firing line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The line of positions from which fire is directed at a target.
• noun The forefront of an activity or pursuit; the vanguard.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military The line from which soldiers fire their weapons at a target; especially the front line of troops in a battle
• noun A row of shooters
• noun idiomatic The vanguard of an activity

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the most advanced and responsible group in an activity
• noun the line from which soldiers deliver fire