The ammo counters in UT arent moving parts, just a sheet on the side that gets a new texture slappen on it each round fired, its far from the same thing.
For a better comparison, open the INF-Ued, and then the mesh browser, and look at the reload animations, you can see that the magazine is animated to fall aslong as its in view, then it magically reapers in the free hand, and then its animated so the hand slips it back into the weapon, its the same clip used as the one that was taken out.
Allso, look at the Unreal Automag, the muzzle flash is modeled, and if you play in software mode, there will be a BSP hole where you can see its just moved back and forward into the gun.
Its the only way you can do it, as far as i can see, as the full mag is too long to be inside the weapon.
One could allso make it a series of textures outside on the mag, that changes like the ammo counters on the UT weapons, but it will look fake, and just cause even further slowdown, IMO, it issent worth it aslong as there is an none-trancparent alternative.