We have an irate client who says our matchlock is not .58 caliber as advertised because the actual size of the bore is .615.

Upon inquiry, the proprietor of a local black powder shop had the following to say:

"Caliber size refers to the size ball used when shooting. The actual bore diameter of a
matchlock is sized to allow for a patch around the ball to create a proper seal against the inside of the bore. A gun designed to fire a .58" diameter lead ball projectile will have a .615 bore diameter to
allow for the patch."

So the question is- does the bore size or the projectile size determine the caliber of the gun?

Our gunsmith insists they are .58 caliber because that's the size of the projectile, and the local gun shop owner agrees. The client insists it's the other way around, and that we're "insulting his intelligence" by claiming otherwise.

Who's right?

Is there a standard that everyone uses?

We need to get this sorted out ASAP before we have more problems....

Your expertise would be appreciated.

I think the simplest method is to advertise that the gun "fires a .58 ball" (or bullet, or minie, et cetera) and avoid the caliber issue completely.

Your gunsmith is right at least for modern manufacture.

Who/what/where in caliber sizing:

For example, let's use a .308 aka 7.62 rifle. The bullet is a .30 cal bullet but the barrel MUST be bigger or the bullet would heat up and seize/catch in the barrel. Plus you have the modern grooving (such as 1:12 twist) cut into the barrel making it even wider (to provide bullet spin with less friction).

The barrel of a smoothbore must be larger (.615 is reasonable) to accomodate the patch. If it wasn't, you'd end up with a jammed bore and very possibly a burst barrel.

In addition, without some extra width, the PSI in the barrel would far exceed industry recommendations and BOOM aka split barrel.

Most common ball calibers are:
.50
.58
.68 (paintball)

The organisation that regulates modern manufacturing is via the guidelines provided by Sporting Arms and Ammunition Manufacturers' Institute (S.A.A.M.I.). Their data is what reloaders compare their data (powder amount) so that they don't exceed industry standards risking firearm failure. Admittedly old caliber data is by pressure testing old barrels and trial & error. My reloading books unfortunately don't address blackpowder sizing.

Friedrich

Though I still haven't found a period reference to how calibers were determined in the 15th century, by the end of the 17th century, they were referring to how many balls there were to the pound, with no reference to the actual physical size. This convention continued on into the mid 19th century for most civilian arms and military arms until the advent of the rifled musket and minie ball.

Dru

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We've taken the safe course of rewording the matchlock ad to say that the gun shoots a .58 caliber projectile. Let 'em figure it out from there.

Thanks for the input.

Actually, our gunsmith insisted that the gun be a "standard" caliber so it would be easy to find ammo. I guess in his world, the caliber is gauged by the projectile, not the bore size.

Jeff did some more poking around the net this AM and it seems that everybody makes their own version of the "standard", pretty much confirming what Dru said.

I'm not losing any sleep over this, believe me!