You are right, I stand corrected.

I read about the Chinese bombs in "Atomic Adventures" by James Mahaffey, but reading again, I see that only the Chic-4 bomb originated from the US W-33 warhead, which was copied by the Soviets and became their 3BV3 bomb. All these had a yield of about 10 kT. As I mentioned in my previous reply, there were gun-type uranium bombs with up to at least 30 kT.

I think your original point was that in order to get yields 10 times as high, you would need much more uranium, and this would be unsafe.

The Ivy Mike bomb that you mentioned had a fairly crude, but effective, safety measure: a chain containing boron (very strong neutron poison) in the middle of the assembly, to be removed only immediately prior to deployment. I don't see why this would not work with a gun-type design.

Still, you could say that the implosion brings together fissile metal from all directions, and at huge speed too, while the gun-type brings the metal from only 2 directions and at much lower speed, and therefore the implosion can assemble a much higher hypecritical mass. So inherently the implosion design can result in a higher yields. And that is absolutely true, and it is very likely the reason that nuclear powers prefer the implosion design, even in some weapons that use uranium.

From the post-WW2 history, it looks like gun-type was used specifically for smaller yields. An artillery shell should not have a megaton yield, for the simple reason that you want to survive after you fire it.

Yet, from the same book I learned that during the Manhattan project 20000 explosions were used to understand and fine-tune the implosion design, and for each explosion that happened at least 20 were analyzed on paper before. More than 1000 scientists and engineers worked on nailing that design, and it was by far the most expensive part of the entire Manhattan project.

Immediately following WW2, the US switched to a different method of uranium enrichment, that made uranium cheaper to produce than plutonium. I don't know if it was cheaper by mass or by yield, probably the first.

Still, let's imagine an evolutionary path where the US finds itself after the end of WW2, with a tried, tested and practical design based on uranium and the gun-type, and which needed a fissile material that was getting much cheaper. And knowing that it's possible to get an alternate design based on plutonium, but with that required an unknown amount of additional fundamental research and then engineering effort. That would have very likely been the situation if von Neumann did not get involved.

A lot of organizations faced with such a dilemma choose the incremental gains from an existing design, rather than exploring a potentially revolutionary, but risky alternative.

In such an alternate history, would the scientists be able to increase the yield of a gun-type uranium bomb to 100 kT, or 500 kT? The boosted fission design was developed between 1947 and 1949, and there is no reason it would not work with a gun-type bomb. Once a boosted version of a gun-type is developed, a version that uses a lot of boosting and an additional U-238 temper around the core can deliver a lot of extra-yield, without increasing the U-235 mass, and the chance of pre-explosion. I'm sure motivated scientists could have come with many more ideas.

Here's a similar scenario of incremental changes to an existing design: after WW2, Admiral Rickover chose the pressurized water design for his submarines. He was a very smart man, and I have no doubts that he made the right choice. However, there are hundreds of possible reactor designs, and for applications other than submarines it is very likely other designs could be much better, yet 75 years later, and PWR is by far the most widespread design.