Okay two more cents. I hunt pigs a lot, in my experience the best shot is a quartering away head shot with point of aim being behind the ear. The gristle plate youre talking about is a feature found in mature boars (male), Ive never seen it stop a bullet but that isnt the best place to be placing a shot to begin with. A pig's vital area is actually a bit more forward than a deer, so your dealing with possible bullet deflection off a legbone. So- just my opinion, but head shots have served me better. Have I ever had one fail? Yes. Im sure the pig died later but ran far enough off that I never found it. It was a frontal shot, the hog simply wouldnt cooperate.
That being said, I'll share this with you. A man I know shot a medium sized hog several times with his 30-30 and it was still moving. It was in a trap (not a box trap, a lot trap about ten yards square) he was shooting across a creek, perhaps 30 yards away. His theory was that some of his bullets must have deflected on the hog panels that trap was made from. My theory was that he simply missed. When we got the beast back across the creek- which was no small task I might add, we found we were both wrong. He had hit the thing multiple times, just not where it would do much good.