The only possible danger with any bronze brush would be if you stop it's forward motion and then reverse the brush and pull it out while it's still in the bore or cylinder. Damage isn't likely, but when the bristles are bent in the opposite direction, the tips are pressed tightly against the metal in either place. I have doubts that any damage could occur unless using a stainless steel brush, but IMO it's still best to push the brush completely through and allow the bristles to relax before changing direction for the pass back through. I have used worn bronze brushes rotated in a slow drill to help clean badly fouled cylinders (near the shoulder at the front of the cylinder. But seldom is that necessary if cleaning is done regularly as it should be. I tend to use a nylon brush first, if I use a brush at all. The secret is to fully coat the interior surface of the bore and cylinders and allow it to sit and soak and soften before further cleaning actions. A good bore can usually be cleaned with just dry patches unless it's been allowed to go too long. And it never hurts to use a wet patch, a bit of a soak, and then a few dry patches after any long shooting session. If you do that, it won't be necessary to use a brush at all in most cases. My experience, FWIW!