25gr. of H335 is a pretty common load for .223. The case is almost full when loaded with that charge. There is enough space left to dangerously overcharge, but not to double charge. Fortunately, I gave my Mini away some time ago, so I don't know whether a Mini can fire out of battery or not. Another possibility is that he fired a hot round behind a squib.
I almost had that happen to me with my AR this past week. Bullet was lodged about three inches into barrel. Hammer for second shot was cocked, but action had not cycled sufficiently to extract/eject spent round.
Makes you want to wear kevlar gloves and eye protection, doesn't it??
Shoot to kill; they'll stop when they're dead!