Well a couple things, fast firing guns usually have a lower weapon damage, which in turn will lower SF damage and FitSD damage. It makes up for it with RoF but in the end, it will end up being roughly the same (except high RoF means lower accuracy).
Now you’re also seeing fire against flesh damage, which will make FitSD look like it’s doing higher damage compared to a weapon that’s not matching flesh bar. The math just simply checks out though, feel or not. While FitSD is NOT the worst skill, its first point invested is much much better than the following 4 so for most builds that want to do some serious work invest points in better skills. Nobody is claiming it’s TCP bad or anything, so take it if you like it.
So something I just learned today is FitSD 1/5 does the same damage dots as 5/5. The initial (weak) 3% or 15% is nearly worthless, while it seems to take on the full dot of the weapon, grenade, or hardpoint that procs it. This is regardless of how many points you put into the skill. So this is huge for only putting a single point into the skill. However I didn’t test this, and I’m not sure if the one’s testing it have more to look through. I’m sure they will let people on here know once they figure it all out for sure.
But, this would explain why the Nukes aren’t seeing more damage from it when you invest more points, you would have to spot the initial proc of FitSD from the Nuke, the dots will be the same 1/5 or 5/5.