[backwoods-engineer]

Iran is using Uranium. Unlike Pu, the shape of the U235 doesn’t really matter once it reaches critical mass. A simple rail-gun will set it off.

It's easier than that: with the right casing (proper amount of beryllium and heavy enough to contain the initial neutron storm), all you have to do is use an explosive to plop one sub-critical chunk of uranium into another bigger chunk, that becomes critical when "assembled".

Remember, in WWII, "Fat Man" (a gun-assembly uranium bomb) was considered to be so reliable it wasn't even tested before being dropped on Japan.

[chimera]

The gun-assembly 235U bomb was named “Little Boy”. The “Fat Man” was the plutonium weapon used at Nagasaki. Check out their form factors for the naming conventions. The Trinity test article was The Gadget, since it was not yet weaponized.