30 years ago when I was a kid I fired 10,000's of 38 spl's in a Smith 686. There would be the usual burnt powder, carbon and burnt lube forming the ring. A few swipes with a bronze brush and 357's would chamber just great.
BUT
Years later I bought a borescope when I started rebarelling my target rifles. Started scoping everything I owned. Looked at my Smith chambers at 20x power with the borescope. That ring looks downright nasty. Even in a mild cartridge at low pressures like 38 spl you're still dealing with 20,000 psi, high temps and both corrosive and erosive effects on the steel. My Dan Wesson 44 mag that had 10,000's of shorter 44 spec through it had the same thing. Ditto for my Python although it probably only had 10,000 38 spl through it.
I think we're seeing both corrosion which shrinks the bore (e.g. think of a rusty bolt which is tight not loose), and erosion which is loss of metal. Either way the changes are permanent changes to the chamber and throat and not just the build up of gunk that can be cleaned away.
It doesn't mean much for the average shooter or the average gun but may be an issue for very high volume shooters or guns. It's an issue for me as I intend to keep both my Python and Dan Wesson till my kid inherits them. Neither are safe queens so both now get only the long cases 357 mag or 44 mag.
I wish it weren't so but 5 minutes with a borescope can really change how you think about barrels, throats, cleaning, hot loads, chambers etc. etc. Rebarreling a rifle or semi auto handgun is no big deal but putting a new barrel or cylinder into a revolver esp a semi collectable one is a whole nother ball of wax.
Bottom line, never, ever buy a borescope.... sometimes ignorance is bliss, sold mine.
