That's waaaayyyyy bigger than the rounds for the Galactica appear to be. If you look closely at some of the scenes where the turrets are shown in action, you can actually see a couple of spacesuited [CGI] crewmembers moving about inside the turrets through the transparent canopy.

 

It can't be very far. Using the CGI crew for scale, total barrel length (from somewhat inside the turret casing to the end of what are apparently the accelerator rails) can't be more than about 20 or at most 30 feet (6-8m).

This is why I propose that the guns be tapered-barrel conventional cannon built up using 3D manufacturing from artificial diamond (so as to handle INCREDIBLE detonation chamber pressures) with only secondary boost provided by those (ludicrously short) external accelerator rails.

Kurt: Have you thought of using the GURPS vehicle rules to model the Galactica? GURPS Traveller uses a version of these rules that smooths out the provision of fusion power to weapons and other major systems, by assuming that the calculated stats for each system includes a "slice" of power plant capacity in it's base stats (e.g. the cost, mass and volume percentage by which the ship's main power plant would need to be increased in order to support that system's normal operation). Of course, you would still have to wing the tapered-barrel cannon a bit . . .

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