You can shoot a Pig with the same caliber as you shoot a deer with (assuming .243 and above) if you take a look at this post:
http://thehighroad.org/showpost.php?p=5713016&postcount=193
The pictures are:
and
If you look closely, the "Shield" that you speak of, is HIGH above their vital organs anyway. You want to shoot lower than you would a deer, that's the significant difference. The first photo is obvious the central cortex, or brain stem. Shoot there (smaller target) and instant ground action.
The last pig I took was back in May, and I shot it in the central cortex. Dropped dead right there... took him with a 40gr Vmax .223 round.... was it ethical? You to decide. I knew my shot, I knew my distance, I knew death was right there. All the conditions were perfect, and so I took the calculated shot. If it wasn't the only one there, and there were others, I probably wouldn't have shot it with that bullet... and gone with a different one, that expanded, and not fragmented.
The point is, shield or not, shoot to kill, and the second picture will show you where the heart and lungs are at if you're using a .243, 270WIN, .30-06Springfield, or any other caliber (I left .22 calibers out because I only shoot varmints with my .22 calibers). Use your best judgment.