Hellguns have collapsible stocks.
Aren't grav and volkite weapons used only by astartes?
I do believe I have seen melta models that do in fact have stocks.
Hellguns do? Thats news to me. It's also surprising seeing as the current standard one is gyro stabilised according to the 40kwiki. None of the Cadian ones have stocks but that's probably because like the bolters it's hard to make the sculpt work.
Volkite weapons were a Pre Space Marine weapon so no I don't think so. Could be wrong though.
The only Melta weapons I've seen with stocks is the old metal (now resin) SM biker with one and I think one of the old metal Catachans. All the art work I've seen of them is stock less and all the plastic ones I've seen are as well. Again rule of cool in a sci if world used to justify anything.
Oh yea and Eldar Shurikan catapults don't have stocks either.