Gas blowby does cause erosion. A few rounds-no problem. A few thousand would be another story, especially if rapid fired. (A hot barrel erodes faster than a cool one.) Try and see if you can recover a bullet and see if the jacket is expanding to fill the grooves. You can tell this under a magnifying glass if the land markings (the grooves in the bullet) appear to be cut in sharply and there is no blackening at the edge of the land markings.
I am not sure I can describe this kind of erosion, but if you look at any .45 Auto that has had a lot of GI FMJ fired through it, you will see dark areas on either side of the lands. That is erosion, caused by gas at high pressure passing through the gap created when the jacket does not expand to fully fill the groove. (The .45 is notorious for this because of the heavy military FMJ bullet and the low pressure. Think of trying to push dough into a bread pan and not pushing hard enough to get it into the corners.)