Hey Eat. Your mail box is full. Time for some spring cleaning.

Greg

This whole thread is about dispelling mis-information.
Trolls are trolls, they will always be trolls.
There's no fixing them.

Yama, It's cool. We were just discussing the military M-16/M4 bolt. Yes, the civilian market moves faster with development than the military world. Unless they find it prudent to move faster on something. (Especially during wartime)

lysanderxiii & I both seem to agree on it. So, we're good on this.

No worries WV, yes the military (ours) is behind the times where the M16/AR15 is concerned.

What is the original safety factor of the bolt since its being quoted so many times?
2:1, 3:1?

t don't know. lysanderxiii would know.

I do however figure that one of the reasons they kicked the pressure up & added a steel penetrator is due barrier blind issues, as mentioned in the Roberts Report.

Just ran across this looking for something else:
Topic Never gets old


Magellan was wrong. The Earth is flat. LOL!

I ask again.

What is the original safety factor of the bolt since it has been quoted so many times?
1.5:1, 2:1, 2.5:1, 3:1?
Surely one of the experts has to know.
I cannot find anywhere that states a "safety factor" of x:x

Here's what we have so far.
Pertaining to AR15 bolt strength of the .223/5.56 along with with the variants we have been discussing 6.8, 7.62x39 and 6.5G
The bolt strengths are not the same because the web that the lugs attach to are undermined on the back side by the bolt recess.
The 5.56 web is apx .070", 6.8 .045", Grendel and 7.62x39 .035" thick.
The common way to figure bolt strength is lug width x length x 7(number of lugs) X yield strength of material however the web thickness removes some of that strength.
The 5.56 and 6.8 bolts have .125 recess removed from the .277" lug length.
The Grendel has a .135" recess removed from the lug strength.

We know that there is a safety factor built into the bolt system.
But know one seems to know what the numbers are.
We also know the despite the material specified how well the actual final heat treating is done determines how well the bolt will perform as designed.
Such as what LR stated about the G bolt problem and how some of the non mainstream builders that build parts for that particular caliber using 8620 and having heat treating issues.
From this we can ascertain that the more major player rifle builders are going to build better parts.
This is why in the 6.8 we do not see bolts made to mil-spec dimensions like the Stag, DPMS, Wilson Combat, Daniel Defense, LMT, Noveske having any problems.
Bolts like the ARP Super bolt and LWRC Advanced Combat bolt are yet another step above.
But even bolts from PSA, Anderson or AIM are not breaking, I have found no record of any 8620 6.8 bolts.
In all fairness any substandard bolts break and wear prematurely such as the 5.56 8620 Stoner bolts from Midway.
While in the 6.5G and 7.62x39 the parts are not being built by the major rifle builders and parts are failing from any manufacturer, of course with different frequency but no one seems exempt.
I have been researching this and see breakage across the board.

From all this we know that while running the 6.8 SPCII to 58500 psi with the ability for that to peak at 60,000 psi as has been noted, we are still not breaking anything.
Leading to us ascertaining that the 6.8 while adding stress to the bolt in the AR15 even up to 60,000 psi.
Does not exceed the design limitations of the bolt in a hunting, target, plinking, CQB role.
Where as anything larger than the .420 bolt face of the 6.8 does exceed the limitations of the bolt design in as much as there is bolt failure across the board even with the reduced pressures.

We have also learned that the leade, cone angle and groove to land ratio all DO play a part in how a chamber handles pressure.
As well it helps determines the pressure curve and the creation or control of pressure spikes.
We have learned the SAAMI does indeed consider the appearance of pressure spikes when testing a limiting factor of a cartridge and will limit pressure in said cartridge accordingly.

As well we have eliminated the whole hoop, tenon, blah blah blah discussion as the bolt is the true limiting factor in the platform.
As such the bolt, case, primer would fail 10's of thousands of psi before an actual chamber failure or even any type of destructive chamber event.

What powders have been tested to reach these 56-58,000psi chamber pressures? I'm sure they could dump just about anything,  but what proper powders were used?

