An assault or battle rifle would do just the trick in closer surroundings, with a sniper rifle, you would have to:
A. Hold up your heavy rifle
B. Make sure to have a steady aim (if you are firing it while standing it can be very hard) by controlling your heartbeat and breathing
C. Aligne your scope with the target
D. Inhale, pull the trigger, exhale.
And fire power isn't that much of a deciding factor, because a battle rifle could just as well be using the same 7.62mm bullet as your sniper.