I have seen this happen and not happen on the same rifle depending on the powder used. It happens often when I use Alliant powder and not when I use Hodgdon or IMR. Alliant also leaves particles in the bore. But if the load shoots accurately, I don't concern myself with it.

Looking at your buddy's case, it appears that he may have a tighter chamber, which could explain why sooting happened on yours and not his,shooting the same round.

But, as alluded to above, if your factory rifle shoots around 1 MOA or less, a little soot means nothing....Jim