Well, we are looking at mainly  7 years since the SPC II break through/testing.
Not too many people have a Piezo or strain gauges laying around. As for SSA & Wilson Combat commercial ammo. you'll have to ask Art K who owned SSA before the buyout (said to have created the SPC II designs [Which is said to be very close to the original Murray design.])  & WC.


.

AA5744
AA1680
AA2015
AA2200
AA2230
X-Terminator
LT-30
RE7
10x
1200R
AR Comp
H335
4198
8208
N530
Norma 200

CFE and LEVER will not go over 55,000, cant get enough in the case.
1200R is the least temp stable of them all, be careful with that one near max in high temps.

These have been recently tested in loads that have been around since the associated powder has been out.

Work up please!!!!
If your not sure what your doing DONT!

H322 used to be popular. But, it has a slow velocity.

I can't believe the mods have allowed this shit show to run this long in the tech forums.  What an embarrassment.

There has been info in this thread. Damn good info. Have to take the good with the bad.

I was working at Barrett when the SPC II came about. That was a huge cluster for the overlap in chambers we had going on with the M468's

So, you can produce documentation that proves the exact composition of the steel your bolt is made from?  You can produce documentation that shows the exact temperatures of the heat treatment?  I know for a fact if you have a proprietary bolt, you have no idea if it was made to the drawing, because you don't have the drawing to check against.

And, by "documentation that proves" I mean documents that would convince a jury in a court of law, not a company advertisement.

No, generally, nobody in the civilian gun buying world has anything, other than the sellers word, that ensures they are getting parts made "to print".  That's why buying from trusted companies is so important if you want quality (and safety).

When the AR15 bolt was designed nobody did a stress analysis on it and nobody did a detailed fatigue analysis on it so the original safety factor is immaterial.  You don't need a safety factor if you know the actual stresses.

We know the actual stresses because we have a recent stress analysis on the bolt with military loads that reach 60,000 psi.  This shows stresses will exceed the yield strength of the bolt lugs in certain places.  Because the bolt thrust of a 6.8 at around 58,500 psi is going to be at least the same and more likely higher than that of a 5.56 at 60,000 psi, and there is material removed from under the lugs, we know that the bolt lugs of 6.8 are also going to have areas that exceed the yield strength of the bolt material.

As to high velocity factory loaded ammunition.  The factory can do things hand loaders cannot, as they have access to more information.  Not all lots of powder have exactly the same burning characteristics, and can use powder lots that show lower pressures with faster loads.  LCAAP does closed-bomb tests on incoming lots of propellant and holds lower pressure lots for tracer, as the longer bullet tended to make the cartridge more prone to over-spec pressures.

Hand loaders cannot cherry pick powder lots.

Also, Western Powder's reloading book has some errors...I question their wisdom in giving out some of this information.  From their reloading guide:


This is incorrect.  NATO pressure limits for 5.56 mm NATO Ball and 5.56 mm NATO Tracer are set at 380 MPa (55,114 psi), maximum average pressure.

The maximum average pressure plus three standard deviations is 420 MPA (60,915 psi).

This is from STANAG 4172.

You know it would be nice if we could just get a max inner dimension of a 6.8 SPC case.  Best I can figure from web  research is around .369...

- I still find it strange that people some how think that 58000 PSI is some how different than 58000 PSI....  Why did SAAMI put a 55000 limit.

- Putting faith in a powder company that states that a weaker system can safely run at the same pressures they list as max in a AR10/Bolt rifle.  The 6.5 Creedmoor speeds from a 24" tube that are at or below what Hornady factory ammo will do out of shorter barrels (18.5 -21 Real world chrono'd 140's @ 2640 from an 18.5" tube 2750 from a 21" tube) doesn't give me much confidence in their "published data"

- Hodgdon list's simular 6.8 speed's at much lower pressures.  

