[2019] Assassin

I don't know why you wouldn't want the heirloom. It's a class-based retrain of the Slug Rifle, and has the Frost Element at Legendary, making it easy to hit multiple times.

Also, given the RNG nature of Forging Weapons, it's natural to sometimes get Heirloom Rifles or Shredders instead of Sniper Rifles. If you want to guarantee a Sniper Rifle, there is a Forge Talent that will help you do that.