FMJ has a solid copper shell outside with a lead core. It's designed (like everyone else has mentioned) to go straight thru with minimal mushrooming.
The copper tipped bullet folds inward upon impact, causing the lead core behind to mushroom exponentially around it. Putting the round into the shoulder caused the round to nearly fully mushroom. I don't know if anyone has seen a 180 grain bullet that has been fully mushroomed - it's about 2 - 2 1/2 inches across plus 7.62x54 has lots of power behind it.
I can't begin to imagine the impact a round like that has on a human body. . . . . . . and sure hope I never have to.