Over the course of GTAV modding, I have noticed a lack in add-on weapons, most weapon mods you find are just replaces for weapons, not to mention the lack of documentation of them as well.
I have released 2 weapons that are add-ons, and let it be known to you that I have also created many other addon weapons that I have not released yet. Most are not released because they use models released from this site. I would like to offer my services to anyone who wishes to make their weapon a working mock of the weapon rather than just simply replacing the weapon. At some point I do plan to release a tutorial or documentation on creating addon weapons.
Below is an example of a Kar98k mod created by @kingfire .The mod originally replaced the Heavy Shotgun.
Take a look at the comparison:
If you are interested, feel free to contact me in any way.