From what I know (and from what is mostly reflected in game), a bullet’s damage is based on the square root of its muzzle velocity in joules. For a while, 50 BMGs were the exception to this, doing roughly 66 damage (less, actually, because their guns had a damage penalty).
Well, now they follow the pattern, and do 134 damage. Which is /great/ if you want things dead, but terrible if you want something to leave a corpse, like a Shocker Zombie.
EDIT: Also, try another vehicle. Like say, a Humvee or a APC. I just tried, and they spawn with no weapons attached, even with the Spawn Command.