The 30,000-pound GBU-57A/B Massive Ordnance Penetrator is not hard enough to take on Iran's current nuclear facilities, said Pentagon sources to the Wall Street Journal.
Right now, the current GPS-guided GBU-57A/B has a 2.7 metric ton high explosive warhead, and can penetrate 200 feet (60 meters) of 5,000 psi reinforced concrete, 26 feet (8 meters) of 10,000 psis reinforced concrete, or 130 feet (40 meters) of moderately hard rock.
But that's not hard enough. They need more cowbell, according to Defense Secretary Leon Panetta. While the bomb—which is designed to be delivered using B-2 stealth bombers—can seriously damage Iran's underground nuclear facilities, it wouldn't completely destroy them. He said that they are still developing it:
I think we're pretty close, let's put it that way. But we're still working at it because these things are not easy to be able to make sure that they will do what we want them to. But I'm confident, frankly, that we're going to have that capability and have it soon.
The Air Force started receiving the massive ordnance penetrators in September, but more are coming in case things with Iran get really nasty and serious clusterfucking ensues. [Wall Street Journal via Defense Tech]