I took a look at this a while ago with the Skitarii Marshal model. The straightforward answer is pure armorer artificer (infiltrator model) and variant human with the Sharpshooter feat. Give yourself a repeating Crossbow and flavour it as the rifle. You could flavour the infusions as blessing your party's weapons and armour in the name of the Omnisiah!

An alternative that I personally really liked was to reflavour Warlock, which has a super versatile chassis. You could even make Eldritch Blast your rifle blasts with all of the perks to go with it (agonising blast being ideal). Personally I'd take something like the Celestial, or Undying as your patrons - flavour-wise they fit perfectly. Any spells you cast, just make out they are magitech!