Magnetic particle inspection is a non-destructive test (NDT) that is done to find microscopic cracks and imperfections in the metal. The process involves electro magnates and tiny magnetic particles suspended in an ultra violate light-sensitive liquid. It can also be done with magnetic powder with similar results.  Colt fires a high-pressure M197 Proof Load cartridge with each bolt and barrel and then does an MPI test to look for any cracks or other microscopic issues with the metal in the barrel and bolt. The image below shows what cracks look like during the test. It is a screenshot from this video by MaterialsScience2000.