(Not directed towards ColtM4 as I agree with him)
I see alot of REMF's in here that speak about that which they do not know. The USMC places great value on marksmanship, and the M16 platform is truly a marksmans rifle. It makes for easy follow up shots, extremely accurate firing, and strong enough rounds. The people who seem to bitch about it the most are generally not military personnel. If you clean it, she will kill for you.
Now, onto the op's original point. What do you think the M14 is? Its a refined version of the M1. Have you ever held an M1? Its heavy. Now imagine carrying 10 20rd mags full of .308 and a select fire M1 around all day long along with a pack, wow, that would be heavy. The M1 was a great platform, but the M14 was greater, and its even making its way back into the Corps today. Its the new DMR and makes for a fine weapon.