So my Hail did, at max damage, 229 damage to a depth 28 zombie.
My Nova did 273, at max damage, to a depth 28 zombie.
So I punched in calculator math and, in the "hardest depth" (which you should account your gear for mostly. In other words, depth with most health etc), the raw damage from the bullets from the Hail did 16% less damage than the Nova's bullets.
So... without statuses accounted for, the raw damage is roughly 15% less damage in comparison to a Nova when using a Storm / Magma / Hail.
So are we safe to never say "30%" and "25%" less again?
hmm, yeah but try to test it in other depths to confirm it.
plus i didn't know the community said it was around 25% ~ 30%.