[Gideon7]

Nobody has exploded a U235 bomb (Little Boy type) since the 1950s (AFAIK)

https://en.wikipedia.org/wiki/Gun-type_fission_weapon#Comparison_with_the_implosion_method

All bombs since the 1950s are implosion type using Pu.

The reason is because U235 bombs are just too unstable and dangerous to handle.

[grey_whiskers]

News to me. Recall that the Hiroshima bomb didn't have a prototype tested first, as it was felt to be so reliable.

Unless there are details of the internal geometry such that it is easier to have an accidental supra-critical mass (though not dense enough for a sustained chain reaction), with a consequent lethal flux of gamma radiation in the near vicinity.