So, as the games documentation cleary states that, everything is affected by gravity. Well, I was in the Apache the other day and landed on one of the buildings at a flag post and seated the gunner position. I fired off in the distance, and it appeared that there was no bullet drop at all. I sat there a good 10 minutes (not being useful to my teammates at all) just shooting and watching bullets. Asside from a bit of variation of in where the bullets left the barrel of the gun, it looked as if the bullets straightlined past the game's viewing distance. With any other gun/device, given enough distance, you can clearly see gravity take affect after some distance. Am I examining this wrong, or is there anything else that is does this?
According to this page, the bullet-drop on gun-projectiles varies from gun-to-gun
Bullet drop is -15 m/s for all the weapons except for AS VAL and all sniper rifles (excluding SKS) its -9.81 m/s
Shotgun shells have their own drops which are mentioned at the chart.
I can't find any information about bullet-drop for vehicle projectiles, but it's not unreasonable to believe they also have separate drop-rates. I'll update this answer if I find more info.
Anecdotally, I can state with near certainty that the helicopter and AA projectile-drop-rates are much less than the other guns, possibly non-existent.