It isn't accuracy or stabilization that the land / groove ratio affects, but rather pressure.

If you hand load, look into your manual for loads for bullets of the same weight but of different construction.

There are loads for rifle bullets with a lot of bearing surface that will reach max pressure 100fps before another bullet that is banded like a Barnes. The Barnes will be 100fps faster than the same weight bullet due only to bearing surface.

Too much bearing surface on the barrel works the same way, it increases friction, so it increases the amount of pressure it takes to move the bullet to a given speed.

This is what the old fast twist barrels had that Remington used in the tests and that were produced by several makers for the 6.8spc with the SAAMI chamber.

What you had was a perfect storm of mistakes that made those barrels reach pressures of 7K-10K more with the same ammo as the spec II can run at the same velocity.

The tests have been done, there is a PDF in the thread that you can look at and see the exact and vast differences in pressure using the same ammo in different chambers.


The question of the pressure differences has been answered.

The question now is what bolt thrust is safe, how was it arrived at, and what chamber pressure does it equate to.

As Pav said its not changing the height of the lands.
Its changing the width.
Anyone with QL can change the bore area .001 and see a difference of 1400 psi for example.
Bore area is only part of it. Engraving force on the bullet is the other reduction and much tougher to calculate.

I will put up more on this tonight, however not the proprietary specs of the barrels we are using that these guys have come up with leading to the results we have.

I should have addressed this earlier, sorry.

Those are external case head measurements which can be used for an approximation but are not near as reliable as the internal square area of the actual case head radius.
As has been discussed in the thread and an agreement as to its use been reached.

Also please take note I am using the lower of the 2 internal case head measurements in the numbers for the 6.5G.
Dan Lilja says its .370, I am using .350 which is the lowest measured by someone I know that section a G case.
If I was trying to shed the worst light on the G I would have used the higher number.
When in all actuality I am trying to be as concise/ fair/objective as I can.

I thought people were trying to specify rifling number and shape of lands and grooves, not depth of the lands and grooves.  If you're having to resort to reducing land depth in attempts to bring down the pressure, and increasing chamber pressure to over 58,000psi, your throat life and bore life are going to deteriorate more quickly.

It's another argument to pursue lower pressures, not higher ones.

Where am I violating the COC again?  Just because one can't deal with critical thinking and clear identification of unsafe practices, then cries loudly about it, does not equal a COC violation.

Remember that OP stated that 58,850-60,000psi is fine with this cartridge.  

just for shits and grins, I sectioned a Hornady Grendel case and it measured .370

don't celebrate yet cause I also sectioned a LC 5.56 case......................

wait for it......

.324

I find it very interesting that a 6.8 case measures less then a LC 5.56 case....

This thread has just gotten stupid....

Sigh.
Where did you measure the case?
At its widest point?
For the sake of the discussion I used the submitted 5.56 drawing, the measurements of the 6.8 and the 6.5 all in the same place.
Your argument here puts the G and its related bolt thrust in a very dim light.
Are you sure you want to do that?
I also could have used .300 for the 5.56 that so many places on the net reference.

Every brand of brass will have a little different measurement. All cases are thickest at the bottom but they are also widest at that point.
The guy that drew the exaggerated case drew it to make his point. If he had shown the case to scale no one could have seen why pressure is exerted from the widest point.
We have agreed that pressure is exerted from the widest point.
However we have also agreed that pressure to the bolt face, bolt thrust, is reduced by case tension to the chamber wall.
This is why I used the measurements I used for comparison.

SPC II Schematics with a throat:



FACT 1#  Guess it doesn't apply to current mil loads. But, then it went from a 1:14 twist to 1:12 chrome lined to 1:7 along with 150+ other changes.
``
As you can see we went from a 6 groove 1:9 twist with a .05 freebore to A  de facto barrel standard of 4 groove 1:11 (or 1:10) with a .100 freebore. Which b-t-w allows for longer COAL. Which is the basis for Facts 10 & 11.

As per the letter; You can decrease pressure slightly just by reaming to SPC II.
Current factory loads loads use SAAMI COAL & pressure limits. Although, there has been 1 factory load of 2.3 COAL for a 140gr Berger bullet in the past.

As for 12. True. Opposite for a decrease in temp (winter shooting). FWIW - test have been done on barrel temp too. Reason to do ladder loads at different temps.

