You can do anything that the user could do
The difference being on what type of systems the user has installed, like the weapon only being able to shoot if the user has his finger on the trigger, or only shoot on a mental command, or even if the smartgun system is external or internal, which would allow the user to unplug the smartgun system and simply lose the bonuses.
But if the user has installed a remote firing system, you could easily make the gun shoot whenever you want, as it overrides the manual firing system.
A smartgun is basically a portable computer, or commlink, that can shoot bullets.
The user, however, cannot remove safety measures from the gun, like shoot when not being held, or dna-scanners, or systems that prevent the gun from shooting previously-defined allies. Which means that you also won't be able to overcome the safety systems with a single mark.
You could not hack into a grenade and make it explode as the safety systems prevent it from exploding within 5 meters of the user.
The Gun&Run has an Advanced Safety System that prevents anyone else from operating your gun. Your only option as a hacker then would be to brick it.
A small list of things you could do:
- Eject the clip
- Turn on/off the manual-trigger safety;
- Change the loaded ammo type if the gun has multiple of them;
- Turn off wireless, so the user cannot benefit from any wireless bonuses;
- Make him lose aiming bonuses;
- Turn off the camera, so he can't shoot through corners;
- Turn off aiming assistance modules, like wind speed and distance metrics;
- Use it as a jammer (see the Jam Signals action, page 240);
- Snoop, so you can see (and even stream) all data that is being transferred between the gun and the user. This could be extremely useful as evidence;
- Trace the wielder's location (see the Trace Icon action, page 243, requires 2 marks).
- Reboot the device (requires 3 marks);