The sniper has a "one shot, one kill" move that's normally the right move for trying to snipe individual targets.
Weapons with the 'ordnance' tag use gear every time you attack with them, regardless of what move is used - even if no move is used and you just shoot it at a tree, presumably. None of the sniper's weapons have this tag, so they only use Gear when you're using one of the more sustained combat moves: Assault or Covering Fire.
The choice of move seems to have a lot of latitude. When I played with John, I was playing a special forces Operator. I was infiltrating a building, and came across a surprised enemy with an AK-47 - John let me shoot and kill him without even rolling damage.
As a special forces operator, he figured I was trained for this precise thing, my gun was at the ready, my target was surprised and had no cover, so John just declared my target down.
Moments later, I was engaged in a firefight with a guy who was up some stairs. Here, the outcome was much less certain - the target was responding to the sound of my weapon, had his weapon at the ready, and there was some distance between us to make misses far more likely. John declared that this was an Attack: we were both just trying to kill one another; both of us just went straight to rolling damage (3D6 Direct, I think).
If, on the other hand, there had been some way for me to get up to the second level, I had thrown a smoke grenade, there was some more interesting tactical element to the gunfight, or I was perhaps trying to drive him back, John might have ruled it was an Assault. I'd have rolled 2D6 to see how the combat went on top of us rolling damage, and spend Gear.
If, instead, I had been firing steady bursts to prevent the enemy on the second level from drawing a clear bead on me (or my companions), it would have been Covering Fire, potentially allowing a teammate to flank him while he's hiding from my hail of fire.