In February 2018, the service had announced that it was buying an unknown number of additional complete MOPs from Boeing, which would also have likely have been in the GBU-57E/B configuration. The month before, Bloomberg had reported that a " fourth upgrade " program for the MOP was complete and that existing bombs in the stockpile were also being retrofitted.

It is unclear how many MOPs of different variants, or in total, there are available for operational use. As of September 2011, the prime contractor, Boeing, had delivered 20 of the bombs , but it is known that there have been subsequent orders and that a number of the weapons have been expended in various tests since then. It is likely that the total number of available weapons has swelled in recent years.

On Oct. 28, 2019, the Air Force announced that it had awarded $90 million contracts to both Superior Forge & Steel Corp. of Lima, Ohio and Ellwood National Forge in Irvine, Pennsylvania for the production of an unspecified number of case assemblies for the BLU-127C/B warhead. The service separately confirmed to The War Zone that this is the warhead at the core of the GBU-57E/B variant, which has been in operational inventory since 2016. The B-2 Spirit stealth bomber is presently the only aircraft capable of and clear to employ the MOP operationally, though B-52 bombers have dropped some of the weapons in prior testing. The future B-21 Raider stealth bomber will also likely be able to carry these weapons.

The U.S. Air Force is continuing to update its stockpile of 30,000-pound class Massive Ordnance Penetrator bunker busters, or MOPs, and bring older MOPs up to the latest standard, also known as the GBU-57E/B. This comes as Iran has announced that it will begin injecting uranium hexafluoride gas into centrifuges at its underground nuclear facility at Fordo – the kind of prospective target that drove the development of the MOP in the first place – a significant breach of the controversial 2015 international agreement over the country's nuclear program.

It's not entirely clear how the GBU-57E/B variant differs from previous MOPs, either. "The E/B variant enhances the ability to penetrate concrete structures and deliver the high explosive payload," Ilka Cole, the Chief of Media Operations in the Public Affairs Office for the 96th Test Wing at Eglin Air Force Base in Florida told The War Zone by Email. Eglin is also home to the Air Force Life Cycle Management Center's Armament Directorate, which is responsible for managing the MOP program. The MOP effort dates back to at least 2002 and early publicly available graphics showed the bomb being able to burrow down up to 60 feet to get to their target. In 2007, an official Air Force news story said that this capability had expanded to up to 200 feet. Whether or not this maximum depth accounts for penetrating through layers reinforced concrete is unclear.

DOD via GlobalSecurity.org A very early graphic from the MOP program showing the bomb able to penetrate up to 60 feet under the ground. By 2007, the weapon was reportedly capable of burrowing nearly four times deeper than this.

The story from 2007 would have likely been referring to the first and second warheads, known as BLU-127/Bs and -127A/Bs, though it might have been referencing work on the subsequent BLU-127B/Bs, as well. GBU-57/B, -57A/B, and -57B/B were fitted with the BLU-127/B, -127A/B, and -127B/B warheads respectively. What subsequent improvements Boeing made to the GBU-57C/B and -57D/B variants are unclear, but, as The War Zone previously reported, the latter type did at least include an improved fuze. The "C/B" part of the BLU-127C/B designation is arbitrarily assigned and does not automatically mean it was first used in the GBU-57C/B, either.

Based on the fact that the GBU-57E/B model was in inventory in at least some numbers in 2016, it is likely that these weapons were developed as part of the Enhanced Threat Reduction IV (ETR-IV) upgrade program, which Boeing won the contract for the year before. Evaluation of bombs with the ETR-IV updates also continued into 2017, according to the Pentagon's Office of the Director of Operational Test and Evaluation. Whatever the GBU-57E/B's exact capabilities are, it would seem reasonable to assume that it can penetrate deeper than the MOP variants available in 2007. We know that the inert BLU-127C/B, at least, has a "booster" in the forward portion of the warhead, but it is unclear if this refers to a system that physically propels that section, such as rocket motor, for added penetration or an explosive charge that simply accelerates the detonation of the main charge. The added penetration would be necessary to strike at especially deeply buried and hardened facilities, such as Iran's nuclear facilities at Fordo. The main portions of this site are reportedly around 295 feet underground and further shielded by multiple feet of reinforced concrete.

Google Earth A satellite image showing what is visible of the facility at Fordo from above ground.