Massing fires means getting 2 or more firing units to attack the same target.

These firing units are generally batteries or platoons, but I have "massed" 2 demi-platoons- in Afghanistan, we split each firing battery into 3 2-gun "platoons".

The thing that makes it tricky is that, by being in 2 separate locations, you increase the # of variables- massing is an exercise in mathematical elimination of error- getting everyone on a common database. There is not magic to it, just mastery of gunnery.

When I was a LT in the late 90s, we practiced it extensively- massing the BN every time we went to the field (battery based organization, with only 3 firing units, made it not much of a challenge), and practiced passing tactical control of the BN from the BN FDC to the BTRY FDCs, so all 3 of us learned how to control the massing of the BN.

The cannon units did it when I was in Korea (01-03), and my BN at Bragg did it exactly once (OCT 03) that I know of. Of course, with the demise of DIVARTY and the focus on distributed operations (if you're doing FA stuff at all) made massing a BN less of a priority. When I was in command in A-stan in 05-06, I had the only two platoons (one wasn't even from my BTRY) that had overlapping range fans- and we massed them. It took some rehearsing, but it wasn't that difficult. I imagine that, as long as two firing units have overlapping range fans, it is still being practiced. It would be if I were running things.