Is snap shot classed as a primary weapon shot? For example does it allow you to trigger Fin and add a blank result?

why would it be a primary weapon?

Secondary weapons are every attack other than your normal printed red number. The upgrade slot means nothing.

A secondary weapon is defined as an upgrade card with an Attack header.

The Finn example wouldn't work for two reasons, firstly because it's a secondary weapon and secondly because adding the blank result is a modification, and snap shot blocks modifications.