Bullet for bullet the G3A3 is the most powerful weapon in BF3 next to sniper rifles.

The G3 rifle is one of the most widely fielded Assault Rifles in the world.
Multiple versions of the G3 exist, and it has been used in countries around the world, including Sweden where it was known as the AK4.
This particular G3 is the A3 variant, with a fixed stock.
The long barrel and firing heavy 7.62x51mm NATO round enable the G3A3 to reach out at longer ranges than most other Assault Rifles, but at the cost of heavy recoil.

http://www.youtube.com/watch?v=d17Xb2WaJ3o