I think modern US designs are fusion boosted, making them immune to predetonation. Older experiments are increasingly dangerous, especially high-yield uranium guns.
But if you pile a bunch of such cores up together like they were cannon balls, that inherent safety goes right out the window.
But, as a thought experiment, what is the difference between a pile of boosted-era plutonium cores; and a weird but moderated reactor? Specifically, does an arbitrarily-sized pile of weapons-grade plutonium marbles have the same inevitability of criticality you expect than if those are designed for use in boosted triggers?