Fired as part of Operation Upshot–Knothole and codenamed Shot GRABLE, a 280 mm (11 inch) shell with a gun-type fission warhead was fired 10,000 m (6.2 miles) and detonated 160 m (525 ft) above the ground with an estimated yield of 15 kilotons.

Original Caption:

Frenchman’s Flat, Nevada - Atomic Cannon Test - History’s first atomic artillery shell fired from the Army’s new 280-mm artillery gun. Hundreds of high ranking Armed Forces officers and members of Congress are present. The fireball ascending.

Thought this was interesting, not small arms related, but we don’t normally think of nuclear weapons outside of the nuclear triad.