What happened in his opinion was that the barrel got so hot that it shrank and the chamber was so hot that it caused the powder in the case to change it's chemical make up, I am assuming that he knew what he was talking about.
I'll have to call BS on your gunsmith. If barrels got so hot the chemical composition of the powder could be changed, belt fed machine guns would be blowing up every time they went into combat. Also, anyone who has any understanding of the properties of metals (which most gunsmiths should) would know that steel expands as it gets hotter so the chamber couldn't shrink. I'd find another gunsmith.
