There's 2 sections to the front cockpit transparency. Only the section above the pilot's head fires on ejection, while the front section also fires if you use the fracture handle. That's the bit that would deposit big chunks of perspex in your chest.
Interestingly, I seem to remember the aircrew manual saying that there was "a high likelihood of serious injury to the front seat occupant during any ejection" or words to that effect.
