mete, how would one know? Actually the chemicals in a tracer are activated by friction and the presence of oxygen so I'm sure there is some glow in the barrel but more out of the barrel in the air.
Standard MG ammo loading for tracers is 1 in 5 (tracer, four ball, tracer, etc.). They are used for aiming your "cone of fire". Now get this, the "training" load for the 25mm chain gun on the M2 Bradley is 100% tracer! ...and that "will" erode you barrel faster! Also tends to set the training range on fire in the Fall!
Once while observing M1 tanks on the training range, an Abrams took a shot with the tungsten penetrator used for training (depleted uranium used in combat). The penetrator (normally quite invisible since they travel at over 3000 ft/sec) hit a berm made of dirt and rock, ricocheted almost straight up, glowing like a red hot poker! Happened at dusk so you could see it quite well.
Bruce