- rule 5 of the tech forums should apply to this and just about every thread the OP starts.  Cut and paste hardly constitutes "first hand experience"

Some more information about recommended pressures:

Any time you do something more than once you will be subject to statistical variation.  When SAAMI recommends a maximum average they understand that the results will look like the top graph.  (The x-axis is the actual pressure and the y-axis is number of occurrences.)  The SAAMI listed Maximum Probable Lot Mean (MPLM) is what is considered the maximum safe limit for loads, given there will be pressures that are higher that the average pressure and the fact that all the possible pressures in any given lot are not tested (they can't test every single round in the lot).  This is shown in the second graph.

By suggesting anyone load to the Maximum Probable Sample Mean (MPSM) you are going to be using the third curve.  If your average pressure is at the MPSM pressure, no matter how well you try to control things, half of the loads will be above the average.  This ain't smart.



This goes for ALL reloading, not just 6.8 mm Remington, 6.5 Grendel or 5.56/.223.

Also, the C.I.P. maximum load for .223/5.56 x 45 is 62,350 psi, this pressure is the equivalent of the MPSM pressure.  This is NOT the maximum pressure you should load your ammunition to, this is the maximum pressure any cartridge within the lot should reach within the statistical spread.  The C.I.P. recommended average pressure, the pressure you should try and get with your loading, is three to five standard deviations below this, or around 55,000 psi.

Speaking of "real world experience". . . . Anybody have velocity data for Hornady FACTORY ammo in ARP barrels? In barrels from other vendors?

I agree with both the G and the 5.56, the chambers/barrels were not improved since SAAMI.


However the 6.8 is a different story.
Yet again.
With the pressure spikes associated with the wrongly submitted drawing to SAAMI the pressure was limited for this reason.
SAAMI does consider pressure spikes when assessing the capabilities of a cartridge.
If not for the pressure spikes associated SAAMI could/would have rated it higher.

The fact remains that we have been loading to these pressures for 7 years with no ill effects.
No one has said you should run at max pressure with every load you do.
Of course anyone with any cartridge should run the powder/primer/brass/projo combination that gets the speed required for the job at the least pressure.

Irrelevant to the discussion as "factory" ammo is not what is being discussed.

So there is no specific "safety factor" for the bolt.

As well you are ignoring that these loads and pressures have been ran for 7 years now in the applications we are using them in.
With no ill effect.
All you guys could dig up was 3 bolt failures in 7 years, 3.
NO chamber/barrel failures.
This out of all the commercially produced bolts from all the aforementioned manufacturers.
That is a pretty long term quality vs use vs application study.

"We know the actual stresses because we have a recent stress analysis on the bolt with military loads that reach 60,000 psi. This shows stresses will exceed the yield strength of the bolt lugs in certain places. Because the bolt thrust of a 6.8 at around 58,500 psi is going to be at least the same and more likely higher than that of a 5.56 at 60,000 psi, and there is material removed from under the lugs, we know that the bolt lugs of 6.8 are also going to have areas that exceed the yield strength of the bolt material."

Now to state this as a limiting factor you would need proof, not a guess.
As the bolts are not failing.

Have we not all done a stress test on the bolts running them for 7 years from all manufacturers?
With canister powder variations.
With all the different commercial parts manufacturers.
Ran in a commercial hunting operation with high round count uses with loads to the stated pressures for 7 years.

That's some damn good quality control we have from the makers of 6.8 parts now is it not.
This is the benefit of having the major players back the cartridge.

What is the only AR15 variant caliber not based on the 5.56 case that is currently used in weapons in actual military use?
What is the only AR15 variant caliber not based on the 5.56 case that is currently used in SAW's in actual military use?

Your points do however suggest that perhaps the 6.5G should have its pressures lowered though.
Because it is prone to breaking bolts and as such does exceed the pressure containment capabilities of the weapon system, the bolt being the weakest link.

In response to my personal bolts, yes I do run an ARP bolt, and have ran a DPMS.
I have full confidence in all the manufacturers previously listed.

Here I find myself in a debate with mostly the Grendel crew, kind of like the iron pot calling the stainless kettle black.
Your bolts break, your cartridge has a containment or quality control issue.
Not the 6.8 at 58,500 psi.

