt ever existed, so many questions could be answered.

Jon;
My take is these overbore Lefevers were made that way for their gauge. In their early days I highly suspect that waterfowl guns accounted for a large portion of their sales. Early catalogs recommended using wads two sizes larger than standard for Black Powder use, but only one size for smokeless. My personal belief is that many waterfowlers shot brass cases & these two sizes larger wads was to accommodate the brass cases. Smokeless was more often loaded into paper cases thus the smaller wad. I think that a lot of the early guns were simply bored for use with the brass cases & oversized wads. Another thing I suspect but can't prove is that when they specified a wad they were speaking of its actual dia. Thus one over for 12 would be an 11ga or .751". In later years wads would be labeled for their intended use & a card or filler wad labled as 12 would actually measure to about an 11 & an 11 ga wad labled as for use in 12ga brass shells woud measure more like an 11/1 size. This would fit right in with Lefever's recommendations.

2-piper
Sorry my use of the word tolerance was wrong. I was refering to the .729 to .739 and possibly should have said "range". Your explanation of the 12/1 etc. markings was just what I needed.
Thank you

Bob

I suspect that you are right about the brass vs paper aspect of those days. I am sure brass shells held court for many years after paper came into use in wet environments. Still I wish I could come across a 7.5 pound .750 bore gun instead of almost nine pounders. Toting one of the nine pounders is not that much fun. Might as well tote a true 10 bore.

Backworth Bob,
Other than the proof marks applicable to the allowable proof diameter range [measured 9 inches from the breech face],chamber length and if appropriate the word," choke," U.K. proof houses add no other marks concerning the barrel dimensions. It is the gun makers responsibility to ensure that the barrel wall thickness is sufficient to accommodate resultant proof stress and if applicable any recessed chokes[back bored].
A word of caution is perhaps in order. Back boring English guns greater than the maximum diameter allowed under the rules of proof renders them, "out of proof" and subject to reproof in the interest of user safety. Under .U.K. law sale of guns out of proof is a serious offense.

I hate to beat a dead horse, but British Proof only pertains to the inside of a shotgun barrel. The things British gunsmiths do to the outside of a shotgun barrel makes a British Proof mark absolutely useless unless we know nothing was done to the outside after proving. Most guns in British auctions that warn "wall thickness below recommended minimums" have perfectly legitimate proof marks, but have been struck to thinner wall thickness.