I think that could be the problem...Rifling on a true round ball barrel is deeper than what the Lee's like...Barrels with button rifling will be about .006 deep, or shallow...I'm thinking the deeper rifling allows gasses to get by the bullet, causing erratic behavior...

I have seen that happen many times. I have seen tight 100 yard groups fall apart at 150. When I was doing the load development for my 460 gr paper patch bullets I was getting 2" groups at 100 yards. I went to 150 and I didn't even hit paper. I moved my load down from 90 to 80 grains and I was back on track. I shot a couple groups at 200 that were sub 5" and I stayed with that load. Ron

With that twist rate a conical would never have the ability to stabilize properly. Though with enough powder and a distance of only 50 yds., those targets would make sense.

However, when you lengthened the distance the bullets were in such a state of unstability and I am surprised they didn't keyhole. Just my thoughts though.

Don't hunt past 50yds.