It was stated we have no I idea what pressure we have been running, pages ago. So tests were done to confirm.
All results came out right where we thought they were.

Just to answer here.

ARP 16" recon.

All ammunition I have fired has been about 40fps or so higher than the factory listings. The factory Hornady I had was the 110gr stuff. It was IIRC, clocking what the 20" barrels were shooting them at, about 50fps faster than the factory stated data. There were a few factory loads that were 50fps and over the listed numbers, and some that were about the same. Temp and chamber / barrel used in the testing makes a difference. The SSA tac loads were very close, but they were tested in spec II chambers and 1/10 or 1/11 barrels, usually both.

Thanks, PW!

Does STANAG 4172 not pertain to M855/SS109?
Not the new standard set by M855A1 ammunition?
Which generates 3000 to 5000 psi more than standard M855 or the STANAG 4172 specification.

Which takes it to 63,000 to 65,000 psi.
This new standard does require bolt replacement at 6000 rounds.
However this is under full auto application is it not.
M855A1
or
"And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i."
M855A1 article

"Though the M855A1 is more expensive to produce, its performance is considered to compensate. One possible danger is that it generates much greater pressure in the chamber when fired, decreasing service life of parts, and increasing the risk of catastrophic failure of the weapon, though this has yet to occur."

So our own military is running the 5.56 up to 65,000 psi in battle conditions.
SAAMI rates both the 6.8 and .223 to the same pressures (old 6.8 chamber/barrel config.)
Yet you still claim we are being rouge running the 6.8SPCII to 58,500.

But is most likely 95% of most of these rifles diet...

I get it... 7 years, thousands of rounds a year, no broken bolts...  Now depending on the context this sounds impressive.  It also leaves a lot of assuming...

I would call that an assumption of the most grand order.

A trip to the 6.8 forums would show how much reloading is going on, constant development with new projectiles and powders.
Dr Lucci at Wild River Ranch uses handloads almost exclusively.

The performance gains of hand loading make the 6.8 unique compared to any other caliber in the AR15 that has the proliferation of factory ammo the 6.8 has.
But only the 5.56 has more available ammo now. I my humble opinion.
None have the available different ammo and especially projectiles designed specifically for the cartridge.

A more accurate guess would have 50% of the 6.8 shooters hand loading.
How about a poll here and at the 6.8 forum to find out?

I bet those rounds have allot of bolt thrust at those pressures.

YES. it does for SS109.  However, current pressure levels in the current US specification for M855 (58,700 psi) are above what are in STANAG 4172.  STANAG (NATO STANdardization AGreement) 4172 listed pressures are what all the NATO countries agreed to (55,000 psi). Just because one country unilaterally decided to bump the pressure up does not means that is the new agreed on pressure.

The M855 specification has a max average pressure of 58,700 psi, has been since 1999.

The M855A1 specification has a max pressure of 62,000 psi, and has been out since 2009.

Neither of these pressure are "NATO pressures", they are "US Army pressures"

Since 1999, the US Army has seen the number of bolt failures go from almost none, to such a rate as to put a 5000 round life on the bolt.  Read the report I linked to way back. Now, the Army is even toying with the idea of putting a round counter on the M4 to monitor the rounds fired.

Just because one organization does something not to bright, does not mean it is safe.  The M855A1 has other problems as well, the exposed steel nose of the projectile has a tendency to ding up the aluminum feed ramps.  The Mk318 does not require such ridiculous pressures to achieve performance and does not damage the weapon...

Ahhh, yes, we all know that everybody that shoots 6.8 is actively posting away....

Ever think that only those most enamored with 6.8 would post about it and of those enamored with it would hand load it, so the on-line number will be skewed toward hand reloading.

I agree US Army pressures not NATO on the M855A1.
However "not to bright" is a personal opinion. Oddly you have touted the prowess of our military in the area of the M16 and now chose to say they are "not to bright"in their choices.
They are seeing wear and cracking at a higher rate than with M855 but I have been searching and do not see a rash of bolt failures as you would elude to, but rather bolt replacement recommendations based on inspections.
So given that you would have to agree that the 6.8 at 58500 psi is a safe operating pressure.
Because we have seen that if the weapon system is stressed to the point of being at the edge the capabilities of the system the system will exhibit failures at the weak point of the system.
As such the 6.8 at 58,500 does not exhibit these failures which would appear if the weapon system was stressed to this point.