The design of the  bolt has not been changed since it was originally designed back in 1958-59, other than opening up the face to accommodate larger head diameters, which if anything weakens the lugs by removing material from underneath making them more prone to flexing.  The only process changes were the dropping of the hard chrome plating due to hydrogen embrittlement and the addition of shot peening to improve fatigue life. There has been an additional steel used in the commercial field, AISI 9310, which has pretty much the same physical properties as Carpenter 158.

The 58,500 psi loading is still above the original design load and seeing very nearly the original design proof load.  Bolt thrust loadings of 4000 lbs result in stresses at the base of the locking lug that exceed the 931 MPa maximum yield strength of the bolt steel (per heat treat specification).  This will result in low cycle fatigue.

Yes, the AR15 bolt is overloaded at 4000 lbs bolt thrust and you are advocating an additional 15% load.  Then you add on the environmental variable of ammunition temperature, that can add an additional 5000 to 7000 psi to the chamber pressure.

I shall quote a few relevant snips from the report where I got the stress figures:

From:
Failure Analysis of the M16 Ri?e Bolt, V.Y. Yu, J.G. Kohl, R.A. Crapanzano, M.W. Davies, A.G. Elam, M.K. Veach
Department of Civil and Mechanical Engineering, United States Military Academy, West Point, NY 10996, USA

Please explain how exceeding the yield strength of any bolt is good for it.

Oh, and I haven;t heard a technical point from you, you have shown that you do not know how to calculate bolt thrust, or how gas pressure works, you keep trying to deflect the argument that your supposition is ill-advised by bad mouthing another, unrelated design, you believe that just because in is in a general reloading manual, it is automatically recommended for your particular application (just so you know, there are other bolt designs that are stronger than the AR15, 58K would probably be fine in a modern well-made Mauser-type bolt action), you live with the notion that just because you have only one or two failures, that proves it's safe, and you seem to think that every bolt failure that occurs will be posted to the web so we can see it.

(If you want to discuss the bolt loadings of the Grendel cartridge, start another thread, it's not helping your case here.)

Try and find the maximum internal diameter of the case as the pressure act on the entire case not as these knuckleheads gentlemen think just at the base.  It may be difficult as the maximum diameter occurs up about .2" to .25" from the bottom.

Please detail to me exactly what is incorrect about this statement.

When the AR15 was developed, what were the only two cartridges used in it during this time period?  When the design was put into production what other cartridge designs were being used in it?  After the addition of shot peening and the elimination of the chrome plating in the early 1960s the bolt was never changed or redesigned.  The heat treat is the same, the material call out is the same, the dimensions are the same.  If you have information to the contrary, let's have it.

Even if I don't have the exact maximum (which I'm pretty sure I do), it still shows that its more than .290 or what ever he used.  To think a case with a case head of .442 (SAAMI) has less of internal dimension @ its widest point than a LC5.56 case, and still somehow have room for more powder is silly.  I knew his #s couldn't possible be correct as soon as he posted them given the calculations I did based on the Case head itself.  

This whole thing is smoke and mirrors.  

I think we all get there are guys on this forum that absolutely love their pet calibers, but good god man, the lengths they will go...  

Practically begging people to adopt the their brand, crazy....  let him get his 7.62x39, I have one and its a blast.  Nothing like going to the range and blasting steel @ 200-300yds and not have to worry about policing and saving brass.

As for the Grendel, your absolutely right, deception always works better with distraction...  and to that note I have an email into Hornady about their factory loadings and how much pressure they produce, more because this thread has got me curious more than anything else.  I will be sure to post the results.

Well LR2 lets go on a 6.8 history lesson.
Bullet engraving forces.
This must me pasted in the browser to access the file.
http://www.researchgate.net/publication/235142167_Rifling_Profile_Push_Tests_An_Assessment_of_Bullet_Engraving_Forces_in_Various_Rifling_Designs/links/010828e40cf2c5839ebbea3b.pdf
"That report is why I was playing around with rifling configurations, chamber designs and cone angles in 07 and 08. To clarify what they are calling a forcing cone we call a the throat or throat angle, what we call the cone is the chamfer directly in front of the case mouth that leads to the freebore .2775" dia area where there are no lands.
see page 27 and 28 for the summary. A .6% bore area increase may change engaving force 20-25% which may change peak pressure 2-4000psi. reducing the width of the lands by 50% could change engraving force 40-50% which may change peak pressure by 4-8000psi. Changing the engraving force/start pressure setting on quickload can drop the peak pressure by several thousand psi."

