Miles, the only time I have seen a Nosler Partition deflect from its course was when my wife deliberately put one in the tailbone of a cow elk that was stumbling downhill--about to die from another 150-grain Partition through both lungs.
I don't know why my experience differs so much from yours, but then life is full of surprises.
JB, It's been quite a while since I used Partitions. But... I never saw them perform all that different than other cup and core bullets other than they were harder to get accurate loads with. A good example of lead core bullets deflecting was a deer I killed this year (after it had been shot in the knee by a guy in my party). I was shooting a 30-30 with LeverEvolution 160s. The deer was maybe 100 feet away going through four foot high swamp grass. I had a decent chance at a back of the head shot so I took it. The front of the deer when down and the back up a little. The bullet hit one of the last two ribs down 3-4 inches from the top of the back. It should have penetrated just behind the diaphragm maybe catching a little liver. Instead it bounced up off the rib through the chops and out without touching the spine. I have seen many variations of this. Scapula hits, leg bone hits, rib hits, spine hits etc. Hit bone at an angle and it seems pretty easy to deflect bullets. That's why what I have seen with the Barnes is so impressive to me. I have only ever seen one barnes deflect, and it was not much. Deer was at 140-150 broadside. Shot from nineteen feet off the ground. Exit was maybe an inch above the entrance. Just barely enough to notice if you were looking. I look. Carefully at every one. My experience has been that they go through in a straight line that's very unexpectedly straight.
I have had two lose petals in deer. One was a 300 WM with a 150 X at 50 feet. In the mouth, shattered teeth, through the neck at the atlas and a petal lodged in the spin just in front of the pelvis. The other a TSX 130 in .270 at 80 feet was also in the mouth shattered teeth, out through the atlas. One petal was out a little off center on the neck a couple inches below the shank exit. The other petal made it through a lung and out the side of the rib cage in the middle.
Less those, all the rest of my experience has been uniform.
I intend to design a test to look at the what it takes to tumble one on the way to the target, and to examine the possibility of it re-stabilizing in some fashion traveling butt first.
