To be clear, the bullet resistance is only on the boars, and particularly the very large ones.
The "shield" runs from the back of the shoulder, around the chest to the rear of the offside shoulders. It's made of very tough cartilage and on large boars can easily be over 3" thick at it's thickest point.
The shield will very much stop a lot of bullets especially if the shots are not at close range and often minimize penetration on others.
Avoid hitting it if you can and most calibers will do fine but unless you're taking head shots I would not recommend any sort of FBJ or other non expanding bullet as you'll end up losing a lot of them.
Between Mario and I we killed four this week with the AR, the biggest two around 200lbs sows and then I killed a huge boar that soaked up a running away shot from the .375 Ruger with 250gr GMX that hit him halfway between his anus and nut sack and stopped in his chest. It slowed him down enough I thought he'd fall with each new step but finally at about 450 I put another as he started to angle ever so slightly right through the mid rib and into the spine between his shoulders that planted him stone dead.
This was a very large boar around 500lbs measuring 52" from nose to the base of his tail. Unfortunately one of his 4" plus cutters was broken off so what could have been a monster trophy ended up coyote bait except for the loins and tenderloins.