I tested all other guns with Advanced Round ammo to see if it was ammo type. I think it's just that Dark Matter doesn't have a sprite for its projectile where other guns do. Either that or the round travels so fast that it can't render, I can't tell since either way it's invisible. I did apply heavy_shot 3 to see if I could slow the round down for it to render, and it certainly travels slower than a default marlin round, which is visible.