big gun

NOUN /bɪg_gən/
7 LETTERS · STARTS WITH B

big gun is a valid 7-letter word worth 10 points in Scrabble and 15 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
B3I1G2G2U1N1
3 + 1 + 2 + 0 + 2 + 1 + 1 =10 PTS
DOUBLE WORD → 20TRIPLE WORD → 30
MEANING

What does big gun mean?