Watched one yesterday. 3 guns with 5 soldiers at each one firing 32 shots each.
It was interesting to see how they did it. An officer shouting out orders for "Number 1 Fire, Number 2 Fire" etc.
Then each person in the troop ensuring it was loaded and signalled that it was ready. Along with the troop leader signalling the gun was loaded and then ordering their troop to fire.
I could have sworn there was a double fire - as the officer wasn't that clear with 2 and 3.
And I think there was someone with the officer keeping count. As it could have been possible to lose count.
Interesting to watch though. I am sure it's faster in real life when they are firing a barrage