Definitely more on the style/silly as opposed to practical side of things, a guy has made a 3D-printed nerf flintlock. It even loads using a ram-rod.
Firing video:
https://www.reddit.com/user/CarnyCreations/comments/j1td0b/3_rounds_rapid/I believe it’s still WIP, but the creator’s expressed interest in selling the files and possibly its printed out parts themselves in the future.