Is those made out of parts or meshes? My recommendation for modeling / building guns in studio is to make them large, that way you can do all the tiny details possible / its easier to make. After you finish a large scale model of the gun, you can use the model resize plugin like this one for example:
If possible though, when it comes to creating guns overall, I recommend taking the time to learn how to use blender. This allows for much more complex modeling, and can get you more accurate results for what you want due to the tools provided on Blender. Other than that, I am really impressed with both guns being that they were made in studio 