Chuck, old time m/l rifle barrel makers would check for straightness with a bow and a string of black cat-gut. With the breech plug out, of course, the string would be passed through the bore end to end, then attached to the bow which would stretch it tight. Then, looking through the bore towards a light source the string could be placed alongside the bore as it was rotated, revealing any lack of straightness. Barrel would be bent until the string revealed a truly straight bore.
I'm sure this method required somewhat of a learning curve, but it seems to me it could be easily employed to check for bore straightness. I've always looked at the concentric circles of reflected light, but I admit it's got to be off pretty bad for that to work, for me.
Best, SRH
