Re: Resizing brass from 308 t0 260??
Riley's concern seems the most probable culprit. When brass is downsized, it gets thicker. Your new necks as loaded could be too thick for the chamber neck, interfering with bullet release.
Maybe, maybe not, but:
That you didn't check this before letting your son fire the ammo, disqualifies you as a competent reloader, and certainly from father of the year in 2012..
Have a gunsmith look over your action ring and barrel/barrel nut for potential damage.
Buy 260 ammo, or at least use 260 reloading brass, until reviewing/understanding basic reloading.