Changing weapon models

I'm kind of struggling to figure out how to do this, and have been for some time. The instructions on the tutorial page are unclear for me on how to do anything, so I thought that I should just ask directly for information on how to do this specifically. I would like to replace the clone trooper rifle on an "extra" custom model I've added which comes from the game's model directory with the dlt-19, or an equivalent which is close if that doesn't exist within the assets already. Any help is appreciated, thank you.