Trying out this mod in sandbox i found out that my tanks would only use bullet ammo instead of flamethrower ammo and tank cannon shells

That's right. I have added a note about that to the Information page and the mod's README.md file.

The reason is RoboTanks use a hidden static turret entity to do the firing and draw enemy aggro, and the static turret only fires bullets. If there is interest, I could look into adding flamethrower capability. I do not know how I would add cannon shell firing since there is no static turret that fires those.

Oh thats a pity... I would have loved having a fleet of tanks shooting cannon shells. Well, actually it would be cool if they also throw flames but as I want to use this mod in PvP I dont knoiw how efficient that would be. Is there really no way to give them cannon shells?

There isn't a vanilla turret that fires cannon shells, but I see there is at least one mod, "Cannon Turret" that does so. I'll look into using that somehow. It will take some time of course. :)

Thanks for responding so quickly and for your great work! Dont worry, I am patient and excited :)

I've got a preliminary version ready for beta testing. Are you interested in helping to test it before release? If so, you can get it from the github page:

https://github.com/smcpeak/factorio-robotank

Use "clone or download" on the right to get a zip file, unpack it in the "mods" directory of Factorio, then rename the directory to RoboTank_1.1.0. (Move aside the old version of RoboTank.)

See the included README.md file for instructions.

I also made a mod with cannon shells that do not cause friendly fire damage:

https://github.com/smcpeak/factorio-smart-cannon-shells

Same installation procedure as for RoboTank, except rename the directory to SmartCannonShells_0.1.0.

I've now published RoboTank_1.1.0 with cannon shell firing capability and SmartCannonShells_0.1.0 on the Factorio mods site.