Every Bullet Is A Tracer: Justified for non-slugguns, inasmuch as modern slugthrowers kick their tiny dust-grain bullets up to such a high velocity – a respectable fraction of c – that they plasmate the air they hit on the way to their target.  The bullet doesn’t glow, but the resulting plasma bolus does.

(The disadvantage that this has, that firing gives away your position unless you turn the muzzle velocity way down, has been noted by the relevant tacticians.)