It's always the simple things that elude me...

I've managed to blunder around the Data Editor and figure things out on my own so far but this has been dogging me for over twenty four hours. I've created a custom unit based on the Thor and, very originally, named him The Hammer. Everything duplicates nicely. However, as soon as I remove the Thor weapon and set in his custom - and brilliantly named - Hammer Cannon (Thor's ground weapon with the ground target requirement removed), I lose the "muzzle flash". Every other animation is there and the weapon functions as desired, but the little muzzle flash that comes from the arm cannons is gone and only that.

I can't figure out what's missing that this small piece of the puzzle falls out. Actor? Effect? Behavior? Beats me.

It should be in the "Events" of the Actor.

Basically the Events are like small triggers that activate when abilities or behaviors or whatever are turned on, off, started, or ended.

After some meddling, it would actually seem the Thor "Hand Gun" impact and launch models have no association with the customized weapon. Not sure how to go about correcting that.

Muzzle flash and explosions on the target are mostly attack actor stuff...

Have a look at the thor's attack actor. Most important are the 3 separated fields on top of the table.