Hopefully this will be the ultimate bolt life discussion thread...

[prepare]

Something else of importance that is invaluable IMO is the amount data available on the actual mil-spec bolt/BCG since its inception. Everything from manufacturing processes, destructive testing, service life, failures, and millions being fielded across multiple wars.

Nothing else compares to the amount of data and lessons learned about the evolution of the M16, M4, AR platform and its components.

[Pasta123]

You would need to look at the yield strength and charpy value of every possible alloy and all possible heat treats for those alloys to figure out which will give you the most strength without getting brittle.
One of the few companies that made enhanced bolts made the first few batches too hard, like many they thought the harder the stronger. The bolts chipped around the cam pin hole and corners of the lugs.
If you are only talking about 5.56 bolts then yes, you could make bolts that would last many thousand rounds without too much effort. We were a small shop but machined apx 300 bolts a week for almost 10 years and still have had none break or none reported.

Yes, pushing a new standard for 5.56 barrel extension will be hard today, sometimes standards are best to start early. Pity that the industry in general are short-sighted and always keep design proprietary.

Also, why 7 locking lugs? Any historical reasons?

[Pasta123]

One more thing, I think people rarely talks about heat treatment when they're discussing bolt life, perhaps they didn't know better here?

Come to think about it, you can't separate design with the material you're using, and a good build quality can go a long way.

[constructor]

Yes, pushing a new standard for 5.56 barrel extension will be hard today, sometimes standards are best to start early. Pity that the industry in general are short-sighted and always keep design proprietary.

Also, why 7 locking lugs? Any historical reasons?

The amount of rotation in the cam pin track is limited. If there were more lugs they would be smaller and weak, if less lugs there wouldn't be enough room on in the carrier to rotate the bolt enough.

[Pasta123]

The amount of rotation in the cam pin track is limited. If there were more lugs they would be smaller and weak, if less lugs there wouldn't be enough room on in the carrier to rotate the bolt enough.

I did thought about that before, which seven lugs, with all consideration, probably make the best balance between size and durability. Is it still relevant today, though? The Chinese 191 I've posted above uses four lugs, and the travel distance seems to be similar (unsure).

Of course, that's a different caliber to begin with, and the bolt life may not be that different due to multiple factors.

[Todd.K]

Large lugs don’t necessarily need to fully rotate / engage.

[constructor]

You guys going to convince every barrel maker and extension manufacturer to change the design of the extension and make bolts that aren't compatible ? That will be an uphill battle.

[ABNAK]

I gather the OP is a China-born person who has moved here (?). Interesting to hear his take and suggestions/questions.

One thing Chinese that I would buy without hesitation are weapons (most of them at least). They can chrome-line the f**k out of a barrel and other parts. The AK, SKS, M1911A1, and even a few other guns are good-to-go. Actually even the M14 was okay; the receiver was forged and the barrels were chrome lined. The bolts and hammers had a rep for being "soft" and supposedly worsened an already long headspace. Nonetheless, those issues could be rectified.

The Stoner system has long had "upgrades", be it military or commercial. While the currently accepted mil-spec is 8620 for carriers and C158 for bolts (both phosphate finished) there are no doubt better options if you want to pony-up the $$$. While 9310 is newer I haven't read where it surpasses C158 as far as AR bolts go. I like some of the "wunder" finishes (as long as the bolt itself is mil-spec): Cryptic Coatings Mystic Black, LMT EBCG, even a well-done hard chrome job. Nitride I can take or leave. The LMT EBCG goes beyond just a finish.....it has slightly altered the function, but in a positive sense. It is also expensive as hell. I have a couple complete but I also have the Enhanced Bolt Carrier coupled with a Cryptic Coatings Mystic Black bolt.

[Pasta123]

You guys going to convince every barrel maker and extension manufacturer to change the design of the extension and make bolts that aren't compatible ? That will be an uphill battle.

Not really. Just wondering if 7-lug design is holding the system back, or if this question is relevant at all.

I gather the OP is a China-born person who has moved here (?). Interesting to hear his take and suggestions/questions.

