A brief explanation of MPI w/pics
My day job is as an applications engineer for a valve company. Most of what I do is with high pressure/high temp applications for power plants (steam service). Due to the critical nature of the process contained in these vessels, there are several safety and manufacturing codes that must be complied with. One of these is MPI.
I was recently at a valve manufacturer's facility and in their QC department they demonstrated an MPI for us. To the naked eye, the valve body looked as solid as could be. However, they sprinkled the valve with red ferrous (magnetic) dust. They then used electrofied probes to create a magnetic field across the valve. The dust accumulated in the flaws and they became clear, thus the valve body was scrapped. As you can imagine, you'd want to similarly find flaws in the constructions of the critical pressure-containing parts of a firearm like an AR15. Thus, the reason for and importance of quality MPI bolt and barrel:
http://i854.photobucket.com/albums/a...6092011160.jpg