When choosing Firenock lighted nocks you have an option use the weight match nock system.
There is roughly a 23 grain difference when adding Firenock lighted nocks. It can result in lower impacted points at distances of 30 yards and beyond, and also change your front of center percentages.
There is a simulated weight that attaches to your nock just the same as the lighted nock that will make all of the nocks weigh the same. This will ensure your arrows all fly the same.