>>96756119
See:>>96678527 >>96686074 >>96686468 >>96690616
I'm not well versed in 40k but watching Astarte, their "gyroc" doesn't seem to take time accelerating, it's just a cool visual effect.
So I'm supportive of the idea of using a shotgun without stock loaded with HEMP and call it a bolter.
Now, looking up the internet says a standard bolter carries up to 30 rounds while the shotties only carries ~10, so I also suggest a modified Assault Cannon as a bolter.
As for skill, I'd use alternative specialties from Pyramid #3-65, that'd mean your Space Marine lugging the one handed Assault Cannon (minST 40) could hoot it with Guns (Pistol).
More accurately, you'd use the Payload technique but I'd say that's too realistic for a 40k game, Just ignore, call it the effect of the rocket propulsion if you will.