That is awesome! My only advice would be to reduce the breach rolls a bit. An open-topped gun pit like that would be hideously vulnerable to fire from above. Admittedly, their small size relative to a capital ship would make actually scoring a hit difficult(as would their distance, being all way down on the ground), but I imagine a direct hit from just about any ship-mounted gun would eliminate the whole gun. Maybe also reduce the movement(or lack thereof in this case) modifier die as well, to reflect the difficulty in hitting them?
Aside from that, I really like this! I love how it uses existing game mechanics to bring ground guns into the game, without needing to make up new rules. Hopefully official ground emplacements will work much like this, when they are brought in.