In talking with other gun enthusiasts, particularly of the 18th Century styled American gun, and how that relates to the European gun, I have come to wonder about the architecture of the gun, where it comes from, how the form was developed, etc.
The basis of the gun is the barrel, that is, the barrel is the foundation, the reason for the existence of all the other parts, and the wood that holds all these parts in a proper working relation to each other. The shape of this barrel is what determines, in large part, the style of the gun. After that, the hardware play a vital role in the design of the stock. The buttplate tells us what kind of buttstock, the guard says we will have a step toe, or a straight buttstock.
Jim Filipski and Rich Pierce keep bringing the 'hump' to my attention. This happens at the tang of many early guns. That hump, combined with a banana lock, is very different from from the late 18th Century American gun. But this design is very appropriate for European guns, and I suspect, very appropriate for early American built guns.
That said, I have come to realize that the BARREL is key to this style gun. I have come up with some sketches to explain my theory, but would love this to develop into a discussion about early barrel configurations, what you may have seen, or measured, or built yourself.
In the sketch below, the barrel is viewed from the side in XXII. The barrel has extreme flair through point 'b', the end of the barrel. The barrel flare continues into to tang, with the high point at 'a' and then curves down again. This is where the characteristic 'hump' comes from.
The top view of the barrel, XXIII, shows that the profile does NOT have the same flare as the side view, through points 'e' and 'f'. One purpose is to allow the lock to be closer to parallel with the bore, and the sideplate panel does not flare out either. Second, reducing the barrel flare allows a smoother transition into the wrist area, a slimming effect. How much this taper is removed varies from gun to gun.
One might effect a barrel like this by buying a heavy configuration Swamped barrel, and filing or milling the new profile, as in sketch XXIV. Note that the plug, or tang should be taller than the breech end of the barrel so that the hump can be filed appropriately. In an ideal world, you could make a drawing and the barrel shop would mill up the barrel for you.