HPT and MPI testing. Why?

[shadowspirit]

Some ARs advertise the barrel is high pressure tested (HPT) and magnetic particle inspected (MPI). IIRC, some advertise those same tests for the bolt carrier group and the bolt within the group.

Do performing these test give any benefit? If these tests are performed, do they weed out anything that would have led to a much shorter service life or the risk of catastrophic failure somewhere along the line?

[markm]

Some ARs advertise the barrel is high pressure tested (HPT) and magnetic particle inspected (MPI). IIRC, some advertise those same tests for the bolt carrier group and the bolt within the group.

Do performing these test give any benefit? If these tests are performed, do they weed out anything that would have led to a much shorter service life or the risk of catastrophic failure somewhere along the line?

They weed out any material defects. Like you say... a barrel could burst if a crack in the steel wasn't identified. Maybe lysander can shed more intimate light on the rejection rate and such.

[mack7.62]

It's part of mil spec to keep parts suppliers honest, not required by a good company who tightly controls their raw materials. Knights Armament does not do HPT and IIRC has stated it cuts bolt life in half, they also warrant their bolts for +20,000 rounds.

[markm]

It's part of mil spec to keep parts suppliers honest, not required by a good company who tightly controls their raw materials.

American Mediocrity and Contracts awarded to minorities! I remember this crap in the 90s with Sanchez and Coopers mags. As I understand it they would give the magazine tooling to various brown people so they could make money screwing up the military's magazines. American stupidity at its finest.

[bamashooter]

Some ARs advertise the barrel is high pressure tested (HPT) and magnetic particle inspected (MPI). IIRC, some advertise those same tests for the bolt carrier group and the bolt within the group.

Do performing these test give any benefit? If these tests are performed, do they weed out anything that would have led to a much shorter service life or the risk of catastrophic failure somewhere along the line?

Some (very few) do all components. Most batch test. Primary reason is for liability and it also serves as a good marketing tool.

[lysander]

First off - MPI without HPT is pretty pointless.

If a parts is stressed (HPT), the weak areas will open up and become visible to some form of penetrant inspection. If the parts is never stressed the weal areas will never become visible.

Second, the point stressing the parts is some stuff is just not visible from the outside, (see Figure 1).

And third, the idea that a single 120% stress test will some how hurt the part is laughable. That's not how that works. Steel has a fatigue limit, if the stresses are below this limit, it will not fail due to fatigue. Small arms barrels live in this range. M16/AR bolts are subject to low cycle fatigue. This means the stresses in the bolt under normal operation exceed the yield strength in some areas, and eventually cracks will develop. Measured life expectancy of bolts (military bolts) has shown the first crack can show up as early as 5,000 rounds, and some bolts will last over 20,000 rounds. That one loading that was 20% higher than the average is not going to shift that huge uncertainty zone enough to be noticeable.

(If you honesty believe that an HPT will shorten the life of the part, don't ever go to your doctor for a medical stress test.)

. . . they also warrant their bolts for +20,000 rounds . . .

Please, provide a link to this. I have heard this over and over but their website states something completely different.


Figure 1

[lysander]

It's part of mil spec to keep parts suppliers honest, not required by a good company who tightly controls their raw materials. Knights Armament does not do HPT and IIRC has stated it cuts bolt life in half, they also warrant their bolts for +20,000 rounds.

There is only one place in the world to get Carpenter 158 steel - Carpenter Technologies Inc. I do not know how much tighter you can get on your raw materials . . .

Yet, many good companied feel that there is value added to HPT/MPI their bolts, and still manage to price them competitively at $60 - $80, only $10 to $20 more than the non-HPT/MPI bolts.

". . . penny-wise, pound foolish . . ."

[NoVA5326]

HPT absolutely shortens the life of the bolt, and this was explicitly covered in a NDIA briefing.

What it does get you is assurance it will work out of the box, at the cost longevity. Probably a good idea if you want to be able to spit out a TDP to any vendor and have confidence in a QA/QC process.

You will see bolts for GPF contracts maintain the legacy HP/MPI and some to other units not specify this.

[lysander]

HPT absolutely shortens the life of the bolt, and this was explicitly covered in a NDIA briefing.

What it does get you is assurance it will work out of the box, at the cost longevity. Probably a good idea if you want to be able to spit out a TDP to any vendor and have confidence in a QA/QC process.

You will see bolts for GPF contracts maintain the legacy HP/MPI and some to other units not specify this.

Which NDIA Brief?

Please link.

[alvincullumyork]

It's part of mil spec to keep parts suppliers honest, not required by a good company who tightly controls their raw materials. Knights Armament does not do HPT and IIRC has stated it cuts bolt life in half, they also warrant their bolts for +20,000 rounds.

I thought knights batch HPT their barrels and bolts. They would take certain percentage of a lot and test those and then throw them away.