Wow, that is a wonderful piece of detail. I feel better using the modifier now that I know the historical context. I'm loving the game even more. Thank you Ben and Brian.
The modifier is from a trained drill performed by the attacking battalion. It was an aggressive rush of all hands screened by the smoke generated by the salvo. The salvo drill benefitted the attacker as much as any particular effect the salvo had on the intended target, no need to track the target.