From Spud Wiki
A C Cell is a 'C' size battery.
Use and target performance
They fit well in 1" SCH 40 PVC pipe with a gap of only ~.03" (.9mm).
The thick steel cases make them very robust, and they can often be reused if shot at softer targets, although impacts with unyielding objects (such as rocks and steel plates) will deform them to the point where they won't fit in the barrel anymore.
Alkaline batteries are not good for the environment, and so one should make an effort to clean up after firing.
Diameter: 25.3mm (.996")
Height: 50 mm
Weight: Varies; roughly 68-74g for Duracell alkaline batteries