As to the number of 6.8 shooters that reload.
The respondents so far in this forum are at 78.6% either reloading for the 6.8 or going to start.
21.4% shooting only factory ammo.
This out of 42 respondents do date.
At the 68forum
56 respondents, only 1 does not hand load.
Now considering that only about 1 in 20 ever respond to a poll do the numbers.
The 68 forum alone has 1815 members
Figuring only 70% hand load and not the 95% of respondents that state they do.
That is 1270 hand loaders at that one forum alone.
If each only fires 100 rounds a year that is 127000 rounds a year. x 7 years 889350 rounds fired.
Now considering in a fairly recent poll in the same forum respondents stated that most of them fire 500 rounds a year on average.
That is 4,445,000 rounds fired in the last 7 years by 6.8 commercial shooters that reload from the 68 forum alone.
Now also considering that is the foremost authority on 6.8 reloading this is likely a low estimate of total round count.
As well current testing has shown that the loading listed there do fall between the 55,000 to 59,000 psi range.
That is a pretty damn impressive sample size that has not lead to the aforementioned failures.
Don't like my numbers? Cut them in half, no less impressive.

Yeah, your total number of rounds fired is probably a pretty conservative estimate.

Actually it sounded a lot more impressive when 1 gun fired 1000's of rounds a year for 7 years  (assuming 2000 per year) 14000 on 1 bolt...

Now you are saying 1270 guns are firing 500 rounds per year for 7 years 3500 per bolt...

Cut it in half and that's 1750 per bolt...

And of course we are still assuming each shooter only has one rifle....

All about context and assumption...

BTW:

Asking if people reload in a reloading forum....  That's classic...

Yes very conservative.
I shoot 3 to 5k myself now.

There are a number of them that shoot many, many thousands a year. Tons of professional hog hunters and game hunters are using it for one, and have been for years. A good number are taking them through courses frequently, several members there , one that is a builder now.

Some of these rifles are seeing 5K + a year, and they have been since the early days of the spec II design, since some of those guys are using it for a living, using it to test and develop projectiles and ammunition etc.

They are not reporting problems.

I don't shoot as much as I used to, but I have about 2K on mine, all have been either tactical / combat factory loads, or hand loads that you are deeming too hot, with the exception of 40 rounds that I bought when I bought the gun ( I bought the tac/ combat loads at the same time, a total of about 280 rounds ), and 40 rounds that I just bought to test a magazine and also to see if they shoot accurately in my rifle, since I've not shot them before in it. XM68GD being one, and S&B 110gr FMJ being the other.

Had it since October of 2009, and it's never had an issue, and the bolt looks brand new.

I shoot this gun all year, but I shoot it a lot in the heat of the Carolina summers, where it has laid in the sun in 100f weather for 30 or more minutes, direct sunlight, mag inserted, round in chamber, waiting on another shooter to finish. Then 25 rounds were fired in quick succession , emptied into a target 300 yards away, in about 8 seconds from prone.

The ammo used was a 90TNT hand loaded to 2925fps ( chrono done at 70f ). A total of 350 rounds of it was shot that day by myself, my son, Trevor ( farm owner ), Chris, and Daniel.

I have reloaded that same brass  4 more loadings since, of the same charge, same projectile, and also 110gr Hornady OTM at 2700fps. Brass still looks good.

I have had one issue with brass, and that is a batch that I loaded on the Hornady press, where I had been loading on the RCBS. I had bumped the shoulders too far back, and overworked it. That's the only batch, and it has since been fixed.

Its actually the ammunition and reloading section. Where would you have preferred it? The Gemology section?

Classic, is a devote 6.5G follower constantly trying undermine what guys have done with this cartridge.
While adding little or no technical merit to the discussion.

