Aren't single-shot weapons generally more powerful than their automatic equivalents bullet-for-bullet IRL as well? I realize there are exceptions to this, but as a rule o' thumb...
Er...if we want to get all real-world, the rule of thumb is: a 5.56mm round through a 20" barrel is a 5.56mm round through a 20" barrel is a 5.56mm round through a 20" barrel. Unless some variable (bullet weight, cartridge loading, barrel length, etc.) changes, same bullet at same velocity which equals same damage assuming same hit location.
Assuming same barrel length, a bolt-action gets a
slight velocity edge but it's almost negligible, while it gets a notable accuracy edge that accounts for bolt action being "sniper's choice" overall.
Given same caliber, real-world automatic/semi-auto/single-shot weapons are the same
bullet for bullet but the automatic (or burst fire) ones are much meaner
over time- for reasons that should be abundantly obvious.

http://en.wikipedia.org/wiki/M1918_Browning_Automatic_Rifle