Disclaimer: I'm not a lawyer.
If the guns you are making are "abstract enough" to not be a specific model, then you don't need any license. Guns are recognised internationally and not really tied to a specific company.
Having said that, it's worth being aware of some of the consequences of having guns in your game:
- Your game cannot target primarily children. In a lot of countries, anything made for children cannot have any reference to guns.
- Your game might get different rating on different countries, which might limit the number of people that can download it. Some countries have issues with violence in video games, and might even ban one that is too violent.
At the end of the day, you are making something for entertainment. Nobody will (realistically) sue you because you made a video game with guns. The worst that can happen is your game not being playable in some countries (depending on the amount of violence).
If you are too worried and you want your game to have as big market as possible, things you can do include:
- Make the guns be more "fantasy-like" so it's not easy to tie them with real life ones. Games like "Halo" has done that to some degree.
- Limit the amount of blood that is shown. Some games go as far as to show other graphical elements, like "dust" instead of blood. One example that comes to mind is "Tekken".
However rules of how to control the rating are not concrete. It's all about how a game feels when you play it.