Why is it only AR bolts need to be MPI and HP tested?

jimbobborg · Nov 26, 2016

Another thought to the original question. Were these tests available to the countries that made the other rifles? It seems to me that they didn't do these tests because they weren't available.

dreadpirate · Nov 26, 2016

I wish all manufacturers would mark MPI/HPT on the bolt. Many do, but some don't even though they do in fact test them.

Pinecone · Nov 27, 2016

MPI has been around a LONG time. And used for a lot of things other than firearms parts.

However, MPI only detects cracks the come to the surface.

But again, some of the other designs, are not highly stressing, small cross section parts.

atblis · Nov 27, 2016

AR bolts don't need hp/mpt tested

and

Who says other guns don't get tested similarly ?

outrider58 · Nov 27, 2016

atblis said:
AR bolts don't need hp/mpt tested

and

Who says other guns don't get tested similarly ?

Care to expand on that?

Biggfoot44 · Nov 27, 2016

Help me out with the terminology ; is MPI same or similar to good 'ol Magnaflux?

drewhall · Nov 27, 2016

Sounds like it, that is what Ive been under the assumption of. Just like on blocks and heads.

From Wikipedia:

"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."

If you go to the MagnaFlux website they have flourescent and non-flourescent etc. Seems MagnaFlux is but one brand of MPI equipment.

mopar92 · Nov 27, 2016

Biggfoot44 said:
Help me out with the terminology ; is MPI same or similar to good 'ol Magnaflux?

Its similar. Small differences and to be done properly needs to be done after firing a proof load (IIRC 50% higher than SAAMI pressures) to check it per Mil Spec.

clandestine · Nov 28, 2016

The AR Bolt is thin in many areas. MPI shows near surface defects, NOT just surface defects.

No, not all AR bolts need to be HPT/MPI, but a quality one does. If you want a dependable gun and not a pile of shit, buy a good Bolt.

Stay away from Nitrided and NiB Bolts. They often have tolerance or embrittlement issues.

Sometimes Bolts are not properly degaussed or demagnetized. Check for magnetized properties with a small carbon steel washer. Magnetiized Bolts are not good.

clandestine · Nov 28, 2016

https://www.mdshooters.com/showthread.php?t=200948

BradMacc82 · Nov 28, 2016

mopar92 said:
Its similar. Small differences and to be done properly needs to be done after firing a proof load (IIRC 50% higher than SAAMI pressures) to check it per Mil Spec.

IIRC, the proof load typically is around 70K psi - the typical 5.56 loading is right around 55K psi.

dreadpirate · Nov 28, 2016

clandestine said:
Stay away from Nitrided and NiB Bolts. They often have tolerance or embrittlement issues.

Glad you said that; I almost bought a nitrided bolt.

kalister1 · Nov 28, 2016

There is an old saying: You cannot inspect the quality into the product.
MPI testing does not improve the product, it only verifies the condition of the piece. More important is the failure rate.

Pinecone · Nov 29, 2016

No, but inspecting properly can remove defective parts from the process, so they are not sold. Making those sold, overall, higher quality.

dreadpirate · Nov 29, 2016

I don't know if MPI/HPT is really necessary, but all the top shelf companies seem to do it. I don't see Colt or BCM offering a non tested version of their bolts. Now Palmetto State Amory on the other hand ...

outrider58 · Nov 29, 2016

At least partially because they are the only bolts bought and sold in mass quantities that are interchangeable with all guns of their kind, making them the most copied bolts ever produced. It is the best way(at this point) of knowing what you bought was reliably manufactured.

Why is it only AR bolts need to be MPI and HP tested?

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