I have read a lot of what has been published about the early nuclear bombs of WW 2, and my understanding is that the U-235 or "cannonball" bomb didn't need the exotic trigger materials and was not a high tech design at all. It seems that the Navy weapons people charged with designing the "gun" worried about catching the "plug" if it went through the "doughnut" without going off. They were assured by Oppenheimer's people that there was nothing to worry about. It was only the plutonium implosion bomb design that got into exotic design problems because it required literally squeezing a solid metal sphere into a smaller volume in microseconds, something that had never been done.
There's considerable oversimplification of nuclear weapons design in the public domain.
Keep in mind that the entire might of Nazi Germany couldn't do it.
Those who tell you that an old rusty cannon barrel and a globe of U235 are all that's needed are woefully uninformed.