A little cap-happy there eh JustJim?
Like Innominate said, it's all about where you place the shell. And for those of you firing 37 mm ANTI AIR shells at tanks, don't be too surprised if you can't penetrate thick armour too effectively.
Let's take a look at the armour specifications of the 37mm Ostwind Flakpanzer and the Panzer IV H.
Ostwind Armour, mm/angle:
Front Turret: 25/37
Front Superstructure: 80/10
Front Hull: 80/12
Side Turret: 25/30
Side Superstructure: 30/0
Side Hull: 30/0
Rear Turret: 25/30
Rear Superstructure: 20/11
Rear Hull: 20/9
Turret Top / Bottom: open
Superstructure Top / Bottom: 12/85-90
Hull Top / Bottom: 10/90
Gun Mantlet: 25/round
Panzer IV H Armour, mm/angle:
Front Turret: 50/11
Front Upper Hull: 80/10
Front Lower Hull: 80/12
Side Turret: 30/26
Side Upper Hull: 30/0
Side Lower Hull: 30/0
Rear Turret: 30/10
Rear Upper Hull: 20/12
Rear Lower Hull: 20/9
Turret Top / Bottom: 10/83
Upper Hull Top / Bottom: 12/85
Lower Hull Top / Bottom: 10/90
Gun Mantlet: 50/0
Hull armour thicknesses are about the same since the Ostwind II actually has a Panzer IV chassis, but note that the turret armour on the Ostie much less than the Panzer.
The Panzer IV H fires a 75 mm KwK 40. Take a look at the armour penetration for a square hit at various ranges:
An AP round impacting at 90 degrees will penetrate 64 mm from a range of 2000 meters (~2200 yards)!
I would say that most GV engagements occur at less than 1000m (1100 yards). What this means is that if you're in an Ostie, and a Panzer IV puts an AP shell into any part of you other than your front hull plate at not-too-shallow an angle, you're probably off to Ostwind heaven, or at least taken out of commission.
On the other hand, Ostwinds fire 37 mm AA shells, meaning that they are mostly HE or incindiery, not armour piercing. I would be quite surprised if they could penetrate 30 mm of steel armour plate except at perhaps point blank range. So, I wasn't surprised upon reading JustJim's story that he had to pound a Panzer from the rear 10 times or so before he would deal any real damage. Ostwinds were never made to have any particular anti-armour capability.
The telescopic sight on the Panzer in AH is marked for up to 3200 yards, but it's almost impossible to hit a moving target at that range, since wind will affect the shell trajectory quite a bit if you're that far out, especially noticable if you're firing directly crosswind. Also, damage beyond 2000 yards is reduced dramatically. I usually try to advance to 2000 yards or less to get a decent shot on target. I've yet to see an Ostie that hasn't popped if I get a good hit on his side. From the front you've pretty much got to go for the turret. 80 mm of sloped frontal hull armour is hard to break.
As for .50 cal taking out tanks, it's possible. The turret top armor is very thin, and .50 can penetrate it (apparently it's possible to kill a Soviet BMP with a .50 cal MG). It was also not unheardof for aircraft to bounce their bullets off the ground (ideally paved ground) in front of a tank in order to ricochet the bullets and hit the paper-thin underbelly armour. Doing so successfully would be effective in damaging the engine. Whether HTC has modelled bouncing bullets into the game, I'm not sure. It would be interesting to test though.
