The Gauss Cannon design by user “uhltimate” with twist-to-reset feature calls for 2x 6mm diameter balls to be placed on the ends of the indexing springs for the turning piece. Rather than purchasing the balls or 3D printing them with removable supports, I designed this alternative in TinkerCAD that can be printed without supports. It's just a 4mm cylinder of 5mm height, topped by a 6mm sphere 3mm above base plane (center of sphere is 6mm above base plane). The cylinder portion fits inside the springs. FYI, if you own multiple GraviTrax sets then you probably have excess GraviTrax Launch Pads that you'll never use. Their springs can be harvested and used in this uhltimate Gauss Cannon.

Also see my remix of the switch portion of this magnetic cannon at https://makerworld.com/en/models/207248#profileId-227372.