I second the .20 in the marui/vstanks.

I tried to install a tracer unit on my abrams a while back. I put the lighting unit from a marui tracer(I got one on mistake with an order a while back) inside the ammo well. looks pretty good, it makes the bbs glow for about 3-4feet past the barrel. I don't think the light is strong enough to "activate" the glow when dispersed in the well. It was just one of my experiments before I got the IR versions.