The gun barrel design of the atomic bomb used unriched uranium and not plutonium as stated. The designers were fairly certain that the uranium bomb would work but there was only a limited supply of the fissionable material. Plutonium was more available but the design was more problematical. The test at Alamagordo was a plutonium bomb. The bomb which destroyed Hiroshima was a uranium bomb and Nagasaki a plutonium bomb.