I think I am reaching the limit of my knowledge on this now.
As I said before, I have never made a weapon addon, so I think you need someone who has, to help you further.
The way I understand it though, you need your model file as a proxy anyway.
You might also need custom animations for the holding and use of this addon.
Finally, I think proxies also require a cfgnonaivehicles section in the config.cpp.
I hope someone who has experience with making weapons can help you here, but I don't and think I have helped all I can for the moment.
I will keep an eye on this thread though.
Planck