I made a similar thread on B.net to garner theories on this.
On Anchor 9 (and presumably Zealot) shots from the Grenade Launcher will actually rise when shot out in Space. Its all rather strange. I tested out most other guns and came to find that the Plasma Launcher shots, when unlocked, will rise too!
Grenades and other projectiles will not.
Can anyone shed some light on this, or is it beyond or mere mortal brains to comprehend?
Grenades were propelled by your hand at a specific velocity/direction, the pro pipe/plasma launcher were propelled forward only, so there’s nothing pulling it down. Even though you’re well within Reach’s gravitational pull. 
I wouldn’t think about it too much - the whole space setup isn’t scientific at all. For instance firing a sniper rifle or rockets should absolutely throw you in the opposite direction as fast as the projectile. Not to mention it should be utterly sielent and the jetpack likely wouldn’t work.
But it doesn’t really matter.
> I wouldn’t think about it too much - the whole space setup isn’t scientific at all. For instance firing a sniper rifle or rockets should absolutely throw you in the opposite direction as fast as the projectile. Not to mention it should be utterly sielent and the jetpack likely wouldn’t work.
>
> But it doesn’t really matter.
The man is right about it not being scientific. In actuallity though, you can´t even shoot a gun which uses gunpowder as the primary means of ejecting the bullet in space. Magnet guns should work, though.
I imagine Bungie put an upwards force on the Pro Pipe shots to stop them dropping too fast. Only in space, there is no gravity, so they move upwards.
As far as I know, even locked on Plasma Launcher shots will still rise dramatically in Space. Yet to test this on Condemned.
If you can set it up right, not only does it rise it comes flying right back at you! You should try it out its kind of funny. I’ve killed myself this way once haha