In the CSM codex, the Obliterators must pick one weapon for all the cult to fire in the shooting phase to shoot in that shooting phase. You may not use the same weapon in consecutive shooting phases. When overwatching with Obliterators, I always assumed that you MUST overwatch with the weapon you shot with in the previous shooting phase, but the description really doesn't say that. It says you must use that weapon for that particular shooting phase. It doesn't say you MUST use it until the next shooting phase either. So what is right, you must use the same weapon you shot with in the previous shooting phase or you can change the weapon for overwatch?