Past Times — Poul Anderson

Saunders got up and ran, crouching low, weaving back and forth. A shot wailed past his ear. He spun around, the pistol in his hand, and snapped a return shot. There were men in black uniforms coming out of the jet. He fired again and felt a savage joy at seeing one of the black-clad figures spin on its heels and lurch to the ground.

The time machine lay before him. No time for heroics; bullets were singing around him. He had to get away -- fast!