MG XII : fine to shoot far away, precise enough, lot of ammo.
Bolter can snipe, MG XII have a better magazine. However, you cannot hit a carapace with an MG XII, cannot kill a bullwark with it and cannot clean a room with an auto fire that the MG XII doesn’t have.
Hellbore: versatile, precise, strong vs elites
Like the boltgun. Hellbore has more ammo in magazine, however it is slower than the bolter to kill a crusher… and it cannot pass through the shield of bullwark and has not the full auto of the boltgun.
Plasma: good against single target
Boltgun does it also… but can be also an horde cleaner.
From my point of view, Boltgun can get on same result of these guns…but has more uses at the same time.
Edit: (I’ve forgotten to add)
I don’t try to get a nerf for one gun… I push for more balance between guns.
But I am ready to listen to arguments what is the weakness of the boltgun if we compare it to other weapons.