Any requests?

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[...] but the flintlock wouldn't even show up when opening the rifle with the Model Editor (I could select it via the "Outline" function though);

Invisible meshes are usually caused by the export plugin not finding a material, resulting in errors during the export. Make sure you have a material with at least the diffuse map in it applied to your mesh in your 3D program before exporting and also that your texture is in the same folder as the mesh.

Another reason could theoretically be flipped normals - if your 3D program does not have backface culling activated in the viewport, flipped polygons can be easy to miss.