Kuva Weapon Fusion Calc

Weapon A = Kuva weapon you want to keep. Any Orokin Reactor, Forma on this weapon will remain the same.

Weapon B = Kuva weapon you want to sacrifice. This weapon will be destroyed. Any Orokin Reactor, Forma on this weapon will not transfer to Weapon A.

Tip: Equip Kuva Weapon you want to keep (Weapon A) to your Warframe, then fusion it from there. This way, your Kuva Weapon won't be accidentally destroyed.