The vehicle cannon doesn't have a caliber listed, though the auto-cannon does (a range at least). To synch up better with CSC this should have a caliber listed.
The other thing is that a cannon would typically involve a much larger turret than an auto-cannon. So the turret on a tank would require MUCH more hull real estate than a 20mm cannon does (which can be pintle mounted unlike a cannon).
It may be better to only have lighter weapons listed in the CRB and leave the rest to be described and listed in the Vehicles book with the proper rules. And since none of the sample vehicles have an equipped cannon, there's no issue with removing it.