hmm..... moons compressed down to extremely high density. Wouldn't the mass of all the shells in the magazine create high gravity that would collapse the ship/gun?, or at least create enough gravity to slow down the speed of the shells being shot? All the other objects near the gun would also start accelerating towards it. Think of it like having the mass of the Earth inside the gun. Anything that isn't orbiting the gun at sufficient speed will start falling towards it. With all that mass, you would also need a lot of energy to move around.
Well, after reading it again, he said each shell is 10 tons. But he used 65 moons to create all of the shells. Don't know the mass of the moons or the number of shells. Total mass is less than what I thought, but it is still significantly heavy, making it difficult to maneuver .