470evans,

that what you see is only apparent towards the muzzle has something to do with the fact that your gun is tapered - the closer you come to the muzzle, the weaker the barrel becomes.

Now, whatever I have read here about what some people call OSR are classical signs of a barrel that has been exposed to a level of pressure that has expanded it to a point where it cannot spring back fully to what it was before. Even what some people call a test for OSR (looking inside that barrel at a angle) is a classical test for that with gunsmiths in my country. Now, as I pointed out before, hard bullets can create far steeper pressure curves then other bullets, add to that a round where the bullet's ogive accidentally touches the rifling (had that with factory ammo in my .404 quite often) and you can have such a rapid rise in pressure that older softer steels, especially in thin barrels, cannot spring back fully anymore. And don't forget that some barrels are duds, they might not have been hardened correctly in manufacturing to begin with. With benign loads (and by that I mean a shallow rise in pressure, rather then a weak load per se) this might never be an issue. With a steep pressure curve, however, it will bring out faults.

As for the claimed 'shadows' of lands showing outside the barrel and the like: That is an easily understood phenomenon, and has nothing to do with a bullet pressing out the lands. Imagine a barrel that is cut open lengthwise and rolled out before you. What do you see? Something like the profile of a car tire, the lands stand out, the deeper the grooves are cut the more the barrel has a 'profile'. Now imagine you could tear like the Hulk at the sides of this rolled-out barrel (this represents excessive pressure). The grooves, being cut into the barrel, are the weakest link (the barrel is thicker where the lands remain) - therefore the weakest link (the grooves) will stretch first (but certainly differently, -this for people who now start thinking about cut-rifling versus hammer-forged). Do this with enough force and you will see the claimed OSR shadows on the outside of the barrel. They are simply stretch-marks alternating with stronger sections of the barrel. Hence the visible lands / grooves pattern.