9mm is ineffective.
.45 ACP HydraShoks are the better tool. Lots of stopping power and lots of damage.
If he only had seven rounds, that sure sounds like a single stack .45 ACP. Question is what ammo, and where did he hit the threat?
Nine millimeter is okay, but shot placement is critical.
I'd never heard of a 9mm with only seven rounds. I assumed it was a .45 ACP weapon, and should have dropped the suspect with 1-2 shots. 
Kel-Tec made the PF-9, which was a dandy little back-up pistol, and eminently concealable. The standard magazine held 7 rounds (with nothing up the pipe).
The 1911 .45 ACP carried six, but I carried mine with one in the chamber, making for seven total.