big gun

noun3

1. •
   A large artillery gun or one of the largest-caliber naval guns (as on a battleship).

   • London, Nov. 1: The big guns opened fire on Paris today at mid-day. — 1870
2. •
   [in the plural] A person, group, thing, or course of action that is powerful, influential, or particularly effective.

   • This has been a great day for the big guns. This does not refer to Prof. Lummis and the other big guns of the Board of Visitors, for every day is a great one for them. — 1888
3. •
   A large surfboard used to ride very large, powerful waves.

   • Special boards are used called "big guns." — 1966