Here's what I've observed in battles before and after I had the Platoon Firing tech:
With just the Rank Firing tech, rank 1 fires en masse and then kneels, when rank 2 fires a volley and then kneels, followed by rank 3 firing a volley and kneeling, at which point all three ranks are reloading. Rank 4 does not fire, only the first 3 do. Even though this is better than before I had the Rank Firing tech, when the entire first 3 ranks would fire as one followed by a long reloading period, there's still a period where the unit is not firing because all shooters are reloading.
With Platoon Firing, behavior is a little different. It's worth noting that I typically deploy my line infantry in 3 ranks so that they all can fire. It looks to me as if about 1/4 of each of the three ranks fires a time, so I see some shots from area A, then some from area B, etc. The result is the unit never stops firing. They do not have the same concentration of fire they did with Fire By Rank, but the shots are constant and they seem to cause more casualties as a result. Perhaps that's because targets aren't doubled up nearly as much.
Bookmarks