doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Proof house marking vs: bore size question

Forgive me for the lack of specific detail but I do not have my books with me right now but here are a few observations on proof sizes:
Original dimensions of nominal proof sizes were not all 10 thou apart. They were the theoretical size of a ball of pure lead etc etc.
The proof size marked is a nominal one and unless by design of the barrel borer, has never been intended as the exact bore size, just an indication of what nominal proof range the bore is in.
When proof sizes went to imperial measurements, 10 thou was the figure commonly found around the 12's and 16's but I think you will find it drifts for smaller bores.
New guns were normally bored as close as possible to their nominal size, eg 0.729", but there was no obligation to do so.
For reproof, wall thickness would not sacrificed to the God of bore size and I have seen guns unfortunately reproofed only a couple of thou inside the next proof size up. This was particularly galling in the old imperial days, less so with the new subdivided metric sizes.
The proof houses have often marked reproofed guns as the next proof size up when their actual 9" measurement is very close but not quite there. I have seen guns as much as 3 thou tighter than the proof size they have been marked with.
When the metric measurements were introduced there was a transitional period when one could have the proof marked in imperial or metric. At this point the 0.729" changed to 0.728" as this is closer to 18.5mm. Also the 0.2mm gaps could be represented by 4 thou nominal proof mark steps, eg. 0.732", 0.736" etc.
When the metric proof stamps were introduced, this finally broke the connection with the old bore definition and the metric procession proceeds in 0.2mm steps throughout the bore range.
Just a new observations. Enjoy!

Toby;
Very good discourse on this. I will say that while I have little doubt that the better makers did try to hold their bores close to nominal I also seriously doubt that makers of lower priced guns paid quite as much attention. Thus they probably continued using reamers which through wear & resharpening came in a bit below nominal. This could easily have resulted in a 12 being proofed as a 13/1 & with only a thou or two of honing room before going out of proof.
Rob;
On the 16/1 I get .669" to .677" but either way your .674" would still be in proof.
.677" is nominal 15 gauge size & 16/1 is halfway between 16 & 15 dropped (Not rounded) to three decimal places. Thus (.677" + .662") / 2 = .6695" & with the .0005 dropped becomes .669". A 16/1 proofed bore should thus accept a .669" plug gage to the required depth but not accept a .677" one.

Back in the pre-1954 era, there were plenty of 12's with at least one barrel originally proofed as 13/1. In general, I'd say I've seen more original proof underbores than overbores (12/1).

I'd agree; I almost expect to see 13/1 on the proof marks when I look at a gun of that age.

Jeremy

But note well, just because it is marked 13/1 does not mean it started life with bores 010" undersize @ .719", it may well have been 725" or even larger, just would not accept the .729" plug gage.