I dont think the D model has 20 mm's

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http://www.hitechcreations.com/models/f4u1d.html Vought F4U-1D Corsair
Country of origin:
USA
Crew:
Single-seat
Type:
Naval fighter/attacker
Normal loaded weight:
12,175 lbs.
Dimensions:
Wing span 40'11¾"
Length 33'4"
Height 15'0¼"
Internal fuel:
237 gallons
Armament:
2x20 mm M2 400 rpg
2.20 mm M2 400 rpg
8xHVAR 5" rockets
250 lb bomb
500 lb bomb
1000 lb bomb
Drop tanks