Without a precisely timed neutron initiator you'll get a fizzle. Either way, gun-type bombs require highly enriched uranium. A simple reactor can be literally just brickwork of natural uranium and graphite.
Reactors are much much simpler to pull off, which is why US had the first reactor whole 2.5 years before a nuclear bomb.
Something that explodes with the force of 100 tons of TNT is still a bomb, even if it isn't a bomb as impressive as the one you were hoping for. And getting the reactor you're thinking of to the point that it worked without exploding took a lot more effort than you're suggesting.
Reactors are much much simpler to pull off, which is why US had the first reactor whole 2.5 years before a nuclear bomb.