Fellows,
I have killed a couple dozen truckloads of them over a couple of decades; 189 of them to be exact. Any boar hog over a couple hundred pounds, and they don't often get that big, will develop the infamous shield. The older they get, the thicker and gnarlier it is. That has led me to believe that age more than size has something to do with it. Now I also raised wild hogs. BTW they get tame in three days. They are a lot smarter than people give them credit for. A neighbor had an old boar, and that animal looked like an out of place miniature rhino, has shield had shields! My favorite boar Gordy, had a big thick shield also, though he still had all his hair compared to Grandpa down the street.
I guarantee you that anything less than a 40 caliber minie type ball would be marginal. I would would bet that a pure lead ball in 40 would be stopped, especially since most people shoot above and behind where a hogs heart is, which happens to be where the shield is thickest. Think about why they have the shield. They brawl like two wrestlers with knives! Their strikes are predominantly from the neck to the shoulder blades. And believe you me I ave seen the results!
Anyway, If they are slightly quartering away, you can get through the thinnest part of the shield. But as Brown Bear said, a modest sized boar hog's shield will not stop a 45 ball and stout charge of BP.
If you know that you are after a truly trophy sized hog, then arm yourself appropriately for an ethical take. 54 Cal ball, tight patch and accurate shooting will anchor it right there if you do your part. The important part is as to load appropriately as Mr Getz mentioned. If you want to shoot a big bore, load it big. Anemic loads will work on deer, but not on pigs. BTW pigs don't bleed well, and are hard to track.
And lastly, I've been charged a few times, mostly because I was standing where they wanted to go. It's not like they see all that well poor buggers.
Best regards,
Albert “Afghanus” RaschCharged! Hog Hunting at its Best!ΜOΛΩΝ ΛΑΒΕ!