Check the twist rate. The older 444s were 1/38, the newer are 1/20. The 1/38 might be too slow for a 320gr.
Also if the bore was coppered up, it will soak lead and wreak havoc with accuracy. And slug diameter might not be a great fit to the bore. Might want to slug it and measure.
Usually jacketed bullets are more forgiving than lead regarding good bore fit.
My 45-70 Marlin bore was really rough, and was rasping off so muuch lead my bullets were sideways at 100 yards. I fire-lapped it and all is well now.
