Later developments in metallurgy made it possible to make cannon slightly more slender without loss of strength. Furthermore, artillery theory developed over time, and in the latter half of the 1700s it was found that shorter guns, using a less powerful charge, were just as effective in ship-to-ship combat as the earlier longer guns with a bigger charge. These two factors combined to make the later weapons somewhat slimmer than the earlier ones. (I'm talking "long guns" here, not carronades, just to make that perfectly clear!)
The maximum diameter varies as I have said, later weapons being slimmer. This difference wasn't all that great however, early guns might have a "wall thickness" of 5/4 of the bore, late ones 4/5 instead. If you go with the 3-2-2.5 rule I have indicated above you will not be far out, whatever gun you are making.
Far more interesting as far as modelling is concerned than the exact diameter is the total length of the weapon and the distribution of the "steps" or "sections" and their attendant reinforcing rings. It is these details that make one gun model recognisable (I was going to say "instantly recognizable" but thought better of it...) from another of the same caliber. There was a great variety of gun models, each country was likely to be having a number of different models in service at any given time. No universal formula can really be given for the length of the sections, only rules of thumb. For a lot of weapons plans should exist in the national archives or the relevant museums. Note that the height of the "step" varied from model to model, some have a very pronounced difference in diameter from one step to the next while others do not.
Total length: Later guns should be shorter than early ones, as a general rule. Large-calibre guns should be shorter, relative to the bore diameter, than small-caliber ones. Check references!
The reinforcing rings, well, they weren't really, they were decorative. On the weapons I have seen up close the rings protrude maybe 1/4 - 1/2 inch from the surface of the gun, this wouldn't make a heck of a lot of difference to the strength of the thing!
Typically there was a ring at each step, plus one behind the muzzle flare and a ring or a set of rings at the breech end, but some gun models had several more rings at various places along the barrel. The ring might be a simple protruding band, or it might be a more decorative design. Rings might be single or double.
19th century guns usually dispensed with these rings altogether.
As far as model-building is concerned DON'T OVERDO THE SIZE OF THE RINGS!!!! They are not all that noticeable when looking at the real thing from a few meters' distance! You can see them, but that is more due to the effect of light and shadow than to the protrusion of the rings from the gun's surface.
The shape of the flare also varied considerably. Some had the classic,
elegantly flared shape, some were plain cones, while others had more of a
"large protruding ring" at the muzzle rather than a flare as such. The details
of the muzzle end itself also varied, some having a plain face while others had
one or more decorative "steps" in the muzzle face. Check the references for the
particular model of gun you are modelling!
{Ståle Sannerud}
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