Why are bullets certain caliber?

Factual Questions
41

The two links you provided on the subject, I found very interesting and informative.

I can’t find my copy of Ezell, so I don’t have anything more to contribute to the tumbling discussion. (I think I lent it to a Marine buddy of mine many years ago, and we all know what slow readers they are…)

However, back to the original question regarding the actual caliber of bullets: does anyone have any response to my beer and peanuts treatise on how calibers may have been determined in the first place? (Post #22)

42

I thought it was a pretty good attempt at how one might go about determining a new calibre now, working from the bottom up. In fact some of your ideas match the concept behind the H & K G11 with its caseless ammunition. Getting a rifle to be capable of 6" groupings at 500 yards isn’t difficult; it’s making it easy to shoot that accurately that’s hard. The G11 was designed IIRC to fire very fast three-round bursts and took a sort of shotgun approach. An average soldier was supposed to be able to put the burst inside a 50cm circle at 600 metres. Not that accurate, but three bullets spread within that circle meant that one of them was likely to go through an important bit of a person with the circle centred on them.

But I’m digressing. The fact is most calibres are more or less accidents of history, each calibre evolving from the previous, and you have to start back in the days of black powder and muzzle loaders. Pressures generated by early black powder wouldn’t have been consistent, and later “corned” powder would have allowed better control and more velocity, and everything would have had to be fine tuned by trial and error. There were no standard ball sizes - you got a mould to go with your arquebus and had to cast your own balls! Standard sizes were introduced with the “caliver”, an abbreviation of “arquebus of calibre” from which we get the word. Those sizes would have been very much larger than modern calibres.

Then came rifling and the advent of the long bullet rather than the ball, but the tools and mandrels for making barrels were inherited from the past and so the calibres stayed the same. Smokeless powder was the real game changer, pushing muzzle velocities up so bullet weights and calibres were reduced. .30 calibre seemed to be the magic number for a great number of manufacturers. The USA replaced their 45-70 Government with the .30-40 Krag and later with the .30-06. The UK replaced their .450 Martini-Henry with the .303 British. Winchester developed the .30-30. The exact reason why .30 calibre has been popular for so long isn’t at all clear, but I’m willing to bet it wasn’t a bottom-up approach like the one you outlined.

43

It occurs to me, looking at the standard sizes of drop-tower shot, that there’s a few familiar numbers in there: .22, .270, .300. I wonder if the modern rifle calibres come down to us from development work with smokeless powder being carried out using lead balls as test projectiles? Shot (pellet) - Wikipedia

44

Scanning the table of shot diameters in your link to shot towers, it looks like you may be on to something here. There are certainly a lot of very familiar calibers for shot size in that table.

This then raises the obvious question: if the original calibers were derived from shot sizes, and shot sizes are determined by the copper mesh size, why did the shot manufacturers select those particular mesh sizes?

For example, why would someone have selected a mesh size of 0.38? I can see the logic of 0.33, this being one third of an inch, but 0.38? Or 0.303?

Or having started at a mesh size of 0.22, what would be the logic to then go to 0.27?

Why these very small incremental variations?

45

They probably used the same caliber not because they were testing with lead balls, but because they were testing new rounds in existing weapons. If you fit a new firing mechanism into an existing weapon, you can just modify the stock a bit and maybe tinker with the back end of the barrel, and maybe add some rifling. It’s a lot less work than making a whole new weapon from scratch, especially considering you’ll need to retool your shop a bit to be able to make a different size barrel.

A lot of cartridges were created in the mid 1800s this way.

Like I said earlier, though, each bullet has its own story. Here’s another example. After the U.S. Civil War, the army had a ton of Springfield Model 1861 rifled muskets (and all of its variants) lying around. However, the war had shown that cartridge rifles were much more effective (from 3 or 4 shots per minute with a musket to 15 to 20 shots per minute with a cartridge rifle), which made the Model 1861 obsolete. Rather than build new rifles, a guy named Erskine S. Allin came up with a fairly simple way of modifying the muskets so that they could be turned into breech loading cartridge rifles. The modified rifles cost $5 each where new rifles cost $20 each, so this was pretty significant. Since the model 1861 was a .58 caliber, the new cartridges had to be .58 caliber as well.

In practice, these new cartridges tended to have fairly horrible ballistic properties. So they very quickly modified them and came out with a better bullet. The problem was that their new bullet was .50 caliber, so their solution was to create inserts that could be put into the barrels of the old converted muskets, converting them from .58 to .50 caliber. Under real world use, these inserts tended to fail fairly often, so a few years later they started making new rifles completely from scratch instead of modifying old ones. The new round that they came up with was officially named the .50-70 Government cartridge (.50 caliber, 70 grains of black powder).

46

I think we are making some progress here; pulling together the comments made upstream regarding the history of both the manufacturing processes and the specifics of the weaponry, there does seem to be a logical progression.

This progression does seem to produce a logical explanation for the bore sizes of munitions.

If we start from the original smooth bore muskets, we know that they had a bore of 0.5 inch, and used a spherical ball.

The half inch ball, and the corresponding bore size, was logical and reasonable as a fraction of a standard unit of measure; ie: the inch.

However, with the increased use of muskets, and the corresponding demand for balls, it would have been logical and reasonable to try to economize on the use of lead in the production of balls.

Similarly, it would also have been logical to consider means by which to reduce the consumption of propellant.

So, it would be logical to halve the size of the ball, and the corresponding bore of the musket.

The volume of lead required to make one half inch spherical ball makes two 0.38 inch spherical balls.

Similarly, that same volume would make one dozen 0.22 balls.

Spheres of these diameters could conveniently and efficiently be manufactured in a standard shot tower.

With the introduction of cartridges, it would be logical and economical to use existing musket barrels, at least during the transition phase.

Therefore, cartridges would be designed to fit existing barrels; and cartridge manufacturing processes would be tooled to make cartridges of these calibers.

Tooling for the new rifled barrels would be created to make the new rifled barrels to correspond to the manufactured cartridges.

Once the volume of such weaponry was sufficiently large, these calibers would have been established as the defacto standards.

The other odd calibers may be simple variations on these units due to some identified manufacturing efficiency in materials or process.