In the
Naval Gun Projects thread on Secret Projects forum, there appeared a few pictures of the United Defense NG^2 155 mm naval gun.
Advertisments in Proceedings in the nineties, apparently, and that's all the info available. This drawing should be roughly to scale.
Looking the drawings over, it seems that the gun tube could be elevated up to vertical: This might be a VLG (Vertical Load Gun) with a stealthy gun shield. Which means that it weighs roughly as much as the MK-45 naval gun.