In a sheer stroke of dumb luck, I stumbled across some of the first data from StarHawk's private beta. As a result I've compiled this graph for you.
Now I know you're asking what does it all mean. Good question. First let's start with the source data (which I'm not linking to, it should be out to everyone soon.) This is based on play data from November 15th to November 18th of 2011. In that time there were 5286 weapon pickups (excluding repair tool pickups) and 950 deaths (excluding 4x4, knife, etc). That means for every pickup the players had an ~18% chance of killing another player with it.
From there it is broken down further, every time a rifle was picked up there was a staggering 45% chance of a kill. Compare that to homing missiles at about 5% and you can see that StarHawk has a thriving ground game. Absent from the data was any pistol kills, which may mean the player doesn't get one, or it is so weak that no one uses it. Also the grenade numbers may be skewed as the player may spawn with a couple of grenades like in Warhawk (similarly with the swarm missiles).
Just based on this data, grab your rifle, shotgun or mines early, they are well worth it. The rocket launcher just doesn't kill as often, which may indicate that breaking locks is easy in StarHawk or they simply are hard to handle against your fellow troops.
There appears to be a few other secrets in the stats, I'll keep mining away.