On mercenary's i saw a guy using Piers anti-material rifle break it's shell in one shot to the head every time(the whole shell) which also makes me wonder if you have piercing skill lvl'ed up if normal guns could do it.
My partner and I had a lot of fun with that particular enemy. When we were playing Chris's story and one of those popped up, I cracked its shell with the Anti-Material rifle and then we just ran up and started smacking it around. It took all of our stamina but it was satisfying!
We sleep safe in our beds because rough men stand ready in the night to visit violence on those who would do us harm. http://i47.tinypic.com/xds7cn.jpg
A shotgun will work wonders on that shell from my experience. I played as Chris and I loved fighting those things, although three at a time did get slightly hectic. I've had two shotgun shells to his open front area kill one of those before.
PSN:omegachuck23 " Failure will spell doom for the human race" Roy Campbell