A practiced eye can detect bends in a tube by looking at a straight line with a backlight, but not overly bright. Many old-time shotgun barrel men reportedly used a shop window by looking at the cross member which divided the panes.
I have not straightened a barrel by this method, but have tried it & can readily see how it works. In my machine shop days, I bent quite a few parts using the arbor press method. You always have to go a bit beyond what you want to allow for Springback.
I certainly did not mean to imply that checking for straightness was a Bad Idea, just that there are also other factors involved, so proving the barrel to be straight is not an absolute guarantee it will hit where desired.
Also over the years, I have seen quite a few doubles, particularly lighter ones, which had the barrel OD enlarged a bit at the muzzle to strengthen the choke area. This gives them a bit of a swamped effect. Looking at them with a straight edge might cause one to think they were bent, when in fact they might not be. Certainly each & every factor has to be looked at.
