Though you should hold your fire if the tank is facing you, hitting the vision slits and hull MG dead-on often kills a crew man. For shots to the sides, T34 turrets are very vulnerable to AT rounds from a perfect 90 degree angle which typically kills either crewman in the turret.
The safest spot to hit at any distance is the chassis, just above the threads but below the upper hull. On the back end of the tank you will disable the engine, just below the turret you may even one-shot the tank as the ammunition rack explodes.
For shots from the rear I often go for a mobility kill first, taking out the engine before focusing on the back of turret, cupola or turret ring with the intention of disabling crewmen or gun systems. A tank with no gun and no engine is effectively neutralised, even though it won't net you any points until the tank explodes.
AT rifles take more patience than any other weapon system. It is a very inflexible role in that while you are not engaging tanks you are not able to do other things very efficiently. You can provide a warm body on the cap zone, but your gun is way to cumbersome to fight enemy infantry. When you actually do engage tanks you need to find the best spots for engaging tanks, which is usually in weird places, far from your spawn and your teammates. Have nerves of steel and hold fire until you actually have a good opportunity to damage the tank. As a tanker I have no fear of rapid firing AT rifles, they cause no damage. I am more worried about a single shot from nowhere taking out my commander, a skilled AT gunner will quickly strip a tank down so as a tanker I often have to flee.
As close as you can be, with as narrow angles as possible (straight on from the side being the best overall option), that is how you destroy tanks before they realise they are marked for death. Enemy tanks I can easily avoid since I can easily spot them. The hidden AT rifle is a potential nightmare in the right hands.
