The Truth About Magnetic Particle Testing

There are several debates out there in the AR15 world that add up to not mattering to the everyday shooter.

The simple lack of a third position on the selector and the fact we have to buy our own ammo tends to nullify the 4140 vs 4150 debate. You will have to really work at it hard to get a barrel hot enough that it matters with semi. I don't think you can get the barrel hot enough that the differences between 4150 and 4150 CVM show up.

Another "mark of quality" is the magnetic particle (MP) test. This test is familiar to hot-rodders and engine builders as magnafluxing. It's a special magnetic dye that you apply to a part under a magnetic field and it's attracted to cracks, allowing them to become visible.

Why would this test matter in the real world of guns? It really doesn't. The military requires that every single rifle procured be fired with a proof round. This shot is well above maximum pressure. If the gun doesn't blow-up, it passes. The Army added this as a requirement after it got many M1903 rifles that had poor heat treatment that would explode.

Things were fine with this test for a long time, now enter the AR. The M16 would pass the proof test and then break shortly thereafter. This adds MP test to the list of things to do. Seems that the proof charge was cracking bolts and barrels so that they would fail under normal use later, but not enough to fail at the proof test. MP testing spots if the bolt or barrel has actually passed the proof test.

The problem with the M1903 was quickly identified and corrected, but we've kept the proof check for 106 years. The reasons for the cracks in the bolts and barrels during the proof test were found and fixed in about 1969, but we keep the MP test. Also note that there have been studies of guns that never were proof tested lasting just as long as the proofed ones. The MP test is BECAUSE of the proof test, and it seems the proof test is no longer really necessary. But if you proof check, you need to also MP test to make sure the proof didn't break anything.

If you don't proof test, then an MP test is meaningless. If you proof every single gun, you need to MP every single gun too. If you only proof one gun per batch, then you only need to MP that gun.