To me, the chamber throat and the crown are always subject to wear from various things. The guys have given pretty good senerios of what will get these two areas.
Heat, rust, scraping with hard metal objects is not good for a bore. Typically barrels are fairly soft in makeup. A bit of exception to the relative softness of barrel, is the LW brand that is considerably harder.
Hot rounds(mainly rifle types) that have a lot of powder capacity for bore size, will kill a rifle throat in a few hundred rounds, especially if shot repeatly, quickly, and get the barrel overly hot. The 22-06 comes to mind in that regard.
Some folks will use a bore guide that keeps the rod off the throat, and environment dictates that design of said guide. I prefer a coated rod myself, but others are ok; if used with a proper guide.
One thing about a bore guide, if the brush will go thru it, that guide really is useless. A brushing guide should in my opinion have a brush release area, and closely fit the cleaning rod of the day. Cleaning from the crown side is more critical than from the chamber side to me. Obviously machining the guide concentric to the bore has its challenges, as does the methods of applying the guide to the firearm, especially if muzzle first.
Sometimes if one uses a brush on a rod to clean the barrel from the breech, once the brush gets thru the crown area, the rod drops down on the crown, then the rough edge of the rod/brush interface bumps over the crown on the backstroke and dings that crown that someone spent hours on cutting. Some benchresters will remove the brush or jag on the backstroke, painful but they feel necessary in their game.
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