"T-Shirt Cannon" is an object that appeared in "Laser Light Cannon" in Greg's storage in U-Stor and is used in "Shirt Club." It is used when Buck Dewey is shirting T-Shirts to advertise Greg's business at random bypasses, (which Include Jenny, Sour Cream, Lars, Ronaldo, etc.) and then they start wearing them.
This was then later used by Steven to shoot at a large crowd of people using Buck's T-Shirt design which dramatically shot Mayor Bill Dewey in the head and caused everyone to flee from it. It does not appear to have great force as it did no physical injury to anyone.
Currently it resides in Steven's possession and it was temporarily in possession by Buck Dewey.