Originally intended for being deployed by United States Air Force bomber aircraft in large numbers, the Mark 28 is much smaller than the high-yield C23 atomic bombs stored in Fort Constantine near Washington, D.C. both in size and destructive capabilities, yet considerably larger and more powerful than the mini nukes used by the M42 Fat Man tactical nuclear catapult.
During the Sino-American War, a large stockpile of Mark 28 was stored in the bunkers of the Sentinel site southwest of Boston where military engineers and scientists were attempting to adapt the bombs for use as missile warheads in ICBMs. Their experiments yielded mixed results and were ultimately cut short when the Great War broke out and a single strategic nuclear bomb impacted not far from the bunker complex, devastating the entire region and turning it into the Glowing Sea. Still many Mark 28 remain intact even in 2287, safely encapsulated in sturdy metal crates and kept out of reach of mankind for over two centuries by the natural hazards of the Glowing Sea. However, this changed quite recently when members of the Church of the Children of Atom gained entrance to the weapon storage facilities in an attempt to use the warheads for their own ends, while at the same time the Brotherhood of Steel arrived in the Commonwealth and tasked one of their Knights with securing the Mark 28 for their war against the Institute.
The Mark 28 can be found in and around the Sentinel site in large numbers. The majority lies neatly stacked in their depots but some are scattered throughout the facility and its surroundings, presumably still lying where they were abandoned in a hurry when the Great War forced the personnel underground. Almost all of the Mark 28 encountered in this way are secured in cubic metal frames, with only a few standing around unprotected. No matter in which state they are encountered by the Sole Survivor, none of the bombs can be interacted with directly. Shooting them has no effect aside from alerting nearby enemies, quite in contrast to the howitzer shells in Nellis Air Force Base.
They are only seen in action if the Survivor destroys the Institute with the help of the Brotherhood of Steel, in which case Liberty Prime will make liberal use of his inexhaustible supply of Mark 28 during his march on the C.I.T. ruins. Contrary to his modus operandi in the Capital Wasteland ten years earlier, Prime Mk II now relies more on his nuclear arsenal than on his eye laser weapon and hurls the warheads at pretty much anything he crosses paths with, even if it is but a lone low-level raider.
It should be noted that, while the Mark 28 are much bigger than mini nukes, their in-game blast radius is not significantly larger than that of a regular Fat Man shot, especially so if the Sole Survivor is a well-versed Demolition Expert. Care should still be taken to stay out of Liberty Prime's target areas when he runs into enemies, as the bombs will utterly annihilate everything caught in the blast, including the Sole Survivor.