However I am sympathetic, failure to feed, prone to bolt breakage, no major rifle builders, limited industry support.
Inability to be a battle cartridge due to the aforementioned issues.
I understand how difficult that must be.

Its really a fine little sporting cartridge however not one I would ever recommend for CQB or home defense.
It frankly does not have the reliability for either.

So I get it, you can use my shoulder brother.

really...

that's all you have?

a few months ago it was who had the most factory ammo, now everyone reloads anyway.... go figure

I get that you have something to prove, (still mad my 12.5"G hangs with your hotrodded hand loads with factory ammo) your as bad as what you whine about.  Want to add something technical...

measure the widest part of the inside of that case.  Maybe the we can calculate an accurate bolt thrust.

oh yea, 35000+ rounds on a single rifle...     Stop and think about that claim for a minute....

The 6.8 does have the most and most varied factory ammo.
As well as the best support from major industry in the factory ammo area.
I also said perhaps 50% reload, 50% use factory ammo.
Many more people look at webworld info than post.
Some never go to forums.
I personally have a healthy supply of both.

Bolt thrust has 3 schools of thought.
One like the one Lilja uses says that the pressure exerted on the case head at the point the radius meets the case wall is the width to use.
The second school of thought is the widest point of the case should be used as the whole case is a pressure vessel.
The latter does not take into consideration the contact between the case and chamber wall and that it pretty much cancels out the portion between where the radius meets the wall and the widest point of the case.
The only time this is not true is when the chamber is oiled intentionally as in NATO testing or in case head separation.
These 2 ways are the most accurate to calculate bolt thrust.
The 3rd is to use the whole case head exterior measurement, this is the least reliable although easiest to measure.

In my previous comparisons I used Lilja's method, As he has been touted by the G crowd as one with great knowledge in the barrel/chamber world.
So I used the method of the guy the G guys trust, trying to be objective and not show the G in the worst light as I don't need to. Not good enough for you I guess.
Just for an FYI Harrison at ARP also agrees with LI3 that the whole case should be considered a pressure vessel.
I think this is true, however I also feel as I just said the the added width between where the case head radius meets the case wall and the widest part of the case is canceled out by the case expanding against the chamber wall.
This would all be negated by case head separation or oil on the case or chamber wall of course.

Now for comparison purposes as has been used in this thread it really does not matter which method we use as long as all 3 cases in question are measured in the same place.
It will still come out the same with the G having the most bolt thrust, the 6.8 second and the 5.56 the least.
That is until the M855A1 ammo pressure is used then the 5.56 at 63000 and the 6.8 at 58500 the 6.8 actually has less bolt thrust.

But all this is really semantics in the real world as the 6.5G/7.62x39 are the ones that break bolts with regularity.
As well as the 5.56 at 63000 exhibiting cracking and pitting prematurely when compared to it at 60000.

awesome non-answer...

Been thinkin'.

I wonder how many rounds of allegedly dangerous very high velocity SPC II rounds Hi Tech Rancher & other testers Hog deprivation people go through a year?

Eh Yama & Wofie, I wonder if they will still be talking about the SPC II rifles blowing up all over the place in another 9 years?
Chicken Little & the Millerites comes to mind.
I guess it's something to do.

Anyhow to conclude;
Use a ladder load & check for pressure signs.
If you test/develop your loads at 70oF, your pressure & velocity can be somewhat different at 30oF (lower) & 110oF (higher).
Something to keep in mind.

Most of my loads are worked up in 90f+. I chronograph them at that temp when I work them up. Then, once I've found the sweet spot, I'll chronograph the load on a day that is within a few degrees of the record lows for here, and I also do a few at 75f, which is the number I usually post online, and near the average temp for here.

I've noticed very little difference with the powders I've used.

Compare that to some I used in the .308. I had some doozies, namely BLC-2.

That is a pretty temp stable powder. Which powder?!
I'll have to restate my statement. I forgot about temp Stability of different powders.

For the 6.8 I've used only extruded powders. .with the exception of AA2200, but it looks really stable so far.The others were reloader 7 and H322.

