In the original CounterStrike game, you charge the M4 by pulling the forward assist to the rear. It always reminded me of starting a lawn mower.
Its usually the mechanics or shapes of guns that bothers me, no so much that 5.56rifleA does more damage than 7.62rifleB. A certain level of physics has to be sacrificed in order to make good/balanced game play. The only things that really bother me in that regard are heavy machine guns like the M2. The .50 cal's mounted to tanks and humvees in BF3 should be pink misting people, instead they take a couple hits and then dive behind a tree to take cover.
Also I like to play the single player stories in CoD just to see how ridiculous they get. Case in point, first mission of MW3 the entire russian armada parks withing hand grenade range of NYC. Should they really be surprised that someone swam out to their IBCM sub and proceeded to destroy everything with it?