A few sets of gauges could work for the bolt in particular.
Dimension
Materials
Treatments & finishes
Assembly
Those are the parts of any specification, military or otherwise. I am unclear on why there seems to be a discussion wherein material testing and dimensional verification are being looked at as either/or. One does not preclude the other.
Its been a coupe years now, but I was told by an manufactor/contractor employee before that pretty much nothing ever fails MPI testing, as in they had worked for several years and a bolt had never failed MPI testing. I’m running MPI tested FN Bolts in my 4 ARs, but they were cheaper than most commercial untested bolts to purchase.
I’ll preface my post by saying I have little AR exsperience but spent a summer working in a machine shop and we magnafluxed (gearhead speak for MPI) parts if the the customer requested it. I think it’s important to point out that the MPI is only as good as the effort put into it by the guy doing it. Doing a bolt must be a bitch becouse it’s so small with all those little machining cuts.
With the money guys spend on there guns I would think a “premium” bolt with fancy hard to machine steel thats been cryo treated and MPI would sell well.
You are missing a HUGE, very important piece of info when you make a leap like above.
How do you know if HPT/MPI would have failed those bolts? You assume the material and manufacture to be equal so that you can pin lack of HPT/MPI on the failures.
The truth is that the low cost manufacturers may use cheaper materials, may not heat treat as precisely, and do poor shot peening (if at all). I would take the non HPT/MPI C158 bolt (of know quality) over a 8620 bolt that passed HPT/MPI.
As it has been explained to me, the failure rate of bolts and barrels not made from their representative correct materials and test properly in accordance with the military specification(s) would be so high as to be actually cheaper to simply start with the correct material anyway.
This is why it is so important for those who do perform HPT and MPI to be clear about their methodologies, proof load used, rejection criteria, etc.
Some of this (in addition to shot peen method and other expanded datapoints) is addressed in the new Chart, but not all.
In the instance of the H&K there are other things going on as well, but I would consider it safe to assume they are using a good material of known quality. Disagree? This is also just running off of a comment made by Rsilvers. If the reports of 416 bolt failures with low round counts are factual, what would be the first step to fixing it? Finding where the problem is and that would involve some testing as well. (I would personally pin the failures on a few other things, but HPT/MPI goes an awful long way to show bolt issues)
The Del-Ton failures are more than likely material alone.
And I would trust the same non HPT/MPI C158 bolt (of known quality) over a 8620 bolt that passed HPT/MPI any day, but does not mean the C158 bolt is good. Using the right materials within the dimensional tolerances of the design and proof tested tells you everything you should know. Removing any one of those opens you up for trouble.
Now what about an alternate means to do so? Batch testing goes further than most, any other ways?
Yes, “we” is AAC. I do not have it in my sig as I did not want to use my sig area as a banner ad for AAC.
I can’t follow what you are asking. What do you mean about “passing everything else?”
What do you mean about “good bolts not being correct?” If you are asking if there are bolts which have passed MPI but are dimensionally incorrect, of course they exist and are probably not rare.
Untested and tested bolts will break, after 5,000 or 10,000 rounds. You seem to be saying that only untested bolts break. All bolts break.
Gauging is the best way. Colt has a govt inspector on site who gauges certain things. Other companies? Don’t assume they do. Maybe they do, but if you are going to demand MPI, then they have that many less dollars to do gauging and test firing. Everyone has a fixed QC budget and best not to waste it on tests which do not find as many bad parts.
But to answer directly, there is NO company which checks every part for all dimensions. That would take forever. The best companies do ISO-9000 style statistical sampling where they test so many parts and the number they test is based on a number of factors - such as batch size, and how likely the part is to be bad. Some companies test more than others. Expensive MPI tests take away from how much other testing one can do. I really am convinced that it is a marketing expense.
Basically once the design is proven, then you can move away from certain tests. Just to give you an example of a more destructive test - drop testing…
When you have a new gun design, you can drop test it many times. It may break. You change the design and do more drop testing. Say it passes this time. You now know that the design is strong and it passes a drop test. You don’t drop test every one you make after that. And true, you never know for sure if the next one would have broke or not.
I know this is not a perfect analogy because drop testing does physical damage, but my point is that, once every unit you test for a while does not break, then you should probably stop that test and spend time on things which are more likely to find bad parts.
I am claiming that this is not 50 year ago, and AR bolts are to the point where you can test thousands of them in a row and not have any failures, so one is better off spending the dollars for that test on additional dimensional testing by upping the number of statistical samples you take which will increase the probability of locating bad parts (dimensional tests commonly finds bad parts, so it is a much more productive thing to spend time on).
Why not do both? It could take 50 or 100 man hours to test everything there is to test on a rifle. Realistically, you can budget up to an hour or two at most, so you really have to spend the time on what finds the most bad parts. MPI does not find the most bad parts.
“Probably” not rare? Could you please provide some empirical evidence that a credible company producing MPI/HPT bolts are passing a statistically significant number of dimensionally incorrect bolts?
Yes, all bolts break. But when? Do you have empirical evidence to suggest that the likelihood of a non-test bolt and a tested bolt fail at the same rate? If there is a difference, is the difference in failure rates greater than or less than the rejection rate of companies that do MPI/HPT?
If not, it sounds as though your entire argument is based on hearsay. Furthermore, I’ve not seen anybody suggest that companies shouldn’t test for dimensional accuracy.
You can inspect an HPT part visually and with gauging. AAC does it both visually and with gauging - 100% of the uppers.
If you think your favorite brand CMMs every dimension on every bolt - well, that is not going to be the case. That would be too expensive. And no one is demanding it. Do I think there are a lot of MPI bolts out there which are out of spec? Yes. I think there are a real lot.
I avoid things cryo treated. I don’t believe in the Easter Bunny either. The exception is I will buy Krieger barrels.
But the HPT/MPI is not about testing the design, it is about testing the quality of the material.
Exactly. You can’t visually see metallurgical flaws.
That I can agree with.
End of thread.
All the dimensional gauging in the world isn’t going to detect something like an inclusion in the steel.
A mag-particle test will.
I thought thats what my post said but he snipped it out in his response… :eek: