You hit the nail on the head, it is an anti-materiel rifle, designed to take out hard targets and the engine blocks of light armoured vehicles, not a tank which would be to well protected to be affected by such a small round (relatively speaking of course). Ghosts, warthogs and other such vehicles should be affected by the sniper rifle, but the tank and wraith should be able to shrug of twelve rounds easily before even having to worry about it. As for the smaller calibre weapons, you might be able to chip the paint and bust the windscreen but that's about it.