With the minigun that I made a couple years ago, I had the rounds disappear ~20seconds after being fired. If you haven't hit anything by that point, you probably won't be hitting anything ever. It was running ~600rounds per minute, I think, which put 200 rounds in the air from a single gun at a time, and I never noticed slowdown if vessel labels were turned off. With vessel labels on there was a definite framerate hit.
It might become an issue if you have several things all firing at once, but with just the one there wasn't an issue, even though each round wasn't just a "dumb" config vessel--each timestep, each round was doing calculations to move itself (to avoid plasma trails in atmosphere) and determine how much it should stretch (to give the effect of the round travelling several meters during one frame of video).