Weapon (Longbow), Legendary (requires attunement)
You may speak the command word as a bonus action before loosing an arrow from this bow. If you do, where the arrow strikes a portal opens with a corresponding portal directly in front of you. The portal lasts for 1 minute.
The bow can create up to three portals a day. Spent charges are restored at midnight.
Not quite a portal gun, but similar principle. I can’t remember what inspired me to do this one, and I can’t decide whether to include it or something similar in my upcoming Sentient Skies module.