Keeping with the current weapons, I think the gun should keep in the same style as the other guns as scifi/futuristic style. But I think if it were a rifle, that might overlap a little too much with the bolt rifle. Since they are somewhat similar weapons, that might not be a good idea.
So I think it should instead be like a futuristic flare gun or revolver. Since the "stake" arches with gravity, it sort of reminds me of how a flare gun looks when you shoot it. Maybe something like a flare gun, but revolver style and longer barrel.
It'd also be cool if the stake initially glows (or emits some kind of sparks) after it's shot but eventually cools down if it's stuck in the wall or something.
Maybe make the sound of the weapon like a sort of hollow but meaty sounding "thunk", similar to the grenade launcher but with a more fizzling type sound on top of it. Again, like a flare gun.