Worth reading for all those with hit reg frustrations. I have had similar issues even with a ping under 10 and on low ping local servers. I am putting it down to accuracy of guns in different stances and other connection based problems.
Network Smoothing Factor:
This slider controls interpolation, a technique which determines where a particular moving object or character is currently located in the game world, based on actual locational data periodically received over the network. If an enemy soldier is running along in front of you, interpolation allows their movement to appear fluid, calculating their movement across the screen even though your computer is receiving only periodic updates of where they actually are. So instead of seeing a person jerkily warping from point to point, you see smooth animated motion all the way along. By reducing this slider, you can reduce the use of interpolation, and in turn potentially increase your accuracy by reducing latency; you're more likely to shoot at where someone actually is at any time. The downside to reducing the slider is that it may make character movements more jerky in the game. This depends on a range of factors, including the quality and latency of your own connection to the server, and those of the other players. Start by lowering the slider to the midway point. If after a period of gameplay on your favorite servers you don't notice any jerkiness in friendly or enemy movements, then lower it slightly again. If you do, raise it slightly until things are smooth.
Gameplay Tips
For those relatively new to the Battlefield series, the key frustration most people voice with Battlefield games is the feeling that they've shot enough rounds into the enemy to kill them three times over, but the enemy still lives. The vast majority of the time this isn't due to faulty hit registration as most people believe. It is part of the design of the game:
Bullet spread and hence accuracy in BF3 is greatly dependent on whether you are firing from the hip (highly inaccurate); crouching (more accurate); or prone (highly accurate). The game is heavily biased towards prone firing.
Moving while firing further reduces accuracy.
Using iron sights (default is Right Mouse Button) is absolutely imperative for accuracy.
Fire single shots or short bursts rather than a spray. You can switch your weapon mode from full automatic to single shot and back at any time (default is V key).
Some weapon mods further improve accuracy, such as the foregrip , or the laser sight which boosts hip-firing accuracy.
Reduce your mouse sensitivity. A high sensitivity might feel more responsive, but reduces accuracy.