Splash damage is currenty negligible to nearby targets or at missed shots, but instead with the current falloff will make the weapon a 4-bodyshot-kill at any/nearly any distance, since it increases the damage just enough to overcome the direct damage falloff. Not sure whether this was the intention, but a larger trade-off / sidegrade could be interesting, though the current version has its very own peculiarities with explosive damage resistances. Description points to the direct damage being reduced to 250 wrongly, as that is the weapon's usual direct damage.
Oops, meant to say that the flat explosive damage is just enough to overcome the falloff, that usually makes it a 5-bodyshot-kill weapon.
Pretty sure it's a bug. I believe I read somewhere that they just copy/pasta'd the Dragoon's explosive ammo on PTS onto the DMR-99 as a placeholder just to get the UI elements and such built around it. They're planning on going in and correcting the damage values before it goes Live. So right now explosive ammo does a flat 250 damage on the DMR at all ranges. And since explosive effects are bugged in PTS's VR, it does 0 explosive damage.
