Originally Posted By col132:
Boy, didn't realize I was asking such a tough question. AR15 site, Bushmaster forum. I'll try to find out and let everyone know what is on the side of their guns.
Military coding: M-16, M-16A1, M-16A2, XM177, etc
A= is Accepted (type approved) change
E= Experimental change
XM= Experimental Model
S= Non-military coding for Semi-auto
thus XM-15E2S is an attempt to make it sound
"governmental" Experimental Model (AR)15 with the A2 improvements i.e. fence around mag release, thicker receiver ring at buffer tube, etc.
Sorry this is not the most technically correct but I don't feel like looking up all the "official" break down of type designations and approved generational improvement coding lingo. Hope this enough to satisfy your interest.