WASHINGTON — Earlier this month the U.S. government conducted flight tests of the B61-12 nuclear gravity bomb over Nevada. <br /> <br />More are required before it enters service in 2020. <br /> <br />According to a factsheet from the National Nuclear Security Administration, the B61 bomb is almost 50 years old and the U.S. holds four variants of it within its nuclear weapons stockpile <br /> <br />The NNSA says the B61-12 will be an upgrade that comes with greater accuracy via a steerable new tail kit. It will also extend the life of previous models by 20 years. <br /> <br />When complete, the B61-12 is expected to be 3.65 meters long and weigh around 374 kg. <br />It'll also have an adjustable explosive yield of 0.3, 5, 10 and 50 kilotons
