firing line

WordReference Random House Unabridged Dictionary of American English © 2025

fir′ing line′, 

1. Military
   • the positions at which troops are stationed to fire upon the enemy or targets.
   • the troops firing from this line.
2. the forefront of any action or activity, esp. a controversy.
• 1880–85

Collins Concise English Dictionary © HarperCollins Publishers::

firing line noun

1. the positions from which fire is delivered
2. the soldiers occupying these positions
3. the leading or most advanced position in an activity