I've been reading about this in my spare time for a couple days! I believe it caused by case necks being undersized for the bullet. Along with case shrink over time! Since new brass seems to be resistant,annealed brass is more resistant, than used not annealed brass! Dirt and residue acts like a release agent. Bench rest shooters have many theory's. Since I have never experienced weld bonding, I'll accept the little knowledge I've gained, and call it good! It's just not that interesting any longer!
Bench rest shooters are commonly seen carrying a tray of primed cases at the range for this reason. Any incidence of weld bond will/can change accuracy as the release on the bullet cannot be uniform or reliable.