One thing Chinese that I would buy without hesitation are weapons (most of them at least). They can chrome-line the f**k out of a barrel and other parts. The AK, SKS, M1911A1, and even a few other guns are good-to-go. Actually even the M14 was okay; the receiver was forged and the barrels were chrome lined. The bolts and hammers had a rep for being "soft" and supposedly worsened an already long headspace. Nonetheless, those issues could be rectified.

The Stoner system has long had "upgrades", be it military or commercial. While the currently accepted mil-spec is 8620 for carriers and C158 for bolts (both phosphate finished) there are no doubt better options if you want to pony-up the $$$. While 9310 is newer I haven't read where it surpasses C158 as far as AR bolts go. I like some of the "wunder" finishes (as long as the bolt itself is mil-spec): Cryptic Coatings Mystic Black, LMT EBCG, even a well-done hard chrome job. Nitride I can take or leave. The LMT EBCG goes beyond just a finish.....it has slightly altered the function, but in a positive sense. It is also expensive as hell. I have a couple complete but I also have the Enhanced Bolt Carrier coupled with a Cryptic Coatings Mystic Black bolt.

Sometimes these people make things really cheap and effective, then inevitably they got too proud of themselves, and quickly went downhill.

I read a book where it's written hundred years ago, where he describes their stuff being "both capable and useless". Accurate description most of the time.

[lysander]

A few things . . . .

1) The life of a bolt is governed by a) wear, or b) fatigue. With wear, the continual rubbing or loading of parts will eventually reduce the the size to where it will no longer function properly. This will probably take a lot of repetitions to ever become a factor, but it is possible to reduce the headspace dimensions to the point that it is excessive. As to fatigue;

There are two types of fatigue: high-cycle, and low cycle fatigue. High-cycle fatigue is the eventual cracking from repeated small stresses and is measured in the 10^6 cycles, for steel, there is a fatigue limit, a point that is the stress is kept below this point, the part can withstand infinite cycles. Low-cycle fatigue is when a part is repeatedly subject to stresses above the yield point, and is measured is 10^3 or less cycles, depending on how far above the yield stress it is subjected to. The M16 bolt design has areas around the locking lugs that are subjected to stresses above the yield point, and therefore always be subject to low-cycle fatigue.



Increasing the radius at the base of the lug will reduce the stresses, but I do not think it is possible to lower them to below the yield limit, let alone the fatigue limit. There is just not enough material.

Fatigue will be exacerbated by small discontinuities, such a corrosion pits, tool marks left over from the manufacturing processes, or micro-cracking inherent from hard chrome plating.

2) The 100,000 round limit I was quoted is based on a bolt made compatible with a standard M16 barrel extension. A completely different design, such as a tipping bolt of a FAL might be capable of such high usage without failure.

3) Lugs that do not full engage create bending moments, and possibly higher stresses. There is also the issue of compressive stress limits. In any case, the lugs size is limited by what will pass through the gaps in the extension.

4) There is a lot of information available free on the internet, it is just a matter of knowing where to look for it, or how to search for it.

5) Cost-vs-longevity. I can buy a regular AR bolt that will last 10,000 to 15,000 rounds for $50. Will I get more than 20,000 to 30,000 rounds out of a $100 bolt? If so, how much more? If it ain't more that 10% to 20% it ain't much of a bargain. The Government uses 25% as a cut-off, it the new part does not come it for 75% of the original design over a 5 year period, it is not worth the change. (Of course, that does include the non-recurring costs, such as engineering and testing cost, which can be substantial.) For me, 10,000 - 15,000 rounds is a lifetime, or two.

6) AISI 9310 is not a "new" alloy. It has been around since the 1940s, and was used in aircraft landing gear and for M60 bolts from the beginning. In fact, it was proposed as an alternate for AISI 8620 in M14 bolts. You may not have heard about it before 2010, but it has been around quite a while. AISI 9310 is also a very popular alloy of high demand gears.

7) The QZB 191 bolt rotates 45° vs the AR's 22.5°. One of the more interesting improvements if the method of getting longer extractor springs.