Yes, it seems that there is huge difference between very big prototype on a secure testing field and battle-ready version sized like 155mm gun. Also the gun itself is top three scandal of that ship.

Basically the main task of that ship was to... save money (don't laugh!) in assymetric warfare against opponents being colonized, to be exact, to deliver long range precise artillery strikes with the cannons, because cannon shell is cheaper, more difficult to defend from and smaller than guided missile. They wanted modern colonial gunboats.

After the railgun failed, they proposed rocket propelled shells for normal artillery. This worked more or less as advertised, but the cost of ammunition was actually huge, not very different than missiles - because when ordered they planned for dozens of ships to be build and ultimately they only ordered 3 and the cost/shell skyrocketed. So they ordered only small amount of those special shells and the ship is otherwise armed with normal shells for its guns (practially useless) and classical guided missiles (which are also getting obsolete, US is even planning to buy norwegian missiles for their new frigates).