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The only known-to-me gun type device ever detonated was Little Boy.
Similar configurations were routinely used for research of criticality issues, when ie. a slug of 235U was free-falling through a ring of 235U, temporarily forming critical mass for few milliseconds. The resulting neutron flux was measured. Sharp spike meant critical amount was reached.
A gun-type device is very simple to construct. The only problematic thing is the fissile core itself, and maybe the 238-U shields and neutron mirrors around it. (Indium-doped steel does its job too.)
There are drastical limitations of gun-assembly device performance, though. The yield is limited to about 20-25 kilotons at most; the speed of insertion is limited, and the assembly will vaporize before the assembly is finished, thus wasting alot of energy. The trick of high yields from the primary stage (we aren't talking thermonuclear now) is to release as much of energy at the first nanoseconds; once the device turns to plasma and starts to expand, the density of the fissile core dramatically decreases, thus becoming subcritical rather quickly.
Another problem of gun assembly is the fissile material. Uranium comes as a mixture of isotopes 235 and 238 (and some 233 and traces of others), which are difficult to separate. Plutonium is easier; comes as 239 and 240, both are fissile, and can be manufactured by irradiating of 238-U by neutrons (a commercial power plant reactor is enough). The problem here is that 240 is fissile, but doesn't release neutrons, thus doesn't contribute to the chain reaction and decreases the neutron population. 239 is the primary product here, but when it stays in the reactor for too long, it becomes 240 easily. In a common VVER-based power plant, where you exchange typically a 1/4 of the fuel quarterly and total stay-in time is one year, you get plutonium unusably contaminated with 240 - too much for a decent bomb. Thus you want to shorten the fuel cycles, which will require more frequent shutdowns of the plant, which will attract the intelligence services. (Guess what revealed nuclear program of North Korea couple years ago.)
You can theoretically use plutonium for a gun-assembly device, but the yield will be even lower - and plutonium has some more undesirable characteristics, like higher neutron production in subcritical state, which poses alot of secondary problems.
If I'd have enough of weapon-grade 235-U (about 25 kilograms - it should be possible to build a nuke with less, but it requires more hightech magic), and a smaller sponsor (a typical sponsored regional sport event would eat about as much money than I'd need for this project, except for the uranium), a gun-type nuke would be well-within my capabilities.
The resulting device would be rather large and heavy and crude, unsuitable for delivery by anything other than a middle-to-large aircraft or a truck (forget about grenades, forget about missiles), but hey - it'd be a nuke.
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