If an Infantry company gets ambushed by an MG post, after going
for cover, they aren't going to be just shitting on the spot. They
should respond with 40mm fired in the general direction of the MG,
follow by a law rocket along with the support fire from the rest of
the company. And while this is going on the sniper & spotter
can move to an area where they can have a better view of where
everyone is before scoping for the MG gunner. 
BTW, today the role of the spotter in U.S military is usually
shared by the 2 men sniper team. This is to reduce the stress to
the designated sniper. However, only military snipers should
practice this habit. Police sniper should never share a single
sniper rifle. 
In U.S, the 'spotter' has a free range of arsenal that he can chose
to carry as long as he is comfactable with it during his mission.
Most U.S army sniper/spotter prefer to carry a M14, M16 &
M16/203. U.S Force Recon sniper/spotter would usually either carry
M4 with scope & silencer( optional ), M4/203, H&K MP5SD,
etc.
Ooh! Just for everyones info, .50cal sniper rifle is being widely
during the start of the Iraq conflict. And its sure as hell was
used for anti-material. Marines prefer to deploy 2 .50cal sniper
gunners & 1 7.62mm gunner as shooter/spotter.