That sounds right to me. Due to limitations in case capacity , you could not put in enough slower powder to use the increase in barrel length. I have had trouble finding much load data for the 500, it all seems to be for people trying to reach max. If I needed that I would get a rifle cartridge. I am using 50/70 load data and it seems to be working fine. If it would go through a buffalo, what more do I need?? I use Blue dot,2400 and xmp5744, and I think they would all burn completely in an 18" barrel. I have to get a friend to chrony some loads, but I am trying to get an accurate 440 FNGC load between 1200 & 1400 fps. Incidentally, just 12 grains of unique will put that bullet through 3 treated landscape timbers, that is 10 inches of pine, and leaves your shoulder smiling.