^ What they said... but also remember, when you're looking around in your main gun sights you're looking from a very small part of the tank... the rest of your tank is still visible and hittable even if you don't have los. It could be that he merely only saw part of your tank and knew you were in one of the fighting positions, and thus was able to howitzer the rounds in. 6 or 7 Km means that the round is going to be coming in almost directly on top of your tank and therefore hitting the weakest armor and having the easiest penetration.
I've managed to get it to where I've been able to barely hit the front armor of a sherman from 1-2Km, but because he was behind a tree he couldn't see or hit me, it took alot of ordnance because of the off-angle, but I was able to kill him.
And for the most part, I've found the fighting positions scattered around the VBases to be really poor positions to use. For one thing, there's 0 concealment and barely effective cover, you're better off scampering into the woods and playing hide 'n go seek.
And everyone is always looking there. The secret to being a successful GV'er is not to be seen. If you are seen you can be hit, if you are hit you can be killed. The trick isn't so much as having a position with absolute command of the ground, and gunning down everything in sight while you laugh at your position soaking up all the incomming rounds instead of your armor... that only happens in the movies.
Treat your tank like a sniper. Find a position, find a target, learn to use your stadia lines properly and 1-3 shots for a kill and then get out of there and find a new position.
Or if you want a real fun challenge, up an M8 with AP rounds and just speed around knocking out the turrets of every tank you see that's trying to sneak their way onto your base.
That's a good way to practice your gunnery, and nobody really expects to see an M8 doing a drive-by of their hiding spot.
But that's only really doable in an M8 where you have enough speed to outrun the turret traverse of a parked tank.