As a former infantry officer, I can make some comments based on training and first hand experience:
1) .303's would have minimal impact on anything but the lightest armored vehicles. I have personally seen quarter inch aluminum armor on a M113 hull be penetrated but still capture the 7.62mm round from an M60 machinegun not letting it completely through. Fired from above into open toped halftracks would of course wound or kill the crew.
2) .50's were originally designed during the first world war as anti-armor weapons against lightly armored vehicle of the period (for Geneva Convention reasons it is still classified to this day as an anti-vehicular weapon not for use against dismounted troops) and I have personally seen a .50 API round go completely through a modern armored personnel carrier hull but bounce off a derelict tank turret.
3) WWII 20mm and 30 mm cannons were designed to do damage from their explosive charge and not kinetic energy like the machineguns cited above. They would damage vision blocks and could damage running gear on a tank but probably not penetrate the armor of the hull or turret. Today's 30mm cannon in the A-10 is designed to use both HE and kinetic energy depleted uranium armor piercing rounds. I have seen films of the gun being tested back in the early '70s where the uranium round was fired into the top of an M48 tank laid on its side. The round went completely through the top and bottom armor as well as a 10 foot dirt berm erect behind the tank. (Awsome fire power but not available during WWII).
4) While training in the use of explosives, we were taught how to use relatively small charges (40 pounds and under) to defeat Soviet armor. 40 pound shaped charges were most effective in actually penetrating all but the thickest frontal armor. Hovever, a 25 pound satchel charge placed against the tank would disable the turret from traversing...not blow it off. Some of these devices also used small amounts of fuel to set fire to the wiring in the engine comparments. A large external fuel can or drum could easily set fire to a modern tank's wiring in the engine compartment. In fact, the Soviets mounted two 55 gallon drums on the rear deck some of their tanks but rigged them to drop off before going into combat to avoid the huge risk of them being ignited.
5) The artillery uses a round for the 105mm howitzers called HEAP-T (high explosive armor piercing tracer) which fires about ten pounds of HE agains the side of a tank and detonates it. Its designed to kill or disable the crew through concussion (rings the tank like a bell) or spalling (flaking pices of the armor off the internal wall of the tank like shrapnel or sending equipment flying across the internal space. These were not available during WWII but the effect is worth noting.
6) During the first Gulf War, I heard stories, supported by gun camera films, of 500 lb GP bombs with laser designator kits being used to kill Iragi tanks because they were cheaper than Maverick missles (less than $100,000 vs. $250,000 or more was cited). Turrets were seperated from hulls and I believe this would have killed both tanks and crews. Here again the level of accuracy mentioned was not achievable in WWII but the effect is well worth noting.
Based on the points outlined above, I'd say that WWII machine guns and cannons would NOT be effective against a buttoned up Tiger. Aerial rockets, which have warheads not much bigger than a bazooka round probably would damage the running gear but might bounce off or detonate without effect against the more heavily armored areas. The larger HVAR rockets could have effects similar to the HEAP-T round I mentioned above. A direct hit with a 500 or 1000 pound GP bomb would probably kill the Tiger. Near misses with bombs depending, on the distance from the Tiger, would have varying degrees of impact on the vehicle's combat effectiveness. Close in detonations would probably damage running gear, disable turrets, concuss but not necessarily kill the crew and even overturn the Tiger.
I don't think that this settles the debate by any means but it is more annecdotal information for the grinder.
