Tracers would make sense for:
- Regular magazines
Piercing rounds magazines
- Cannon shells
Explosive cannon shells
Now, this might cause performance issues in instances with a LOT of machine gun fire, but I doubt this on account of Factorio being very well optimized. Besides, I think the devs aren't afraid to add effects since the smoke effects are getting an update in 0.13.
Lastly, while tracer ammunition uses certain chemicals in real life, I don't think any recipes would need to change in Factorio because it would add unnecessary complication. In other words, it would just be a nice effect to have.
TL;DR Add tracer effects to Factorio.
Thank you for reading.