If you are talking about seeing a actual gun it would have to be a part of the model in order to see it. You can not alter a MDL file without the original file that was used to make it.
Aircraft today are made by using a couple of different programs one of them is called GMAX which is a 3d graphic creating program and it is what a lot of companies use to make games today.
GMAX is not I repeat is not for beginners as it takes a lot of skill and knowledge in order to make anything with it.
I tried it once and managed to make what I called a line shack which I placed at my home base KJAX but for the love of money I could not figure out how to make the P38 GMAX uses what are called polygons and there are techniques that you have to use in order to stretch them out to look like a aircraft no mater what i tried i could not get the effect i was looking for I nearly went bald trying to figure it out and finally threw in the towel. It was probably some simple little thing I was missing but without anyone to help and trying to follow a very thick book was just to much. I still have the book and the original CD GMAX came with if any one wants it I would be more than happy to let you have it free of charge and I would ship it to you at no cost to you.
anyone interested should send me a private message and tell me where to ship it to.
If it is the effect that you are looking follow the first post suggestion.
I have to tell you that the book and the CD with GMAX is very old and I am not sure if it will even install on Windows 10 And I would not be surprised if it has not been updated and have no way of knowing if you can update it without costing you anything.
I found this link that will download and install the latest version of GMAX and it is free.https://www.turbosquid.com/gmax
But like I said this is in no way what I would call a user friendly software if you do not have any knowledge of using what is called CAD computer aided drafting This is not for you.
You will also have to have the SDK for FS2004 OR FSX in order to create anything for either version of FS