A couple of points of general knowlege to pass on to you...
They are supressors, not silencers. An M16 equipped with the supressor of the day could still be heard about 1/4 of a mile away.
Up until the Viet-Nam war, sniper units and doctrine were anathema to the US military way of doing things. Every time there was a war, sharp shooters were broght in and sniper's were utilized. Every time a war ended, the word "sniper" was stricken from every document and banned from both speach and thought. Pretty much
If he was a sniper, it is most likely he was part of a sniping "team", and therefore probably shared or rotated duties with others in his unit, including spotter, radio op, or security element. He probably had a CAR-15 at one point. He probably had several different types of weapons in fact.
You can't really compare the exploits of Carlos Hathcock with LRRP/Ranger/SF types as his military training was that of MP. Yeah, he crawled around in the dirt from time to time, but his primary missions were perimeter security.