In the mod ui i have 4 ammo in gun - when i buy it it is empty. Is there a way for me to fix this or is it a bug?

Please read the following post, perform some testing, and then tell me if you still think this is an issue. As far as I can tell, this is a vanilla display issue, and the gun does have 4 ammo, which you can confirm several ways.

https://umod.org/community/vending-in-stock/37905-guns-dont-come-with-ammo-after-adding-this-plugin?page=1#post-3