Battle Beneath the Earth (1967)

Rogue Chinese communists under the control of a mad general (the rather non-Chinese Martin Benson) are using advanced laser burrowing machines to dig tunnels under the Pacific and then under US cities and military bases so they can detonate nuclear weapons.

As far as plot is concerned this film is a winner, completely ridiculous but a winner. However, it has a rather cheap and 60s spy film cheesy feel about everything.

It is a British film but set in the US, some of the accents and sets are somewhat suspect. The "Chinese" are also generally played by White actors in the tradition of Fu Manchu. Our heroes led by Kerwin Matthews and Ed Bishop go into the tunnels to stop the Chinese before they can detonate the warheads. 

Thus this film has a gunfight around nuclear warheads deep underground. It is total nonsense but... so much pulp action fun!