It’s called Infernal Projectiles.
I always get it (except for her P3 which makes it unavailable). I tried to compare them side-by-side, by looking at the blast area before and after the upgrade gets done. But unless I’m doing it wrong, it doesn’t look like it’s anything significant. Thinking I should be saving the100/100 towards, anything else really.
I might be the wrong person to ask since I prefer snipe Nova. I only get if for bane/scourge but I also noticed that the targeting reticle doesn’t change and I thought I was just seeing things or it’s just bugged and the upgrade really works.
I’m not even sure if it affect the weapon or the ability, but I rarely get it nowadays, even with infested maps where Shotgun Nova shine.