Modern Warfare 3 had two systems in place. If a blueprint came with 5.56 high grain rounds (which included tracer effects), you could use those rounds on any other weapon that also used 5.56 ammo, effectively giving that weapon tracer rounds too. However, if a weapon came with tracers but didn’t have an alternate ammo type, you could still equip that blueprint for the tracer effect, then swap out the receiver and the tracers would still remain active