If you've ever wondered why the old pumpkin slinger was a .69 calibre, rather than something of which the EU would approve, like a .70 calibre, and why it's replacement was a .58 cal. rather than a .60, read on.
Back in olden times, devices like vernier calipers and micrometers didn't exist. This made measuring and gauging barrel dimensions pretty tricky. Of course, if the village gunsmith was knocking out a musket for a local customer, who cast his own ammo anyway, it didn't matter that much - as the customer could be supplied with a bullet mould to go with the gun, and the gunsmith could make them as a matched pair.
However, when whole armies started using firearms, and wanted bullets which could be used by any musketeer, matching became very important. Not that it was critical whether the bore size was 0.303" or 0.304", you understand; just so long as they were all the same. The way they got over this was to set the gauge by what they
could measure fairly easily; the weight of the projectile. Standard weights, believe it or not, go back a very long way. Kings started bellowing for the axeman if they thought coin minters were short-changing them on the weight of coinage, and had standard weights to check the coins against.
These, naturally enough, came in equally handy when checking bullet and cannonshot weights - where the crown had an equally close interest. After all, if you have a standard weight of one pound, it's simple enough to divide it in half with no more than a wooden beam and three bits of string - do the same three more times, and you have an ounce; easy peasy (which is why there are 16 ounces in a pound, and not 10). So cannons and smallarms were defined by (for cannons) the weight of the shot in pounds; and (for smallarms) by the number of bullets, for a particular bore, which weighed in total one pound.
For example, if you take the bore size of a 12 bore shotgun, a lead sphere which just fits in the barrel weighs one-twelfth of a pound. For a 20 bore, the ball would weigh one-twentieth of a pound, and so on.
Hence, the pumpkin slinger is .69 calibre, which makes it a 14 bore, and it's .58 calibre replacement is a 24 bore. The still later 'volunteer' Enfields, at .451 calibre, are 50 bores. The picture is not quite tidy, as some operating leeway had to be allowed. In muzzle loaders, particularly before the Minie bullet was invented, with it's easy fit and scouring effect, there had to be enough slack in military small arms to allow bullets to be rammed home despite the fouling caused by many previous charges of black powder. Once the Minie bullet came in, and even more so after the introduction of breechloaders, the tolerance between bullet and barrel could be held far more tightly - and the bullet could actually be made bigger than the barrel.
In any case, by that stage, Henry Maudslay had perfected his measuring instruments to the point where dimensions could be checked routinely to within one ten-thousandth of an inch - a far cry from 30 years or so earlier, when George Stephenson was mightily impressed that he could barely fit his little finger between the piston and the cylinder on his 'Rocket' steam loco!
But, even so, centuries of thinking in terms of bore sizes were well established, and gunmakers carried on using the system, although there was no reason why they could not have produced firearms with any decimal measurement (in fact, as you're probably aware, shotgun makers still do use bore sizes based on the old system - well, if it ain't broke, why fix it?).
For early muzzle loading revolvers, we have .44 calibre, which is 54 bore, .41 is a 68 bore, .36 is 100 bore, and .32 is 140 bore - quite a drop as compared to early 19th century flintlock pistols, where 15 or 16 bores were run of the mill! The Brown Bess, by the by, a .75 calibre, comes out at an elephant gun sized 11 bore - NOT recommended for use with Minie bullets (unless you have a special deal with your local physio!
).
Maybe it's just a coincidence, but . . . the bullet used in NATO rifles (a puny little pop-gun effort, in my anything but humble opinion) has a diameter of .223" - and, despite the fact that it's
called a .22, so has the humble .22 Long Rifle rimfire round, used for everything from Olympic target shooting to rabbit hunting. Would you believe that size equates
exactly to a 420 bore?
(for the relevant data on the mass/volume of pure lead, and the formula for the volume of a sphere of given diameter, I'm indebted to the engineer's handbook published by
'Machinery', an American trade journal. This Bible-sized tome, complete with Bible-thin paper and tiny print, cost me an arm and a leg when I bought it about 30 years ago - and it's been worth every penny!)
With best regards,
Jack
PS have been signed off by the docs, and start work tomorrow afternoon!
