HPT and MPI: still viable and necessary or outdated bureaucracy?

[rob_s]

A discussion cropped up elsewhere and rather than hijack that thread any further I thought I'd start this one.

If you're going to participate, you need to have at least a passing understanding of the processes being discussed, and be prepared to elaborate on your position with more than one-line posts insulting the other posters while claiming unique knowledge of the secret of life. You should have more familiarity with what is involved than this, but for the sake of readers and those that may have questions, some links to basic explanations:
http://en.wikipedia.org/wiki/Proof_test
http://en.wikipedia.org/wiki/Magneti...cle_inspection

An understanding of how the above topics relate directly to the AR and how they are employed in the process would be helpful. This isn't a bad place to start
https://www.m4carbine.net/showthread.php?t=50559

I look forward to the discussion.

[orionz06]

From the original thread in order, no post has been skipped:

What I wish the chart had is the percentage of important functional parts that are dimensionally correct and correctly heat treated.

This data is unfortunately extremely expensive to gather, but that is one way we screen subcontractors. If they send out of spec parts we replace them with another subcontractor.

You could be buying some rifle that had an MPI barrel and think it is all cool and never realize that your cam pins are slightly oversized. We HPT every barrel but don't MPI them, and would rather put the QC budget into checking parts for dimensional accuracy. Why? Because such testing actually sometimes finds out of spec parts buy one never fails a barrel from MPI (unless it fails, but they never do). So frankly, MPI is Colt's problem. It is mil spec for an M4. It is not mil spec for a 416, G36, or Sig 551. So demand it if you want, but please consider that what really matters is getting bolt and trigger parts that are within the tolerances stated on the engineering drawings.

The issue of the FCG parts is always one that exists, but as you know hard to verify.

I'm curious about HPT without MPI. You stress the barrel but then don't do anything to survey the effects of that stress?

It is inspected.

http://en.m.wikipedia.org/wiki/Proof_test

Can you elaborate on the inspection process you use?

Visual inspection followed by function testing.

Ever had one fail?

No, even proof testing an AR is pointless as they never fail. I believe proof testing only makes sense for new and unproven designs.

From a metallurgical stand point, proofing them then visually inspecting them makes no sense. Microscopic cracks can and will form in metal from stress and fatigue. These can lead to failures. MPI tests for these cracks and imperfections in the metal. So if you're proofing the barrels without MPI'ing them, that would lead to the potential of sending out a damaged barrel or one with other invisible flaws.

It has been stated (and I cannot find the post at this time) that even the top end parts makers have some amount of bolts that do not pass the proof-mpi. Pair this with the recent rash of busted bolt images circulating the forums as of late and I will gladly pay the $20 premium that comes with a BCM or Daniel Defense bolt.

And for "new and unproven designs...", despite that fact that some companies know exactly how do make an AR work, they still test and inspect. Testing is not an extra developmental step, it is a verification that good parts are going into the gun.

One has no idea if a bolt is good or bad based on an MPI test. I have found many bolts out of spec dimensionally that have passed MPI but I am not aware of any in spec bolts which have failed MPI. You would be much better off taking that $20 and having someone spend 30 minutes gauging as many critical dimensions as possible or doing an extended firing/function check. MPI is something Colt got stuck with contractually. The HK416 and Sig 551 - two awesome rifles - are not MPI as far as I know. If you want an M4, buy a Colt M4. Nothing else is an M4. If you think MPI is important for non-M4s, then better have all the pistons in your car MPI and send your 416 out to a lab. Or you could just shoot it.

Let me explain this another way... Everyone has a QC procedure (or lack thereof) and QC budget. I am not sure exactly, but say we spend $50 per upper for QC. If we consume $15 of that on MPI, then that means less of the budget can be used for test firing and gauging. It is easy to just pay the $15 and get to the left of the chart. The chart has no column for number of rounds used in test firing, how many dimensions are gauged, how much high speed video is done, or how the parts are heat treated.

Who said anything about being OK with parts that are not dimensionally correct?

Dimensional tolerances and metallurgical integrity are two completely separate issues. The fact that parts do fail and there is a rejection rate alone suggests there should be hpt/mpi for certain applications.

Correct, both of which cannot be ignored. In this particular application both matter.

Testing is incredibly important. But testing to me means:

Hardness testing.
Gauging.
Function testing.

Ask each company how many rounds they fire on each upper before shipping. That is much more useful than MPI. But live fire testing is also expensive.

I can understand why you would think that way.

I think that way because we employ actual engineers who went to college.

Is that implying posters in this thread are not engineers? I mean the actual posters, not one of their friends.

I'd rather this thread didn't go this direction. If you want to discuss things at this level, or maybe actually explain why your prefer the system you have in more than one-line posts, how about starting another thread?

This thread is for status updates on the new Chart.

To bring this back around but on the subject of bolts...

The new version of the Chart has much more information about bolts. Material, treatment, method of treatment, testing, etc. Not every little detail can be on there, but it certainly has more detail than the old version.

There may well be "better" ways of doing things. It may well be that current metallurgy negates the need for HPT and MPI, but the Chart is about comparing things to a known standard not about believing claims of "better" made by manufacturers.

Ok. I was just saying our decisions are based on science and engineering and would hold up to an engineering design review. HK and Swiss Arms made the same decision.

Let's take the discussion here
https://www.m4carbine.net/showthread.php?t=81215

[rob_s]

Thanks for catching us up OZ.

[rsilvers]

Absolutely important when working out a new design and process for verification. Once none of the parts are failing, the design is proven, and one would phase out the procedure unless bound by contract. We HPT 100% because it is company policy.

[orionz06]

In my experiences, the proof test is still critical. To start the proof test with an out of spec part is foolish, I don't know how someone would honestly believe that anyone would suggest it.

Other things to consider about the proof test and inspection are price. Comparing Del-Ton to BCM, there is a $25 difference, for the sake of argument let's call that the price to HPT/MPI. I am not aware of any other means of inspection as reliable and fail proof as what is done now. You can't argue with overloading the part and then checking it. It will catch the issues just as well as any other means I am aware of now.

It was mentioned that H&K does not do this, but aren't 416's noted for breaking bolts? I know there are a ton of broken Del Ton bolts floating around the net these days.

Does anyone have access to the failure rate of bolts from BCM during testing? Or any other company for that matter? What I have been told in the past, in confidence, only enforces my thoughts above. If there were numbers I think it would go a long way to kill the debate.

[orionz06]

Absolutely important when working out a new design and process for verification. Once none of the parts are failing, the design is proven, and one would phase out the procedure unless bound by contract. We HPT 100% because it is company policy.

But parts are failing, this should be enough to reason that the current means of testing should continue.

[rsilvers]

What do you mean about starting proof test with an out of spec part?

[wetidlerjr]

From my understanding of the processes from this site ( and other sites), I prefer that it be done but I am just a layman (claiming no special knowledge) as I assume most are here. Any testing to ensure that parts on a firearm will not fail during operation makes sense to me. However, I must take the claims that this has been done on the assurances of others that have first-hand experience and knowledge in such matters. YMMV

I do have "unique knowledge of the secret of life" and it is 42.

That's ALL I can add but I will be lurking for more info.

[orionz06]

What do you mean about starting proof test with an out of spec part?

You are caught up in dimensional accuracy, which is important, but I am not suggesting that the proof test is done if the bolt is not the right size.

[rsilvers]

Agreed. But but don't take it for granted that any given bolt is checked for dimensional accuracy. The Internet is making them MPI but no one is making them gauge dimensions.

All bolts break after a certain number of rounds.