Just using the brush followed by sighting down the bore with a backlight at the other end worked well enough as an initial indicator for me. I didn't have a borescope at the time I first discovered this effect.
I would be happier with a close-up picture through a magnifying bore scope. I hope you don't mind if I do not want to take your word for it. I want to see it for myself.
Sometimes you get what you pay for, sometimes you only pay more for what you get.
Three shots are not a "group"...they are a "few".
If the Bible is the literal, infallible, unerring word of God...where are all those witches I am supposed to kill? (Exodus 22:18)