Question on proper use of gun com

[BeckettColt]

I'm currently trying to improve my play, and am running into some trouble. When I watch replays or casts it seems that gun coms are lethal against armies, but attempting to deploy similar methods in my games has universally led me to failure. Does anyone have advice on how to better use my com for combat? Is it all in APM or is there an underlying strategy I'm missing?