If your New Service is all original, don't touch the chambers. However, if it has been refinished, sights filed upon, etc., you can rent a .45 Colt reamer for around $50. I just reamed two .32 H&R Magnum cylinders and a .22 LR cylinder to .22 Magnum. Make sure you rent the handle also. My tap handle didn't fit the reamer and I wound up doing it with an open end wrench.