Hmm, the standard loadout for a real gun is 1:5, IIRC. I've never even heard of firing 100% tracers until now.
EDIT:
Tracers are usually loaded between one in four rounds to one in six rounds. Platoon leaders will sometimes load their magazines entirely with tracers to mark targets for their men to fire on.
Apparently, sometimes they use 100% tracer rounds to guide other gunners to the target. But, as it states, usually it's between 1:4 and 1:6. Anyways, for BtRL, this is all moot anyways, because you should probably stick with the show's reality. Although, I'm curious - does BtRL have the same optical illusion on the recieving end referenced in the wikipedia article?
For those on the receiving end of tracer ammunition, there is a well-known optical illusion whereby the tracer rounds appear to be travelling slowly, but as they get closer they speed up considerably
(Interesting article, BTW, good reading.)