Mike,
S x S barrels are not curved the way you say. this is an optical illusion that is created by the way the barrels are turned, then struck. Plus, it is exacerbated by the fact that you cannot see the same curves on the inside, between the ribs. I have removed the ribs from S x S guns and that illusion goes away. They then look perfectly straight, as they are.
The easiest way to tell if any barrel is not straight is by looking down the bores at the concentric rings made by the light reflecting at different distances from the eye. If the barrel is straight the rings will be evenly spaced all the way around. If not, they will be closer together on the side with the "inside of the curve". This can be done with smoothbores and rifled barrels as well, though it is a bit easier with a shotgun for me.
SRH
