I can imagine this has been covered several times but I am trying to find some clarification.
Can anyone tell me how to replace any of the current weapon models with one of my own?

Peace

they would need to be rigged to the hud file which is an animated set of hands going thru the movements of the weapons operation. You will need a model program that is capable of rigged animation such as Blender, fragmotion, or 3d Max. (there are more, Maya does not export animated X files so not that one.)

Bring your weapon into the program and scale it to the rig size. combine the textures, Adjust the UV mapping on the rig and the new weapon. Delete the old weapon without destroying the rig. Change any animations you want for different features. Export and setup the weapons folder for the new weapon, adjust the gunspec.txt file in there to taste, and then test and correct.

That is the gist of it.

Thanks for a quick response

You are welcome. It is a bit of a process as you can see, We are looking at simplifying this in the future.
But this is very possible with about a week of trial and error if you want to tackle the job.

Pick a model and animation program and work on the theory of things, and start off simple. Youtube has tons of videos on rigging and animation.

Not hard just a detailed process to follow.