C158 Bolts vs. MPI

[SteveL]

Came up a couple of times in the discussion so, M197 HPT is 70,000 PSI. Or at least was in 1994.

Thanks.

[lysander]

There are, as you know, several differing views on HPT.

In the one camp there are those that say that the HPT severely reduces the finite number of pressure spikes that the bolt will withstand. (I about typed that the HPT 'exceeds what the bolt is designed to withstand' but that isn't actually true, the bolt is designed to withstand the HPT. That is this guy's stance:

The idea that HPT decreases the expected life of a part is bunk. The part is DESIGNED to take the load that the HPT delivers. If it is properly designed and manufactured it will take that load literally an infinite number of times before failing. It's called "infinite life calculations" in engineering.

I should have added the following caveat: I haven't seen the design calculations for an M16 bolt. I don't even know exactly what pressure an HPT round is loaded to.........I only speak from my experience as a registered Professional Engineer.

Knight's is perhaps the best known adherent to the HPT testing shortens bolt life, and they seem to have done okay for themselves engineering and production-wise.

What I think would be a winner is batch testing random bolts from each differing lot of alloy. This would reveal any flaws in that lot's manufacture. Myself, I'd hold those bolts back and not sell them. All bolts would be MPI'ed.

My cars have airbags, they weren't crashed before I bought them; likewise my truck wasn't dropped to ensure the suspension could withstand rough roads. Suffice to say, I know that at least one vehicle was crashed, and another dropped, to ensure the durability.

I don't see why bolts would be any different, except, because, you know, 1950's-60's knowledge and technology.

Well, yes and no.

There is the fatigue limit. Any loads below the fatigue limit can be withstood infinitely, loads over the fatigue limit will fail the part through high-cycle fatigue (millions of cycles).

In the case of the M16/AR bolt, the loads are not only above the fatigue limit, in some areas they are above the yield limit. Repeated loading above the yield limit will result in low-cycle fatigue (hundreds to thousands of cycles, depending on the loading)

Below is a stress diagram of an M16 bolt loaded to about 58,000 psi. You see that tiny area of red in the fillet of the lug? There the stress in the steel has exceeded the yield stress and this is where low-cycle fatigue cracks will form. Faster if you have a corrosion pit or something like that. The M16 bolts was designed to withstand repeated loading of 50,000 psi, over the years, the working chamber pressure as been bumped up, and life has been reduced.



Does one shot with 70,000 +/- 10% psi (125%) reduce the fatigue life? Well, the little red spots on the stress picture would be slightly bigger, so yes. BUT, by how much? It will be more than one, because the stress is at least 125% higher. Low-cycle fatigue has an estimate range in the order of 50%, meaning, if a model shows you will get 10,000 cycles before failure, in a population of parts, you could get some fail as low as 5,000 or last until 15,000 cycles.

It just might be me, but I am confident that the life reduction of ONE 125% over-pressure event will be lost in the broad uncertainty band of fatigue.

[jsbhike]

Possible example I think most have heard of.

No first hand experience (reading only), but I would think the steady use of 5.56 proof loads would be similar to .38 Special +p/+p+ through older S&W Airweights. The claim was never a Kaboom from 1 round or even occasional use, but (so the story goes) everything would get worn and loose at a much faster rate than if standard pressure loads were used. Heard a similar story about a cop who had fairly unlimited access to department +p+ 9mm, picked up a Browning Hi Power, and proceeded to trash it fairly quick.

[26 Inf]

Well, yes and no.

It just might be me, but I am confident that the life reduction of ONE 125% over-pressure event will be lost in the broad uncertainty band of fatigue.

The info I quoted was from an engineer on another site. What you posted was very informative (as usual). Thanks.

[Todd.K]

I believe 100٪ proof testing is a relic of a time when metallurgy couldn't always be assured, and when cracks and inclusions in material were more common.

