I would start with about 85 grains of powder and go to at least 245, 295, or 348 gr. In my experience, powerbelts seem to like lower powder charges with 777. Mine shoots 348's and 90 grains the best. This is my elk load, so with 777, you will have plenty of power even if you drop down to 85 grains. Another option is to try pure lead conicals such as maxiballs, no excuse conicals, or buffalo bullets. You got some work ahead of you. There are no shortcuts.