You're inverted. Little boy is the gun bomb. Another interesting anecdote is that Little Boy's design was a fall back. The "Thin Man" was plutonium based and

You're inverted. Little boy is the gun bomb. Another interesting anecdote is that Little Boy's design was a fall back. The "Thin Man" was plutonium based and discovered to be unlikely to yield an efficient explosion, so they changed to Uranium and made the gun bigger.