In the Army, with bayonet/CC training we were taught that if we're using a combat knife or our bayonet unattached, that the arm pit is an excellent place to stab. One, there is an artery there and its very close to the skin. An inch of blade will be a near fatal bleeding wound. If the knife is long enough 6-8 inches, you can puncture a lung or if you're on the right side, the heart if you angle it right.