That is a nice shot. My father in his rendezvous days used to shoot at 300 yard balloons. It was an added competition. I've tagged our 550yard gong multiple times with my Lyman great plains. The balloon shot might be more impressive because that's hold over while I have a spot on the hill side to aim at for drop. If I had to guess the drop at 550 yards I would say it's at least 40 to 50 feet. Probably more towards 40 feet. No way of really knowing I just aim at a certain rock and that's the right drop in feet to hit that gong. It was a fun day at the range zeroing in on that gong. When you hit a foot or two low or high your like that's getting close. It's insanity but sure fun to chunk some lead.