As you already know. (since I repeated it multiple times xD)
The gravity cannon can fire any small ship block as long as it's preferably no longer than 1x7.

This thread is where, current subscribers, can post screenshots of thier own types of ammunition that can be used in the cannon and add to variety! :)

As right now, any small ship block that is one block wide can be used in the cannon, the needed blocks to start off the shell is one small reactor and mass block to get the acceleration going Although more mass blocks can be added, it may take up space that can be used for something else, such as warheads, small rotors, beacons and even pistons, add to that with sensors, timers, even a thruster you can create a large variety of ammunition to be fired from the cannon.

Longer ammunition can be made if you delete the forward piston and the rotor block to gain an extra space of 3 blocks into your shell to fire.

With that said, feel free to discuss ammunition here. :)