Originally Posted by CRNA
Why is that? Just wondering what damage that could cause.
One barrel maker stated he really doesn't understand the concept of cleaning with a bronze brush to begin with because running it down the bore is like lapping it further, especially over the crown.
You can skip to the 4 minute mark and listen to Krieger speak on cleaning and dragging a bronze brush against a crown.
Now as to changing directions of a brush inside a bore, if the brush is a good fit, it should be tight, so tight in fact that I don't think you should be able to change directions. I cannot tell you the mechanism of damage, but I've heard it from sources smarter than me that it is part of the "DO NOT EVER..." list. So, I don't do it.
To me, the real purpose of brushing isn't so much to rid the bore of copper but to break up the carbon/black build up and get rid of it.