After shooting rifles over many years I have seen lots of weird quirks. Although many variables can occur as the result of ammunition changes and such much of what goes "wrong" has to do with the rifle barrels themselves. A pretty difficult task is to visually examine the barrel and the rifling itself until now.
Arguably there have been barrel examination devices of varying quality before the Hawkeye Precision Borescope came unto the market. The Hawkeye borescope is without reservation is a high quality precision optical instrument. A product of the Gradient Lens Corporation based in Rochester, New York, the Hawkeye has several variations and attachments to inspect bores of both long guns to include rifles, shotguns and handguns. Even though a precision tool, the Hawkeye is easily assembled, maintained and applied to it's main function of running bore and rifling checks.
Although there is a running gun battle so to speak going on...