I've never paid much attention to those, but when i saw that sling deals 1d10-2 I damaga, with primitive (5) I realised something is reaally wrong. That means that sling deals on avarage 2,9 damage. That's not enough to harm a fit human which has TB of 3. I'm not even mentioning armour. Thats stupid. Sling were used for a long time as a lethal weapon in ancient armies. And here you have to stand in front of someone for half an hour, and throw a stone (or metal balls) after stone at him to kill him. I know it can be used to throw grenades, but still.
Hand bow - same thing, primitive (5). It cannot be used even as a silent assasination weapon, because darts are too weak to deal even one dmg required for toxin to kick in.
Bow - primitive (6) - useless piece of junk. You'll need a full quiver and a lot of luck to kill someone. I would give it around 1d10+1, primitive (8)
Finally bolas - why it has primitive (1) quality if it doesnt deal any damage?
I will point Slings explictly mention the damage changes depending on what you're using as ammo, which offsets the damage issue a bit. Also, all Primitive weapons still Righteous Fury on rolled 10s, which also makes the potential for damage a bit higher.
Honestly, I don't see an issue with the low damage. You're probably right that slings were used in ancient combat (I don't know enough history in that sense to say) but there's a counterpoint I'd make today: when was the last time you heard of someone using a sling in real-life to attack someone?
There's a reason why people don't walk around using bows, crossbows, etc. in real life, and that's because guns are so commonplace and much better. Likewise in the 40k universe, las-weaponry is ridiculously commonplace, and SP weapons even more so, since this is a civilization that is ridiculously highly militarized. In most campaigns you're never ever going to pick up a Low-tech weapon, and why would you when the standard Guardsman is toting around a laser gun and elites are using fully-automatic self-propelled explosive-tipped grenade launchers? Look at the availability: a Lasgun is just as easy to find as a Crossbow.
No one would use a Hand-bow as a silent assassination weapon, because needle weaponry exists (and is much more effective, considering that anyone you'd want to assassinate is probably wearing at least half-decent armor if not a protective field).
The other thing about primitive, especially in regards to the regular bow, is primitive (6) means that half the time you are rolling a 6, and thus on an unarmored human you're still dealing 3 wounds a turn, and assuming the average humans has less than 10 wounds, that's pretty significant damage. And if they are armored, you're back to the point that you shouldn't be using a low-tech weapon to try to kill them, anyway.
I think you are right about the bolas, though, that seems like a mistake that the primitive quality is there.