doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Question on proof.....

If a gun is proofed at 13/1, at what point is it out of proof?

at .729"

That's what I thought, but suppose its bore was .728 when made, that would have given it a 13/1 proof. So, wouldn't it be allowed .010 before being out?

NO! It was out of proof when it would accept the next larger gage than stamped on its bbl. That's why the system was eventually changed, as unless the actual bore dia was marked there was no real way of knowing if a bbl marked 13/1 was proofed at .720" or 728". Thus when it accepted that 12 gage (.729") it was out of proof.

So what about one that is marked 13B 14M NOT FOR BALL? at this time would it be in proof up to .729? What are the plug size diameters? I have googled but no luck

Before the advent of the fractional bore measurements (13/1 etc), which was in 1887 (same time "not for ball" disappeared), you had a spread of about .020 for each gauge. If marked "not for ball" and not rifled, a 13--under the rules of proof prior to 1887--would have been in proof from .710 up to .728, out of proof at .729 (where the 12 starts).

Larry;
One thing here I am not following you on. For instance I have an older Birmingham proofed 12ga with both bbls cylinder (pre 1887). The bbls are proofed as 13. That's all as far as size. There is no B & M marks & no Not for Ball. Does it not follow the exact same rules you stated, ie in proof from .710"-.728". Seems to me the proof limit applies only to the bore dimension with no reference to the choke.

All keep in mind though & I don't recall the exact date of switch, but the 9" from breech measurement came into being with the breech loaders. Throughout most of the muzzle loader reign, the bore size was measured "At the Muzzle".

I assume that we are talking breech loaders? If so ,13 = .710 . 13/1 = .719 . A gun proofed under British law goes out of proof at the next bore size up .

We found this to our cost when guns like the early Miroku's came in bored to 18.5 mm that's .728 in real money . They were plugged and as they would not take a .729 plug they were stamped at .719, thus only having .001 in proof. After this was discovered the practice was to use an intermediate plug of .725. If the barrel accepted this then it was marked .729 .If not it was marked at .719 .This was only applied to imported guns.

The proof rules in force at the time of proof testing continue to be the rules by which the gun is checked for proof until re-proofed.
Larry has correctly identified that the Not For Ball mark's disappearance coincided with the introduction of the fractional proof sizes. Therefore a gun marked with Not For Ball is easily dated as being proofed before the fractional sizes were introduced and hence has double the proof range of later proof sizes.
However, if the gun didn't carry any choke the Not For Ball was not impressed which leaves you with the need to establish if the gun is prior or post 1887. This can be done by looking for the nominal chamber mark in a diamond which was introduced along with the fractions.
As a matter of interest, in the 1875-87 period if there is no Not For Ball mark but '12M 12B' (for instance) is marked on either flat it means that that barrel was given a recessed or jug choke as standard. Note that both bore sizes are usually the same but occasionally they will differ. It has been suggested that this was caused by a long liquid lunch or perhaps an excellent night previously!

Miller, under the rules of 1887--when the fractional measurements were introduced--there was a separate table for muzzleloaders, which never had the fractional measurements. And that table remained in effect for muzzleloaders. So a muzzleloader proofed as a 13 would be in proof up to .729.