Any requests?

[ferryadams10]

[...] 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);

Open the .dae file in Model Viewer, it will create a .mat file in the map of the .dae.
All you gotta do then is remove the previous .mat file and link the diffuse, normal and (if you have it) the specular map to the new .mat file in the Material Editor.
When you done that, you have to apply the new .mat file to the model in the Model Editor and it will show up in the Model Editor.