Mk 71 was developed back in the 1970s to fill the growing gap in naval gunfire support, as there were few guns in the Navy larger than 5". The Mk 71 offers improved range over a 155mm gun firing the M795 (27km vs 22.5km) and a larger round (118kg v 46.7kg). Off the shelf is a match for any 155mm gun in U.S. inventory firing any type of ammunition (the only acceptation being the LRLAP round).
I would argue that the Mk 71 is better than the AGS because the AGS has no anti-ship capability, and fires only one round, the LRLAP. With only a small amount of LRLAPs being purchased for only 6 operation guns (3 Zumwalts, 2 guns apiece), the cost per round will be abnormally high. To sum it up, I'd like to quote ComNavOps over at Navy Matters.
- Cost – when the Navy committed to the AGS the Mk71 development costs were already long since paid for. The gun was essentially free and a prototype had already been successfully tested. Advanced rounds, if desired, would have entailed additional costs just as for the AGS.
- Explosive Power – the 8” round is so much more effective than a 6”.
- Shipboard Impact – the AGS, as we’ve discussed has a major impact on ship design and ship utilities. The Mk71 is a simple gun and easily accommodated.
- Flexibility – the Mk71 is a general purpose gun capable of engaging both ship and shore targets unlike the AGS which has no anti-ship mode.
Since it is almost impossible to cancel the Zumwalts at this point, it would be best to cancel the AGS and LRLAP, and mount Mk 71s in lieu of AGSs on the Zumwalt-class. They will provide the Zumwalt-class with a WVR anti-ship capability and an improved naval gunfire support capability at a lower cost.
I do not believe that the Zumwalt-class is the right choice for the Navy in light of cost overruns and continued delays. Each mission (naval gunfire support, land attack, and AAW) that the Zumwalt-class is meant to perform requires vastly different capabilities, with AAW and land attack requiring large numbers of VLS cells which will deny space to needed guns for naval gunfire support. It would be more effective to build a class of ships meant entirely for missile warfare (AAW and land attack), while modifying existing and future destroyers to carry the Mk 71 to provide more effective naval gunfire support.