Interesting videos, but very different weapons and targets. The commercial car would have a BAR rating below the armored glass. The guys that shot the glass also admitted to upping the charge. That's one of the problems with black powder weapons: the charges are anything but consistent.
All that said, it only takes 8 damage points getting through the armor to kill an average human (with 4 BOD). That's a completely different yard stick than what it takes to damage tactical armor. To do that, you have to exploit the rounding inherent in the AP vs. BAR system. You are correct that when using TW alone, the damage to BAR 2 through BAR 10 is the "same" (except for the critical hit effects), but that's an oversimplification of AP vs. BAR. I haven't rebuilt the spreadsheet I made that accounted for target BAR yet (I lost the original when the USB drive it was on died). I'll certainly post it on the forum when I get around to it.
True but they were also firing shot, which isn't all that different from a platoon firing rifles. Either way, the car is still getting shot up. I also think that overcharging is one of the nice things about BP weapons. It's kind of like going from standard to armor-piercing rounds in a way. The AP/BD gets upped a little. It can also be lowered if needed.
I know the likelihood of a civil war penetrating a Mech or Tanks armor is close to nil but BT doesn't rely on penetration. It's about knocking pieces off and if a Mechs physically hitting each other can cause damage, surely a big pierce of led will knock off a small piece of BT armor.
This was fun.
https://www.youtube.com/watch?v=JEKyq7BxTKg 
https://www.youtube.com/watch?v=3oX2K0aDYnUI thought this answer was interesting. Most were, "Hell no" type answers. Which I would expect. A few were it might damage the track or drive wheel. Others were yes if it were a siege cannon. This seemed to have some science to it.
Hypothetical Scenarios: Can a 1800s cannon penetrate the armor of a modern main battle tank?
Ian Holloway
, former Principal Lecturer (1997-1999)
Answered 7 months ago · Author has 1.4K answers and 3.3M answer views
It depends on the size of the gun, the type of shot and the tank target.
And there were some very big guns on ships and used as siege pieces.
The effects of smoothbore artillery was extensively tested against armour in the mid 19th century by The Special Committee on Iron. Formed in 1861 by the Secretary of State for War with the concurrence of the Admiralty.
The committee consisted of Captain Dalrymple Hay, R.N. (Chairman), Major Jervois, R.E., Brevet Colonel W. Henderson, R.A., Dr. Percy of the Museum of Geology, W. Fairbairn, Esq., and W. Pole, Esq., with Captain A. Harrison, R.A., as Secretary. this committee sat until 1864 and conducted a large series of investigations and experiments. [1]
The armour tested here would not be as resistant to penetration as rolled homogeneous armour, but it gives us a starting point. RHA is 50–60% more resistant to penetration than iron, at a guess. see [2]
Field Artillery in 1800
[So discounting big guns on ships covered by other answers]
If we are talking about field artillery, so a 12 pounder being the largest gun, that being the largest deployed in the period, then with standard service cast-iron shot the penetration at 50 yards was 1.1- 1.4 inches and about 2 inches with specially treated ammunition -cast iron tending to shatter.
[I have scaled the penetration from larger guns]
This gives us a 12 pounder being able to defeat 12–22 mm of WWII tank armour. This might penetrate an APC today, but not an MBT. It is not enough for the side of a Sherman or a Panther let alone an T-72,. The shot would shatter.
With more modern steel shot, penetration would be about double so at a close range capable of defeating at 1944 medium tank side armour on a good day. Still not enough for most MBTs.
Footnotes
Harrison C A (1866) Results of Experiments with Projectiles of Different Material Against Armour Plate
Development of Warship Armour