Dr Lucci:
"When I was shooting shotguns three days a week for waterfowl, I, too started to research forcing cone angle in the shotgun.
Lo and behold, I found that Briley MFG would lengthen the forcing cone, and thus we could improve the pattern thrown by a shotgun. Its the same principle. When you force a shot column into a sharp cone angle you cause a bottleneck for the ejecta to traverse as it travels through the restriction. This causes a back pressure somewhat like trying to force a slinky through a tapered pipe. At some point of steep angled shape, you are actually causing the molecules of the bullet's rear to be crammed into the middle of its body, (because of the increased inertia) and this leads to a plastic disruption of the projectile. With a much more smooth taper, the bullet can undergo a slight "elastic" transition and stretch a little but not cause that back-pressure spike.

For the benefit of others reading, The SAAMI chamber drawings included an improperly drawn forcing cone angle. When you push that bullet through a shorter, more steep taper, you deform the bullet more, and force the material to "extrude" into the narrower caliber opening of the throat and early section of the bore. This increases the pressure. That's one of the main reasons we saw the "extreme" plated 115 grain bullet shave a little copper off in the taper and throat, and then cause huge spikes in pressure as subsequent rounds went into the chamber.

This isn't really new information, so my hat is off the H for digging and finding it. Is what we have been saying for years. I still see people on other forums claiming that the velocities we get with 6.8 SPCII are "hotrodding," and that modern AR15's can't handle a 58,000 PSI load.

OK, well, I am in my 7th year of reloading 6.8, shoot many thousands of rounds per year, have shot the 6.8 in classes and out of the helo, resulting in hundreds of rounds downrange over just a few hours, and I still have never broken a bolt.
The only "failure" of 6.8 hardware I ever saw was a broken extractor, shooting SPCII loads in a SAAMI, 1:9.5 chrome lined barrel in 2008.
Never again."

"The SAAMI chamber drawing did have a 45 deg cone. The PTG drawing showed a 45 deg cone but the numbers under the drawing that were used to program the CNC grinders were transposed and calculated to an 80 deg cone angle. I think PTG changed it sometime in 2009.
The thing is PTG makes 90% of the carbide reamers used by production barrel makers.
In 06 there weren't many making a 6.8 so I had a barrel just like HTRs. SAAMI chamber, .050 freebore with an 80 deg cone and chromelined. Blew primers the first day."

You gents have dug for what 3 days now and found 3 broken bolts?
The statement was made that an internet forum sample of users was to small a percentage of users to prove anything.
Here-in lies the problem with that.
If you want the most diverse source for 6.8 SPCII handloads your going to go to the 68forums.com
You can find limited loads in manuals but the real meat of it is here.
6.8 Bullet Reviews
There is no specific manual for reloading the 6.8, its online, at the forum where you are also going to see problems if any existed.
This is unique to the 6.8 more than any other caliber due to Remingtons screw up and the carefully redesigned chamber.

Such as this from a different forum.
"Nimrod
06-11-2013, 05:19 AM
I have a .264 LBC from Les Baer. I have had it for a couple years and had very good results with it for the first year, then I broke a lug off my bolt. Every since then I am breaking bolt after bolt, I am on my sixth one and I have even sent it back to Les Baer whom returned it with a new bolt at no cost to me. 28 rounds later, another broken bolt!! I am not running hot loads, this load was a 123gr. Amax behind 27.5gr XBR 8208, about 2420 on the crono. It still shoots great but only for about 30 rounds and then it breaks a bolt. It was head spaced by Les, I am using brand new brass, I checked and I'm not seating any where near the lands, I can't and still fit it in the magazine, and I am using 1.2gr les powder the I used with the original bolt which had many more rounds fired. I don't know what else to do."
The net is full of them if you look, its a real problem.
But 6 broken bolts in one rifle, wow.
As to keeping any other caliber out of this thread, you want to dictate that, go start you own thread, I'm the OP
You cant give the G a reprieve on its lacking bolt strength just because you want to. The G guys came here so the G can be involved.
Its a perfect reference to what is to much for the AR15 bolt system.

Now here's my point.
The 6.8 is not prone to breaking bolts. While the bolt thrust is higher than a 5.56 its still not breaking bolts, they are extremely rare.

