Looking at some old DTIC reports on the M16, going to the A2 length stock had the advantage of "conformity to human factors standards by lengthening stock (alleviating bruised eyebrows, noses, and lips)". This according to USMC feedback on the subject. According to the report:
"The decision to lengthen the stock was made after all portability tests (entering and exiting aircraft/vehicles/buildings, etc.) had been completed and without consideration for body armor, field jacket with liner, load bearing equipment, etc. This change results in the M16 trigger to butt plate length (the test report states 14 inches) being approximately one inch longer than the Springfield, M1, or M14. The Army problem was that the existing stock was already too long for many soldiers (mostly females)."
Out of curiosity, how would using an M16A1 (or AR-15 with A1 stock) result in facial bruising? I've never known anyone that had such issues with the A1.

