School Circle, boots! Here's THE WORD!
Back in 1975, when I was at USMC Officer Candidate School, we were issued M14s, and taught how to use the flash suppressor and front sight to break the wire binding C-Ration cases by combat vets.
For whatever reason, when the Marines developed the -A2, they insisted on a thicker fore-barrel, while retaining the pencil profile under the handguards (the part, BTW, that bends under sling tension). This, presumably, was to accommodate the mounting for the M203, which was spec'ed for the M16A1 barrel profile.
As an aside, here's an Army document giving their opinion of the USMC M16A2 changes:
http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf
A very interesting read.

