The More You Know: MPI Testing


One criteria for choosing a quality Bolt Carrier Group (BCG), barrel or other ferrous metal firearm part is the passing of a Magetic Particle Inspection test. Commonly known as MPI, the process is non-destructive, meaning that the test itself does not have any detrimental effects on the part in question, so it can be applied to a lot, sample size or an entire batch of a company’s product.

A destructive test, on the other hand, would require a randomly selected sample of products that are tested to a certain level of failure and then discarded.

Since an MPI marking or certification is a highly sought after quality control check for would-be consumers, it is important to have a better understanding of what the test entails and what manufacturers observe for both passing and failing parts.

As mentioned above, magnetic particle inspections can only be performed on iron-based alloys: a magnetic field is applied to the piece being tested. So don’t expect to see an MPI marked billet aluminum upper or hand guard.

From the Magnetic Partical Inspection Wikipedia Page –

Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source.

A magnetic field passing through an unadulterated piece of steel. Note the parallel flux lines of the magnetic field.

MPI @ TFB Credit: MaterialScience2000

MPI @ TFB Credit: MaterialScience2000

A damaged or adulterated piece of steel alloy. Notice how the magnetic field is disrupted and no longer parallel.


Credit: MaterialScience2000

Ferrous particles are added to the surface of the part and will gather within the inconsistencies of the magnetic field. The particles can be in either dry or wet form and illuminated for easier observation. Any inconsistencies (damage) would mean a failed MPI test and the part would be discarded


Credit: MaterialScience2000

Magnetic Particle Testing has its limitations, but it is a cost effective, time saving and non-destructive test that can spot otherwise hidden flaws within steel-alloy firearms parts. And that’s the MPI edition of ‘The More You Know’.



LE – Science – OSINT.
On a mission to make all of my guns as quiet as possible.
Twitter: @gunboxready
Instagram: @tfb_pete


  • PeterK

    WOAH. Neat article.

  • Carl Mumpower

    Yes – very helpful. Thank you.

  • Dan

    So anyone else notice the flaccid arrow dripping it’s sauce? No? Fine ill grow up then.

    • Xtorin O’hern

      i can’t see it, but i would like to, assistance?

    • mazkact

      When I took the Rorschach test all I saw were ham hocks.

  • Trevor Patrick

    It’s important to note that while many manufactureres claim MPI tested components, only the more reputable brands (colt, BCM, DD, etc.) MPI each INDIVIDUAL part. Batch testing is no substitute.

    • Pete – TFB Writer

      Good point. Thanks.

    • Phaedin

      I work as an NDT inspector and nearly every manufacturing company conducts inspections in batches. Critical parts are usually 100% tested but small, non-critical parts are not.

      • Trevor

        I would call bolt’s and barrels critical parts, but that is just me.

    • marine6680

      I don’t think they could mark the part with the inspection mark if they did not do individual testing on that part… To do so would open them up to lawsuits.

      I have seen PSA state that their premium line bolts are all tested individually, but their lower cost stuff, while built the same way, is batch tested.

      The bolt markings reflect that. And as I said, I don’t think the liability would be worth it to the company, if they marked the bolts with inspection marks when they are not tested.

      I could be wrong, but that would require active deception on the part of PSA. (And any other company marking every bolt if they are not in fact individually tested)

    • YZAS

      ..and that’s why I went with DD. A company that does that shows their dedication to their product.

  • Also it should be mentioned MPI should always be done after HP testing. The purpose is to detect cracks caused by the HP round.

    • Pete – TFB Writer

      HP = proof load?

  • Collin C.

    Thank you for the accurate, well-written and informative piece. It’s refreshing to see in TFB…

  • kgallerno

    I did MPI testing for a couple of years in my factory. Standing in a dark booth looking for cracks in tranny parts for a few hours af a time was a excellent way to get eye fatigue. After a few hours you get spaced out. Running the Magnaflux as we called wasn’t so much fun. But it was effective way of finding cracks in powder metal parts.

  • DanGoodShot

    I’ve woundered how that worked. Good to know. Thanks Pete!