So this heavy for calibre load worked perfectly, never changing it. Results posted in the bullet performance thread.I agree, the 450 rigby is very capable of pushing those 550 gr bullets, in fact it is a bit of a waste not using the cavernous case to load something that is just more of a good thing, either 550 gr Woodleighs or 515 gr Peregrine monometals. The Lott can also do it but not quite as comfortably, and definitely not to the same velocity. Recoil is probably the limiting factor, not the cartridge. I would favor the extra 10 % weight over 10% increase in velocity any day. Weight of the bullet is more reliable both with the bullet expansion and with penetration as long as you sit in that 2150-2300 fps.