Wow that's awful
for me, anytime I had vertical stringing with my Encores, usually had to do with the buttstock moving on the bags freely. Been using a Bulls Bag last couple years and don't have near the problems with the big boomers.
I ran up to the range for a couple minutes last night. It was already dark so I just balanced a flashlight on a log near the target. Even with the suboptimal lighting arrangement, my reloads were 2 inches at 100 yards. So I think it was just that factory Hornady ammo that was causing the vertical stringing. Nothing against Hornady, I'm sure they produce the ammo to work the best in 460 Mag revolvers, not single shot rifles.
I now have high hopes for accuracy with this rifle. I had never loaded for a magnum straight walled handgun cartridge before. I still have 2 additional powders to test and 2 more kinds of bullets to test. So either I got super lucky, or I still have the opportunity to improve accuracy. Plus I really have no idea what I'm doing with crimp strengths yet and plan to test both crimp strength and seating depth still.