Tracer's do not ignite until they clear the barrel. People who say that tracers will burn up your barrel, eat out the rifling etc are full of poop.
Are they dirtier? Depends on what powder they were loaded with. Factory loaded tracers probably are not any dirtier than any other factory ammo.
Where do they get ignited then, if not in the barrel? From the friction with the air outside the barrel or the fire outside of the barrel?
They do not burn at full strength for a bit of distance though. The powder in the case usually burns a bit hotter than normal powder to reliably light the tracers. With the hotter burning powder it can be a bit harder on the barrel.
chuck
From a former USAF Combat Arms Training and Maintenance (CATM) Instructor. It depends on the type of tracer. The powder rate has nothing to do with "igniting" the tracer. The coating phosphorus or magnesium on the bullet is ignited by friction of the air against the bullet during flight! Some ignite as they are coming out of the barrel. Others (like what are issued troops for warfare, not training but warfare) do not ignite until they are approx 10 yards outside the barrel. They have a burn out rate of around 600 yards or so. Tracers do run hotter in a barrel rthan traditional barrels. Fire 20 tracers and 20 regular ball ammo form 2 seperate AR 15's and see which one is hotter!
Also Allied tracers are Red or Orange in color and Soviet Block countries are Green (usually) in color.