Imbrium said:
If you're using an M4A1 in a dusty/sandy environment and you're diving to the ground every few seconds and leaning the weapon on a wall on its magazine then it jams perhaps every 20 or 30 rounds, with real possibilities of a more serious malfunction, like stove piping. If you're using an AK47 in a clean urban environment, then it shouldn't jam at all.
Sorry to derail the train of discussion but I absolutely have to say wat.
Stovepiping in a rifle is caused by shooting from the hip, usually in combination with weaker loads or having your gas regulator set inappropriately (i.e. venting excessive gas in a hostile environment). And it isn't that serious. Stovepipes are usually cleared just by cycling the action manually. Something like, say, a double-feed would be serious. And AKs don't actually tend to jam because of debris in the action, but due to shit magazines with worn out/weak follower springs and such.
And an M4A1 should not jam that much. It may have reliability issues, but stress tests show about 88 jams for 6,000 rounds fired in 'sandstorm' conditions. 2 jams of those 88 would require skilled maintenance and/or parts replaced. So unless you're stomping your mags before loading them and dumping sand and excrement into the action at every opportunity...
Jams work in a game like S.T.A.L.K.E.R., where you're using a found weapon without replacing individual parts and it suffers from wear over time. In that game, a weapon in good condition would jam every couple hundred rounds or so, and jam-o-matics (every other mag) were the result of guns at 50% reliability rating. Trying to do anything more than weapon degradation would require to whole works (take into account firing mode, firing from shoulder or hip, ammo fired, environment, player's movement, number of rounds loaded in mag, etc.) or you couldn't really justify it. It's the difference between rewarding players for careful, realistic play, and a piss-off game mechanic.