Hello internet people,
I've been running the last couple iterations of this game and talking with John about the rules. The Assault and Covering Fire moves have merged in the latest version of the game. Grab that at http://mightyatom.blogspot.com/. I think you will find that what you are talking about has been specifically addressed.

A quick overview:

Attack - hurt the enemy - no roll, no gear cost.

Assault - to gain a tactical advantage while fighting directly against an enemy - Roll +Battle, Spend 1-gear

Combat Action - Roll +X while in a battle to do something but not trying to directly assault the enemy.

So, in the case you mention, first, you could roll +Lucky to jump from one rooftop to another to get a better view of the sniper, maybe negating his cover. Then, you could Assault the sniper, laying down heavy fire with your BAR (1-gear), allowing your squad to flank him. Then they could just Attack, getting a VOF bonus for flanking him, which is the result of your successful Assault move.