More likely based of the semiautomatic AR-15 seeing that the AR-10 is a 7.62mm rifle
Exactly. The SR looks to be patterned after http://en.wikipedia.org/wiki/CAR-15 which was supposed to be a civilian weapon. These would be semi-auto, firing one shot at a time. I'd expect a Militia like the NCR troops to use this type of weapon.
Me, I've used the Varmint rifle most of the time for sniping, and the Cowboy Repeater for close combat. I got that Cowboy Repeater at level 4 or so. They're pretty common to find on Jackal Gang members south of Primm. I have a .357 Magnum and the weathered 10mm pistol, but never use them.