If you look at the Wolfe publishing website, you can backorder the Sept 2012 issue of Rifle magazine, that describes how not to harm bores, muzzles etc by cleaning. You can get the original copy or also download. The article is page 24, The Other Part of Bore Cleaning. It's the best description I've read, and explains for instance, marks in the mid-bore caused by flexing cleaning rods, wear at the leade caused by tilted steel rods, etc.

In terms of muzzle damage, if you rip a jag back into the bore, it's going to act like a rasp. A smallbore shooter, I know, (which means an indoor .22 long rifle, 25 yard shooter to those not familiar with the term as used in the UK, Australia and NZ), will unfailingly remove the jag from his cleaning rod rather than pull it back through. He's a champion shot and so who would argue with him. I personally lift any calibre jag and/or the brush so that it enters the bore centered. I also use a bore guide every time. If I can't use a bore guide, if it's not a bolt action, I often use a case that's been drilled and tapped for a Stoney Point, (now Hornady), OAL gauge. This can be slipped in the chamber of a double rifle, for instance, and helps protect the leade. Each case is custom fired in the gun before being drilled and tapped.

Some jags are worse by their shape than others and a critical look at them when they're protruding from the muzzle, especially at any sharp edges will give you a new perspective on what to buy.

In my last post, I mentioned that my gunsmith removed 5mm of each of the barrel crowns of my Bock-Drilling, but left the middle support untouched. So there's a slight recess for both barrels. Having inspected it again, I'd say it's more like 3mm, and I'm sure he would have removed only the very least necessary to remove the crown marks that affected accuracy.