The documentation contains the UV maps for the guns: https://rugbug.net/redmatch/resource-packs/files/rifle.html

You create an image using that UV map as a template (don't include the UV map in the image) and save it as rifle.png in the Resource Pack folder. It's the same for sniper.png, revolver.png, and sniper.png

You can ask for help in the #resource-packs channel on the discord server: https://rugbug.net/discord