Lizard: In the top photo you are seeing the bullet base on the left side. The nose to the right. The rifling is on the left half which is the bullet's bearing surface. Most .45 acp full metal jackets are open on the bottom. Not like a minie ball but open as in no copper. You have bare lead core showing. The rest of the bullet, the sides and the ogive, the very top nose, are copper jacket about .010 thick. Some are thinner and some are thicker by a bit. Newer FMJs are electro-plated too. Very thick copper plate on a lead core.
This bullet has a standard gilded jacket of copper. So this open back end is where (in this case) another slug has struck it from the rear. This ripped off the "struck" side gilded jacket and mashed the soft lead core like cookie dough. At high speed fun things happen. It, the second bullet, glanced off but removed some lead but spread the rest out like you see in the photo. The stronger copper jacket on the reverse, or non-stuck side only bulged a bit and stayed in fair shape. Good enough to see the rifling easily. One side looks very much like a bullet and the damaged side looks like smudged lead. This is what it is.
This is a common find on the shooting range. I'm more sure of this than I was on the earlier shot put episode. Turns out the shot put was the detonator to a plutonium bomb, but what the hell, I was pretty close. LOL
OT