Why is it in WW2, the Manhattan Project had focused on creating a gun-type and an implosion type weapon. Almost everyone was confident of the gun-type weapon, that no test was carried out for it and went dropped as planned. However, the Fat Man implosion type had a previous Trinity implosion type test to see whether it could work or not.
What if the Manhattan Project had focused on creating a gun-type only weapon? Do we get nukes faster than the OTL since we focus on producing one type instead of two types? I understand to make the material necessary for it is difficult, but this shouldn't be a problem considering that the US industrial output could make it.
Implosion bombs are far more efficient than gun type bombs. Little Boy achieved an efficiency of around 1.38%, while Fat Man achieved an efficiency of around 13%. Gun type bombs also present packaging challenges because they are giant guns, while an implosion bomb can take the shape of a sphere or egg.
Implosion bombs can also use a greater variety of material. Gun type bombs take much longer to assemble a critical mass than implosion bombs, which would cause plutonium to undergo an incomplete fizzle and blow the device apart while the plutonium bullet is still speeding down the barrel. The same thing probably happens with the uranium as well due to the inefficiency of the gun type mechanism, but it's still enough for uranium to achieve a critical mass and multi-kiloton detonation instead of a fizzle of a few hundred or maybe thousand pounds of TNT.
The advantages of implosion bombs are thus greater efficiency, easier packaging, and the ability to use more material types. Implosion bombs can also be used to help develop thermonuclear weapons.
From a program perspective moving towards plutonium type implosion bombs has advantages as well. Plutonium is produced as a byproduct of nuclear reactor operations, so it's possible to attach a secondary loop to the unit and produce power as well. If the production pile is a heavy water or graphite moderated design then it doesn't even need enriched uranium to operate. In contrast, uranium has to be enriched to be suitable for weapons use. Uranium can't be enriched in a reactor and has to be produced using energy intensive processes.
The British and French nuclear programs were built around dual purpose
Magnox and
UNGG reactors, respectively, which used natural uranium fuel to produce military plutonium and civilian power. Magnox was even given the codename PIPPA (Pressurised Pile Producing Power and Plutonium) during its development by the United Kingdom Atomic Energy Authority.