QL calculations with Bullseye, your bullet, and OAL, in 5" BBL are:
5.4 grains 18.3kPSI, 990 fps
4.2 grains 11.1kPSI, 802 fps
Leading in breech end of the barrel is definitely because bullet can not hold in grooves. 12BHN alloy may or may not hold in 18kPSI, depending on how deep is rifling...