In tabletop, when barraging you can fire a normal weapon first, and then fire your ordnance weapon afterwards.
Here, since barrage is split up into 2 skirmishes, you can't fire your ordnance weapon if you've fired your other weapon first. This hurts a Death's Head build I'm trying out, where the plan was to use knockback bullets (From Walking Armoury) to line up enemies for the Railgun (line, ordnance).