。
She fired a single shot.
Literal
She [topic-は] gun [object-を] one-shot fired.
一発 (いっぱつ) is the counter for shots, blows, or explosions — 発 counts discrete discharges. 銃を撃つ (to fire a gun) is the standard expression for shooting. The sentence is strikingly concise — no context, no explanation, just a single decisive action. 撃つ (うつ) specifically means to shoot or fire, distinct from 打つ (うつ, to hit/strike), though they share the same reading.