Weatherby does not sell an adapter for the 11mm(3/8) Dovetail featured on 18i shotgun models, but they are readily available.
Thanks for purchasing an 18i Shotgun. The 18i series of shotguns features an 11mm(3/8) Dovetail on top of the receiver. If you purchase the correct adapter and red dot mounting system, the pairing should be simple to install and perform flawlessly.
Many manufactures produce adapters for 11mm(3/8) Dovetail to 20mm Picatinny rail.