BUT they don't have to! There are such things as "Dark ignition" tracers in which the tracer element does not start burning until it is 25-50m from the muzzle. A 4:1 ball:tracer mix is recommended to troops as a standard mix for M16 magazines, but all references I've found on US service 5.56mm rounds makes no mention of dark ignition. One veteran soldier was unaware that such rounds existed, but thought they were a good idea.
The same search indicated that both the NATO and US ordinance colour codes distinguished dark ignition tracers by giving them an orange rather than red tip. The M856 round has a orange tip, so hopefully this indicates troops are using dark ignition rounds. Recently I saw the recollections of some SAS and LRDG veterans that served in North Africa. They were very impressed that the Germans and Italians used several different colours of tracers while they had to make do with just one. In many armies squad leaders are encouraged to carry a magazine of tracers for target indication. There is a case for issuing these in a different colour to the tracers used by the rest of the squad. These rounds probably shouldn't be dark ignition. Another tip is to have the last three rounds in the magazine all tracer to alert the firer that he is out of ammo. These rounds probably shouldn't be dark ignition either, and maybe a different colour too.
UPDATE about t Recently I saw a program where an ex-SAS man talking about the GPMG stating that the 7.62mm round in British use ignites at 70 metres. By coincedence Rick Randal contacted me the next day with the following observations.
"US 5.56mm and 7.62mm seem to light up about 10m downrange. I think the delay isn't supposed to be much, as the standards were set during the 1960's, when using tracers for hip-fired full auto bursts was considered a good "night fire" technique at close range. Frankly, tracers are really most visible the closer you get to 90 degrees from the gun-target line -- and I've never had much trouble looking for the disturbed vegetation and dust from the firing position under those circumstances. Of course, I've never done this with 50-75m of ignition delay. . . but that much delay starts to affect the ability of a small unit leader to use tracers as a "short range data link", as the SciFi author David Drake once described it. . . The target of a tracer burst is going to fire down the back azimuth (if they don't panic) -- obviating the utility of delayed ignition in that situation. I liked a 20-rounder with straight trace as an NCO, for fire designation. I could instantly tell by touch what a magazine was loaded with. Never liked the use of the last few rounds as tracer to tell you to reload -- an M16 recoils very differently and sounds (when you're shouldering it) slightly different (the buffer spring goes SPRONK! instead of SPRANG!) on the last round anyway, and I remember the old stories of bad guys using the Garand's clip ejection noise as a signal to charge. Plus the fact that the M16 has a automatic bolt-hold open on last round anyway (which is why it feels and sounds different -- the action is stopped halfway through the cycle). Liked the idea of MGs having a 25 round or so teaser belt of straight trace -- not for "movement to contact", but for disrupting an assault. . . since tracers all seem to look like they're laser designated on YOU, a long "Hollywood" burst of straight trace has been noted to cause assaults to falter (learned that one from a Vietnam 1st Cav vet I served with when I was a private)."
This suggests that conditions in Vietnam have caused the US to issue tracer with no/very short delay. NATO small arms codes use Red for a tracer with no delay and Orange for tracer with delayed ignition. 5.56mm M196 rounds have a red tip, while M856 has an orange tip. I don't know if M856 exhibits any appreachable delay.