That was the one thing I absolutely loved about the Unreal Tournament mod "Tactical Ops".
It was essentially a CounterStrike clone, but with a better physics engine (I still despise the original Half-Life engine) and little things such as buying ammo in magazines. You reload with a half a mag left, you lose that half mag.
Honestly, this is one of the things I really wanted addressed in Fallout New Vegas' hardcore mode. Sure, you've got a 10mm pistol and rounds, but how many mags do you have? Those things have weight too. When you reload do you drop 'em or pocket 'em? Takes longer to pocket 'em. When do you stop to load rounds into your mags?
...et cetera.
"Hardcore", my ass. That should've been normal mode.
.
..
...wait, I think I read that wrong...
anyway, as off-topic as they were, I stand by my comments.
OT: ...what? Have you ever studied how semi-automatic weapons eject casings..?