Gun depression and geometry

There is a oft-repeated story that if a tank can depress its gun more, it can hide behind obstacles better. There is more to this than meets the eye.
In the real world, lets say your tank is in front of a ridge, your gun is at maximum depression, and you are placed so your shell will miss the top of the ridge by 1mm. This gives you maximum cover from cover from folk on the other side of the ridge. Good.
Now let's say you can depress your gun by an additional 5°. Now what happen when you shoot? That's right, you hit the ridge. So you have to move to a higher point to avoid hitting the ridge, making you more visible to folk on the other side. Not less visible.

In the picture above the red tank can depress its gun more than the blue tank. So by moving to a higher position it reduces the blind spot in front of the vehicle (the big blue triangle), which is an advantage. But more of the tank is visible to the folk on the other side of the ridge.

So gun depression reduces the amount of dead ground in front of the ridge. If the enemy is still in the the dead ground after your gun is at maximum depression, you are going to have to drive over the ridge to see him. So in that situation, lack of gun depression can indeed force you to expose your entire tank.
FM 17-12: Tank Gunnery

The other myth is that there is a huge difference in depression between Russian tanks and others.
The Merkava 1 and 2 have 8.5° gun depression, Merkava 3 has 7°, TAM 7°, Leopard 2 9°, Challenger I/II 10°, M1A1/2 10°, Leclerc 8°. The various Russian tanks from T-54 and up to T-90 has 6 to 5°.

So the difference is 1-3°, which makes a difference, but not a huge difference.

Another myth is a little mound of dirt or sand in front of your hull will stop an incoming APDS round. It won't. It will slow it down, but you can shoot a tank thru a sand dune or a building.