doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com MACNAUGHTON Double Rifle - Unknown Caliber

Don't over-think the bore diameter. A soft lead bullet that fits case and barrel well enough ought to be fine - let BP kick it in the backside to bump up. Regulation is where the headache is likely to be.

The stars stamped on the breech are an interesting thing: I've seen them before on ex-military weapons; applied in service to denote some level of corrosion in the barrel - if on the breech, corrosion in the chamber; if ahead of the breech, in the barrel. Whether they have that application in this case I have no idea.

Hello cadet,

Thanks for the reply.

I don't think that's the case here because the chambers and bores are in very good condition.

whats the wall thickness at the muzzles?

Hello Aaron,

Thanks for the repy.

Wall thickness at the muzzles is .130"

Why do you ask?

While a small hole gage is admittedly spherical it also has to be small enough to go in the smallest hole of its range. It too will measure two points 180� apart. It will thus give a false reading with an odd number of lands & grooves. Depending on the width of the grooves it may or may not reach the bottom.
The "Original" question in this thread included the "40" mark on the barrels. This mark applies to the "Bore" diameter, thus the ensuing comments on that subject.

Technically speaking the caliber of a rifle is its bore size. No doubt, in this case, the 40 was the correct marking under the old Gauge system. Makers have taken such liberty in naming their creations that in many cases neither bore nor bullet diameter is actually described by the cartridge's name. A couple of examples which immediately spring to mind are the .44-40 & .38-40 Winchester cartridges.

Hi Mike

I was speaking of small hole gages, not telescoping gages.
I've been working with measuring tools and machining equipment since 1973

I see what you are illustrating with the small hole gages suggestion - and I acknowledge your work-around, but I maintain that a small hole gage mesurement represents a distance across two points. Also note that an engineer or technician can employ a small hole gage outside it's range in order to reach features that are crowded by other features.
In the context of a 500 bpe bore, a telescoping gage could be too bulky, but a small hole gage from the smaller end of the range may be used to read the dimension between a land and a groove in some odd-groove barrels depending on groove width and the width of the measuring surfaces of the gage employed.

I have used small hole gages, telescoping gages, pin gages, and numerous other tools to evaluate the dimensions of bore features.
At times things can end up being a little complicated, and interesting work-arounds like your example can be very useful.

There are also numerous ways to accurately measure the casting shown above,

2-piper,
Of course, properly presented plug gauges give the best measurement, but they are expensive enough that fewer people have them than have small hole gages, which give a decent measurement. The plug gauges come in .001" steps, so even if properly presented can result in .0009" error. With regard to nominal vs actual bullet diameters, other examples include 38 Super, 38 Special, 38 S&W, 38 Long Colt, 38 WCF, and 38-55; not to mention all the nominal designations for 22 caliber rifles.
Mike

Tinker,
I defer to your superior experience, but I have to use the tools I have, to do the best I can.
Mike

And it's always good to look outside the box.
I like your ingenuity.

By the way, I often prefer the small hole gage over the plug or pin gage, which by the way come in plus and minus sizes, so there is a greater range of measurements available with them than you might think.
The advantage to using the small hole gage is that it can/will show you if your bore is out of round, and what the numbers are in that situation. Few bores are actually round.

The .38 line of cartridges started life in the rimfire era using outside lubricated heel bullets which were actually a nominal .38" in size. With the desire for inside lubricated bullets, bullet diameter was dropped to fit inside the case & a hollow base was used to expand & fill the rifling, thus was born the .357" diameter bullet. The .38 S&W began life with an inside lubricated bullet, but apparently simply played upon the popularity of the "38" name. The .38 WCF was the same as .38-40 & also used the popular name of 38 as they were a nominal
.40 caliber. The .38-55 Ballard used as I recall a bullet of about .378" diameter so is a nominal .38 caliber if bullet size rather than bore size is used.

There are plenty of other examples, including the .22's you mentioned, .22, .220, .218, .219, .224 & etc.

I spent 35+ years as a machinist so I have used all these methods of measuring. You will get close with the small hole gauge in the bore with an odd number of grooves. The biggest problem would likely be if trying to check a size against the stamped marking, you might read it a bit oversize. If the bore marked 40 was close to the upper limit it might read as a 39 size.

Not a big deal, but would not prove the proof house's mark to be wrong.