If I’m not mistaken, bolters and rockets work differently. While rockets are directly fuel-propelled and explode (or deliver their load, whatever it is) on contact, bolters are first shot through the boltgun with a solid propellant, like any other pistol ammo, and THEN their rocket propellant kicks in ; plus, bolters have their explosion delayed, as the impact punches through armor in order to maximize the power of an internal explosion.
So rocket launchers are only weaksauce-bolterguns, I’m afraid :(.
Though I kinda dig the idea of a rocket-size bolter which could penetrate a vehicule and then explode, decimating every crewmember.