If you could get isotopically pure Pu-239, which doesn't exist, it would work - but even the tiniest traces of Pu-240 will spoil it. Quoth Wiki: "The presence of the isotope plutonium-240 in a sample limits its nuclear bomb potential, as plutonium-240 has a relatively high spontaneous fission rate (~440 fissions per second per gram—over 1,000 neutrons per second per gram),[20] raising the background neutron levels and thus increasing the risk of predetonation."
Long story short, all those extra neutrons make a chain reaction far more likely to occur - so likely that if you use gun assembly, supercriticality is reached long before the two pieces are physically together and they just blow back apart with a negligible yield. Granted, "negligible" in the sense of nuclear weapons means anywhere from a few to a few hundred tons, and there's still an intense load of radiation and fallout, so for terroristic purposes it might suffice...but you aren't building a city-leveling device with Pu and a surplus artillery barrel.
Anyway, breeding it with a neutron tube would be slow, Like building a skyscraper with a set of Craftsmen tools slow. Reactors are the *only* way to synthesize >mg quantities of any transuranic. If you have those resources, a simple (albeit large) implosion system is pretty much trivial.