in halo (i assume it's the same in doom 3) the gun has a little lcd screen on it displaying the ammo. sounds like a little bit of shader work to me.

in the lower portion of the gun's texture you could have a serious of little images representing the numbers 0-9.

when calculating the diffuse colour of the gun (or emissive) in the ps, you could figure out whether or not this pixel is part of the lcd somehow (off the top of my head, the best way might be to check if the texcoords passed from the vs are within certain predefined bounds on the skin) and offset that pixel by a certain amount depending on the ammo, which would be passed to the shader via a skill.

if you set-up different materials for different parts in MED, you may not even have to check if it is part of the lcd in the shader; just attach that shader to that part of the model.

i've been too busy to learn more about shaders lately so xXxGuitar511 could probably give u a better idea (or better advice) now that he knows what you're going for (assuming my guess was right).

julz