Haha, I wasted time converting those figures back to metric before I realised where you got the figures.
According to wiki its more like 2 kg of TNT.
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Hmm, that seems fairly worthless for shore bombardment, anti-ship, anti-tank.....well, pretty much anything other then anti-missle and anti-aircraft defence.
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I guess it all depends on rate of fire and method of deployment. Note that the 8MJ version seems to be a demo model. They plan to use 64MJ versions in the field. Also they hope to use guided projectiles in the future.
http://fredericksburg.com/News/FLS/2007/01...01172007/251373