So the military loads tracer as follows, 4 full metal jacket rounds then 1 tracer, AKA the 4 to 1 tracer ratio. Then the last 5 be all tracers. For magazines that are multiples of 5 rounds, the last 6 rounds would be tracers because the last 5 rounds are just after the last pattern tracer. I feel like this would be a really simple mod to make, although I know nothing about modding. It would add to the immersion too.

Edit: I'm dumb, I originally said 3 FMJ, 1 tracer. That would a 3 to 1 ratio not a 4 to 1. I fixed this brain fart represented on screen.

Second edit: Added the bit about reload indicating tracers.