Believe, Doug, that you may find that the Lancaster BP double 500 is an "oval bore," and does not/not have rifling. Lancaster used the oval bore (rather than round with lands and grooves) to impart a spin to the bullet and, if I recall, only abandoned the technology in the early 1920s due to the high velocity of bullets not stabilizing adequately.

Don't be put off if you see no rifling as the oval bore works very well indeed for rifles of the vintage of your own.

Regards, Tim