Bear in mind the AH version only allows twin .50s in the tail turret. There is no quad top turret or twin .50 top turret, so you're stuck with twin .303s in the top turret and have no option for single .50 anti-schragemusik in H2S position either. Therefore, don't even contemplate using the guns. The lancs main weapon is speed and ability to dive like a bomb, outdiving the majority of defenders or at least delaying the attack until escorts arrive and enemy fuel is lower.
I take the .303 option with 10,000 rounds and will rtb with only 2000 rounds left. I will fire off as much ammo as I can, only a few enemy will not be deterred by this. I fire at 1k to 1.5k distance and let them know I am awake and ready. I will fire, even when out of shot, simply to stop them re-entering 1k distance or getting into a shooting position. Don't get TOO low in a lanc either, the top turret becomes useless if you are noe. Don't be dying in a lanc with 9000 rounds left.
The way to survive is be very fast when they approach from behind, at 800 yards go to WEP and steepest climb.
Unless you have aimbot or years of experience, take 303 and put on a show, every mile gained counts.