My freind has a Rugar#1 that started out as a .218 Bee. He had it rechambered to the .218 Maushburn His only problem is that when he fireformes his .218 cases and reloads with his improved brass his accuracy goes from .765 fireforming to right at 2.00 with the new case. He fireforms using cci400 WW cases 13gr of IMR4227 and a 37gr. Calhoon. His improved load uses 15gr. of IMR 4227

Any sugjestions on what his problem might be and how to find out whats wrong?

Have you considered that maybe the 15gr load just isn't very accurate in his rifle? (Bad vibration pattern in the barrel or whatever) The rifle is accurate with the reduced load, so I doubt the problem is the rifle.Look in your reloading manuals and try different powders to develop an accurate full-power load.