I shot a groundhog in a large bin of shelled corn yesterday with the Federal Premium 125 grain JHP Personal Defense load in .357 Sig. This load is designed to expand reliably, penetrate over 12" , hold on to its jacket and retain most of its weight. The bullet usually penetrates completely on every living thing I've shot with it (deer, racoons, groundhogs, etc.), and the unusual angle on the groundhog had the bullet penetrating a good 8" of groundhog. The bullet was easily recovered in the shelled corn behind the woodchuck, but I was suprised to see the jacket had been shed.
Now, this bullet has always performed well for me, and I don't really regard shedding the jacket as a performance failure, because this load just kills critters very effectively. I've never had a deer go over 50 yards when shot with it. In some ways I think that losing the bullet jacket can increase a bullet's effectiveness. In addition, I've found this load to be very accurate and to feed reliably in both my Sig P229s.
On the other hand, this evidence for shedding of the jacket does point to important differences between how the bullet performs in gelatin (it never loses its jacket) and what a bullet really does in living tissue.
Michael Courtney