[amluto]

I don't quite buy this reasoning. With a car charger, you indeed need to hold a fairly beefy contact in position so it mates with its counterpart. But in a car charger, there's no functional difference between the cable end and the car end -- sure, one is handheld and one is fixed to the car, but both ends need to secure their respective contacts.

With the gun-type bomb, suppose the final critical assembly has a 2"-diameter "male" part in a 3"-diameter formerly hollow ball. If the 2"-diameter "male" part were in a 3"-diameter barrel then, indeed, some mechanism would be needed to stabilize it. But I see no fundamental reason that the barrel needs to be 3" in diameter -- couldn't the "male" part just as easily be a projectile in a narrower barrel?

(I made up the 2" and 3" numbers.)

[CapitalistCartr]

The barrel is part of the casing, necessary for a sustained reaction.

[amluto]

True, but does the barrel need to be a cylinder? That is, couldn’t the casing be a sphere with a hole cut out and a cylinder inserted? I suppose this could be much less pleasant to machine and to service.

[ChrisTheBore]

No, but they wanted the first bomb to be simple enough in principle not to need any debugging.