Had the pleasure of being able to walk around and inspect a M16 at OshKosh EAA flyin last year this time.
What really struck me was the LACK of armour.
Gunner/loader in back would be vulnerable to a farm kid with a bb-gun much less 50 cal MG bullets. Driver had fair protection, mostly from the sides.
Engine looked to be pretty well protected, and I'd guess the halftrack chassis would be pretty sturdy. However it should be very vulnerable to flat tires on the front. Which should make steering difficult, and cut speed.
I have 3 pics here if anyone is interested.
http://www.users.qwest.net/~jamisoncindy/m16.html