He was with a rifle company, in mortar squad. The company was over-run during an attack, at night: Marines in fighting holes, bad guys running around in the open, a very chaotic situation.

To eradicate the enemy, and save his company, he pointed the tube of his mortar straight up, dropped a shit-load of rounds down the tube, one at a time. The rounds went up (pretty far, but I no longer recall how high up they go) and came back down all over the place.

Risky, but Marines, generally, were in their holes, the bad guy were not, and so were more exposed to blast and shrapnel.

Exactly that, "his own people" including himself, which is arguably more brave than calling more powerful artillery fire on your own position. See PhasmaFelis comment in this subthread; this is the true last resort after final protective fire.

In that situation they flip a coin, you either get a medal or a court martial.