Would this bother you?

[introv78]

My latest PSA / BCG (Nov. 2018) is marked MPI. A bit faint but it's there. https://palmettostatearmory.com/palm...roup-8779.html

Yeah I looked hard before emailing them and was then told by customer service if no MPI marks it was just batch tested. The product page states "C158 MPI/HPT Bolt" so I think either the customer service guy is confused or there is something strange going on. ETA the bolt is stamped c158 but it has no other stamping.

Thanks

[26 Inf]

Purchased a new upper which stated to be MPI HPT and shot peened. I noticed the bolt is not stamped M or MPI and contacted customer service which stated if it’s not marked it was just batch tested. If the items sale page states it’s MPI I expect it to be actually MPI and not batch tested. I have replied back and ask to send me the correct bolt that was advertised but still haven't gotten a reply.


Below is the specs from the sale page-

"Bolt: Full-auto profile bolt carrier group. Milspec Carpenter No. 158® steel bolt, Shot Peened, High pressure tested, Mag particle inspected, Gas Key Hardened to USGI Specifications, Fastened with Grade 8 screws, and Staked Per Mil-Spec. 8620 steel M-16 profile carrier is chrome lined and phosphate coated. "

I'm not a metallurgist or an industrial engineer, so here is what I've found:

Notice the order in which these items are listed: Shot Peened, High pressure tested, Mag particle inspected that is the order in which they occur.

Since steel “looks” fine even when it’s flawed, the MilSpec is to test the BCG with a high-pressure test (HPT) and a magnetic particle inspection (MPI). A high-pressure test is proof-firing an intentionally over-pressured cartridge.

A high-pressure test does you no good if you can’t see if any microscopic cracks or flaws have developed, which is what a magnetic particle inspection does. Think of it as an X-ray for steel. It “sees” hidden cracks.

The proven way to assure that the steel of a BCG can withstand a high-pressure test and subsequent magnetic particle inspection is to subject the carrier to a process known as shot peening. Basically, this is a metallurgical treatment to surface harden the steel by bombarding it with special particles that act as tiny ball-peen hammers.

https://www.americanrifleman.org/art...cs-of-milspec/

So, MPI alone doesn't really tell us much of substance without the high pressure test. High pressure testing of bolts, while maybe not controversial, has some opposition. In high pressure testing the bolt is subjected to an over-pressure proof load, 70,000psi versus the 55,000psi norm. While this isn't necessarily destructive testing, unless the part fails, many believe it reduces the lifespan of the bolt:

HPT is a waste of time and money. It also reduces the life of the bolt. I have stated this analogy before but it is like wrecking a new car into a brick wall and inspecting the frame for cracks. It is an archaic process . The government and industry should do away with it and come up with more productive forms of standardized acceptance /quality control . MPI bolts every 5000 or so rounds would be much more productive. I got news for you guys bolts crack then they break. There is sometimes no way of telling how long a bolt will go after it cracks before it fails. Many of the bolts in your trusted guns out there are cracked.

C Reed Knight III
Knight's Armament Co.
http://www.knightarmco.com/

There you go.

Then we get down to batch versus each piece testing:

If you had 10 bolts that were made EXACTLY the same, but one was HPT and survived the test, then all you've done is proven that the design works and the batch was made properly. Also you took away a portion of life to the bolt that was HPT.

Let's talk bolts. Bolts are made in batches and either not tested, batch tested or individually tested.

Theoretically speaking if Company A makes them out of the same material and makes them the same way as Companies B and C there should be no issues providing there are no flaws in the raw material.

Company A makes 'em doesn't test 'em just ships 'em out. Company B tests 5 out of every 100 made. Company C adheres to the milspec and tests every individual bolt. IF it were my money I would buy from company C for peace of mind. If I was given a rifle with a bolt from company B I wouldn't worry too much. If given a rifle with a bolt from Company A I'd probably send it to be tested if possible.

The most important thing is when you clean the gun to inspect the bolt for cracks, flaking, pitting, uneven wesr on the lugs, and discoloration EVERY time. Generally a good guideline is every 1k down the tube. Buy a spare bolt and enjoy life. (ARFCOM)

Finally, this is this gem:

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

I buy C158 bolts that are listed as shot-peened, HPT and MPI. Based on what I've read, whether it is individual or batch tested isn't a deal breaker or maker for me.

If I was selling them to this crowd, or the folks on other AR-centric forums, I'd ride the 'individually tested by folks who toss and turn at night wondering if they've done enough QC to keep you safe' pony, and I'd ride it hard.

That is the reality of business directed to a market segment.