I think he is talking about hosing an M3 down with pintle gun fire to no avail.
Oh

M3 Main body Armor = 6.4mm
Windshield Cover = 12.7mm
.50cal pulpit cover = 7.9mm
You're shooting with rifle caliber armament with a few (.50cal) exceptions. Adding distance and deflection makes penetration worse as well. The M3 is armoured, lightly, but still armored. You'll notice that the windshield cover has almost double the armor as the rest of the vehicle.
In this scenario it still stands that bullets will absoutley not hit in all the same place even in the most perfect of conditions, and with a single gun, 100rnds...is actually not alot, from a panzerIV's pintle I estimate that to about 3-3.5 seconds firing time for that.
The problem is that they are so many variables that saying "I fired 100 rounds" can not possibly translate into "I should have killed it"
In AH it really, really, REALLY matters where you hit your target, what angle, what distance, and with what type of round.
In short if you're "shooting hundreds of rounds" at a target and it isn't dying. it probably isn't the game, it is how you perceive the game. You're mostly missing, or your hits are not effective.
It could be a game bug of course, but for some odd and unexplained reason the "I did X and it should have died" films seem to not be recorded.