I'd tend to agree all tubes can probably be shown to have some bend, but I'd wonder how the old makers could locate and align such a bend that might not be seen. Once done, how far would that effort move a shot pattern at say thirty yards.
It would be interesting to have an idea how Skeet's smith determines if bend is present. It doesn't seem to be his preferred method, but the reliable solution would be to pilot all choke work from the muzzle end.
I'd guess if he sees trends in barrel bends, however slight, it may be a byproduct of manufacturing. There may be many heating and machining steps that the tubes experience to become a barrel set. I'd think that similar steps could change stresses on the tubes in a similar way on different barrel sets.
The Krieghoff example sounds like a good way to see how visible a slight bend might be, but the purpose may be different than for side by side game guns. I've seen it used more as specialized gunfitting for individual styles rather than to get the barrels to shoot to the same point of impact.
