In the weapon class for the gun the tracer interval is specified. If one gun has, say, every 4th round set as a tracer and another gun has every 2nd round as a tracer, the latter is going to look rather more impressive.
Another oddity. I've found that if a round on the belting has power value of zero, it may not create a hit effect. In my own re-jigging of gun classes I've assigned at least a small non-zero value so as to have a hit effect.
Indeed, of late I've spent much time on better rationalizing the way bullets (including up to the biggest ships' guns) are handled. There are two methods which interact in what I consider to be a weird manner, of which I could write a small book. Not that I'm an expert, and I'm sure to be mistaken in some area or another. But my approach has rectified a number of strange behaviors. For one thing, formerly it was the case that larger gun shells generated just a small hit effect, essentially invisible. Now big shells make a more suitably impactful hit.
I'd like to see some expert eyes give this whole matter some scrutiny, as I feel it's a very worthwhile area in which to rationalize and streamline what could be legacy quirks resulting from the ongoing bolting on of new functionality as the game evolved in its early years.
But alas, this is something for which requisite enthusiasm might only have been engendered a decade ago. Our modding community is now but a shadow of its former prodigious extent.