I have found quite interesting your request.
I have been working the whole day on this.
So I found some weapons that do instant kill on commons:
- M60
- Magnum
- All Snipers (Hunting, Military, Scout, AWP)
- Chainsaw
- Melees
Am I missing something?
Also, explosive/incendiary ammo seems to have the same behaviour (insta-kill) in any primary weapon.