I've always used extruded for the most part. IMR4064 for the .308 , it replaced the BLC-2 and did a much better job.

Varget for the .223/5.56, but I've been using the AR Comp lately.

I'm not saying there were no velocity swings, but the swings were minimal enough that it was not something that was worth changing my zero over, not at the ranges I normally shoot or have the chance to shoot here.

Thanks!
I use Re 7, 10x & AA22oo.

I'm wondering if 6.8 SPC bolt thrust can be calculated from the cartridge case dimensions provided in this link?

The 2 internal case measurement points are not listed there.
But what is your point?
The 6.8 is not breaking bolts.
Look back through this thread, the bolt thrust is there. Measured at the radius, Lilja's method.
The previous comparison was done with all 3 cases measured at the radius.
If I were engineering a cartridge and trying to calculate bolt thrust I would measure at the maximum interior case width.
But we are not. We are making a comparison, so it matters not which measurement we use as long as they are all from the same place.


I would be more concerned with the bolt thrust the 6.5G makes at 52,000 and some of the pressures higher than that in your reloading manual.
They exceed even Western Powders data.
You want bolt life in a 6.5G, don't run it over 48,000.
Bolts are the 6.5's problem not the 6.8.
Its not just the thrust, there is just to little material left when the bolt is opened to .441 and a .135 recess.

You guys are still trying to beat a moot point where the 6.8 is concerned.

Using that data I would say somewhere in the neighborhood of .369...

5.56 proofload @ 70000 psi = 5768  (.324 measured from a sectioned case)

6.8 SPC @ 55000 psi = 5879, 58500 psi = 6253

Grendel @ 51000 psi = 5481 (.370 measured from a sectioned case) Hornady factory loaded specs.

You can see why they don't want to post actual dimension's...  The number don't come out in their favor...

As for bolts breaking, there are people on this sight that claim to have thousands of round on their 7.62x39 bolts (which has considerable more body taper than the Grendel or the 6.8) shooting steel case ammo...  although none have them have claimed 35000+

So your saying the Grendel case is .001 larger than the 6.8.
Not......
Then you drop the Grendel 1000 psi.

Exactly who are you trying to kid?

Here's the math for you.
Harrison sectioned all 3.

Bolt strength-
To compare, a remington 700 bolt =65712 lbs thrust
A mil spec 5.56 =26824
6.8 bolt=24024
Grendel with a .135 recess=21196
All bolts calculated with 160000psi yield strength since carp 158 and 9310 are very close.

Bolt thrust generated by common rounds-Od calculation to left ID calculation to right
If we use the 5.56 as the proper strength ratio then the numbers in red show us max pressure relative to bolt strength using the ID to figure bolt thrust.
5.56-----=6380@58kpsi(4.2X)58000psi-----------------------3828lbs 26824/7.=3832=58060psi
6.8 -----=8121@58kpsi 5720=40857psi-----------------------4350lbs--24024/7.=3432=45760psi
Grendel=7800@52kpsi 5046=33644psi-----------------------4992lbs--21196/7.=3028=31541psi

The above left was figured using the outside case diameters. It is debatable whether the inside or outside dia should be used. The figures above are conservative when using them to figure safety thrust Vs bolt strength.
If we use the bottom inside dia to figure thrust(figures to right), http://www.beyond556.com/bboard/foru...olt-thrust-faq
I see the whole chamber being a pressure vessel.
From what I read the AR15 bolt was designed 3 times stronger than the thrust placed on the bolt.
Then we could argue military use/ use in extreme climates, etc. Most of us don't shoot in 40 below temps or shoot 8 mags full in a firefight. I am not designing or loading anything for military use. We have been loading and shooting the 6.8 at 58-60k as hunters and target shooters since 2007. I think we have proved it works and is safe.

Notice we can run the 6.8 at 58kpsi and have the same bolt thrust as the Grendel at 52kpsi.

This is why the 6.8 is not breaking bolts and the G does.
Its not just thrust, but a combination of thrust and strength.