The thug that survives being shot in the head

Generic tough guy - The idea seems to be that the bullet slowed while passing through the other heads enough that it couldn't penetrate a normal, human skull.

I'm assuming the mutants they were making all had slightly different mutant powers. Super strength seemed to be one of the defaults but I assumed this one had bullet proof skin/skeleton unlike most of the others. They certainly didn't plan on Deadpool's powers that seemed to be a randomly generated healing factor. Francis seemed to have super strength. But to answer your question I think he was simply one of the manufactured super soldiers working as muscle for them.

I just assumed he had a metal plate in his head.

For anyone who thinks he had to be abnormal I should expand on what I said earlier; the scene appears to have been inspired by a Mythbusters episode where a .50 caliber bullet was fired through a line of watermelons. It penetrated the first three and then ricocheted off the fourth. The same exact kind of bullet was used in the movie, it was fired from the same type of gun, and it was also done at close range. They just changed it so that the bullet ended up stuck in the guy's forehead, probably because they liked the visual better and it allowed them to show the audience what happened without having to do a slow mo of the bullet ricocheting.

Here is a slow mo of the watermelon experiment: https://www.youtube.com/watch?v=8KcaDYsAudo

Here is info on the gun and bullet used in the Mythbusters episode: http://www.imfdb.org/wiki/Mythbusters#Desert_Eagle_Mark_XIX

And here is the same info for Deadpool: http://www.imfdb.org/wiki/Deadpool_(2016)#IWI_Desert_Eagle_Mark_XIX

Here is the full Mythbusters scene: http://www.discovery.com/tv-shows/mythbusters/videos/aim-for-the-melons/