Odd, actually. My Cabela's Hawken in .54 is VERY accurate! 250gr. Saboted XTP's over 100gr. 777 is my go to load. It also does real good with a Hornady Great Plains 435gr. over 100gr. 777. Sorry to hear yours does not shoot well. I WILL say, the stock iron sights are JUNK, and will not hold a zero well enough for me. I have offset scope mounts, and put a 2.5x scope on it.
Agree on the sights. I changed those pronto.
Do you the 777 pellets or do you measure each load?