Heavy weapon marine
As you can see the heavy weapon marine for my tactical squad has been given knealing legs (from the command squad set I think). Firstly I think it adds some real variety to the squad in term of height and pose. Secondly it just seems more natural, I wouldn't want to try and fire something that big from a standing position.
Again he just has the DA shoulder pad to which I'll add freehand to make into the Astral Knights chapter badge. I'm thinking of adding extra equipment to the left thigh, maybe spare missiles or just a pistol holster etc.