Originally Posted by KodiakHntr

Well, not really. Not at all actually. The X gets its penetration capability from the fact that it is a long for caliber bullet, and retains all of its weight.
If you look at an expanded X (or TSX) if you can recover one, and compare it against a cup and core bullet, you'll find them very similiar in expanded diameters.



Well, no. The nose doesn't shear off. It disintegrates, throwing lead particles ahead of it and off to the sides. (Google bullet wound channel Xrays to see what that looks like if you want). The "energy dump" is from this front section mushrooming back and coming apart. The partition in the middle of the bullet is what stops it from coming any farther apart. This results in a high speed slow down of the bullet, although it will exit sometimes.