apron of a gun; cover for the touch-hole on a cannon or matchlock
to start (an argument, a battle, etc.)
戦いの火蓋が切って落とされた。
The battle was launched.
選挙戦の火蓋が切られた。
The election campaign was kicked off.
交渉の火蓋が開かれる予定だ。
Negotiations are scheduled to be opened.