You can find a bunch of free 3D models to use here (just make sure you check the license before you use the models in a game you intend to sell). Search for ‘gun’ and you should find a few ...

The FMDA 19.2, released in 2021, is a relatively old model by 3D-printed-gun standards, says one gunsmith who goes by the first name John and the online handle Mr. Snow Makes.