[LMT/556]

Nice diagram lysander, likely explains why LMT radius at the bolt lug/body interface.

[noonesshowmonkey]

Well, yes and no.

There is the fatigue limit. Any loads below the fatigue limit can be withstood infinitely, loads over the fatigue limit will fail the part through high-cycle fatigue (millions of cycles).

In the case of the M16/AR bolt, the loads are not only above the fatigue limit, in some areas they are above the yield limit. Repeated loading above the yield limit will result in low-cycle fatigue (hundreds to thousands of cycles, depending on the loading)

Below is a stress diagram of an M16 bolt loaded to about 58,000 psi. You see that tiny area of red in the fillet of the lug? There the stress in the steel has exceeded the yield stress and this is where low-cycle fatigue cracks will form. Faster if you have a corrosion pit or something like that. The M16 bolts was designed to withstand repeated loading of 50,000 psi, over the years, the working chamber pressure as been bumped up, and life has been reduced.



Does one shot with 70,000 +/- 10% psi (125%) reduce the fatigue life? Well, the little red spots on the stress picture would be slightly bigger, so yes. BUT, by how much? It will be more than one, because the stress is at least 125% higher. Low-cycle fatigue has an estimate range in the order of 50%, meaning, if a model shows you will get 10,000 cycles before failure, in a population of parts, you could get some fail as low as 5,000 or last until 15,000 cycles.

It just might be me, but I am confident that the life reduction of ONE 125% over-pressure event will be lost in the broad uncertainty band of fatigue.

This and the HendersonDefense post are legit the only parts of this thread that aren't just prattle.

If you aren't an engineer, you don't know what "proper" means. You can read the spec, but that's about it.

There's exactly two ways to view this problem: empirically through experience (Henderson Defense) and scientifically through training and study. Any other is snake oil salesmanship.

Canadian C7 bolts use 9310 and have life cycles that make the US MilSpec bolts look like child's playthings. C158 isn't a magical material found only in meteors fallen to earth. It's a steel. There's a ton of flavors of steel, and there are people whose life's work is to learn about, study, and apply their experience and knowledge towards practical ends. We call them engineers.

Meanwhile, 7 pages of navel gazing and "proper [this]" or "proper [that]" and we've had only a handful of meaningful posts.

Nitriding was not a mainstream surface treatment at the time that the AR15/M16 family of weapons was designed. 5-axis CNC machining didn't exist then, either. Neither did cell phones. Mu****as went to the moon with aluminum slide rulers. Things have advanced a bit since then, considering that your back pocket holds more computing power than even what went up in the first space shuttles.

Buy a bolt and shoot the shit out of it. Toolcraft is an OEM, so if you're worrying about "proper [this]" or "proper [that]", pay a company that supplies other vendors.

[mrbieler]

Canadian C7 bolts use 9310 and have life cycles that make the US MilSpec bolts look like child's playthings.

I don't have a dog in this fight, am not a metallurgist, and am not an armorer, but every reference I've seen online regarding the Canadian DND C7/C8 spec shows C158 for the bolt and 8620 for the carrier. Is the use of 9310 published somewhere?

[lysander]

Fun-Fact-for-today: Carpenter developed 158 as a die mold steel, and that still is its primary use.

Another Fun-Fact: AISI 9310 is a very popular steel for machine gun bolts. The M60, M240, M249, and I believe the bolt head for the M134, all use AISI 9310. And, 9310 was approve as an alternate material for M14 bolts.

[Esq.]

Fun-Fact-for-today: Carpenter developed 158 as a die mold steel, and that still is its primary use.

Another Fun-Fact: AISI 9310 is a very popular steel for machine gun bolts. The M60, M240, M249, and I believe the bolt head for the M134, all use AISI 9310. And, 9310 was approve as an alternate material for M14 bolts.

So...my $49 Toolcraft, Palmetto State Nitrided 9310 bolts will probably not blow up and destroy the entire planet anytime soon?