[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.