Anything in a bigger case is.
"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 .035" thick.
The common way to figure bolt strength is lug width x length x 7(number of lugs) X yield strength of material but as I said before the web thickness removes some of that strength. The 5.56 amd 6.8 bolts have .125 recess removed from the .277" lug length. The Grendel has a .135" recess removed from the lug strength."
Should the ratio of thrust to strength be 1:1, 2:1, 3:1?
Figure the strength of the bolts then match the pressures of the 5.56, 6.8, and Grendel to the design ratio and strengths of the bolts.
Do you consider the bolt strength to be the limiting factor in the design of the AR15?
From everything your posting you must.

So you propose all 5.56 ammo should be limited to 52,000psi? If we limit the bolt thrust to that amount the 6.8 would be limited to 40,857psi and the Grendel limited to 34666psi.
I think you will have a tough time convincing all the firearms and ammo manufacturers they are wrong and they should change their specs and designs based on your findings.
Did I read someone said 6k to 7k pressure swings in a load? Its 1500 to 2000 according to AA.

I'm glad we are finally off the dam hoop/tenon crap.
As well top of the morning to your sir, I hope you have a joyous day.

A pressure vessel is a pressure vessel is a pressure vessel.  

Yes, I understand the SAAMI spec chamber causes pressure spikes, which in turn limited velocities.

Yes, I understand the SPC II chamber allows higher velocities for the same pressure.

The difference is not in the pressure containment attributes of the cartridge, but the shape of the pressure curve and the variation of peak pressure across various loadings.

If 55,000 PSI was specified as the limit of the system with thicker chamber walls and a small bolt face then using any other cartidge pushing past 58,000 PSI with thinner chamber walls and a larger bolt face looks to be not so good.

All of this talk of bearing surface, bullet shape, land/groove ratio, ogive, forcing cone, leade, COL, web thickness or whatnot is all just misdirection -- the obfuscating hand waving of a modern day scientism.

Official word from Hornady on their Grendel factory 123grn loads is the try and keep pressures between 50 and 51K...

Thanks, CJ, for taking the time. You didn't happen to ask what pressure Hornady uses for their 6.8 SPC? And what their opinion is on SPC and SCP II?

Also, I suppose you don't happen to have a 6.8 case you can section and measure like the others?

The bolt strength and receiver/barrel extension strength is always the limiting factor in down-range performance, otherwise we would be launching 80 grain bullets at 5,000 fps out of M16s.

The 52,000 psi was based on C.U.P. pressure measurement method, CUP pressures tend to be about 3,000 psi lower than actual.  55,000 psi should be the maximum MAP for anything using a .223 Remington/5.56mm case in the appropriate chamber (.223 in a .223 chamber and 5.56  in a 5.56 chamber), and coincidently that what it is, even with CIP.  (The often quoted 62,000 psi CIP limits is their equivalent of the Maximum Probable sample Mean, which is comparable to the SAAMI MPSM.)  The MAP for M193 with the 55 grain bullet has always been 55,000 psi even in military loadings, and for nearly twenty years that was the maximum pressure for the M855 with the 62 grain bullet.  why they bumped it up is beyond me.

As to other cartridges, ideally, the bolt thrust should be kept low enough that the lugs never get stressed to yield, and that way bolt will last millions of round, practically forever.  However, when dealing with adaptive designs that may not be practical, or, as in this case possible.  In these cases, I would recommend the bolt should be loaded so that the low cycle fatigue life of the bolt is around the maximum life of the barrel, 15 ~ 20k, that way when the barrel needs replacing, you just toss the bolt as well.

Just a cursory glance, and not doing a rigorous analysis, with the 6.8 and 6.5 rounds the two recommended MAP limits should be in that range.  I kind'a think that there was some engineering work done, they probably didn't just pick the numbers out of thin air.  I don't think the industry needs to change their specifications, people just need to follow them.

As to the pressure changes due to temperature.  Different powders react differently to temperature, some are extremely docile, some are touchy.  I quoted the maximum allowable pressure increase, not the average, as when you are looking at safety, you really need to consider the worst case....  

Nope, but I wouldn't be surprised if they kept it under 55000.  If I had a 6.8 case I'd go after it with a dremel, we'll have to depend on someone else for that one.

The only reason I ask Hornady, is that what I shoot and my reloads emulate.  Just curious how much pressure they were loading to, in light of this thread.

Responses in blue sir
Oh and one more thing no one has mentioned here yet.
I and most of the other 6.8 "hotrods" true our receiver face to ensure even contact and pressure dispersal across the bolt lugs to get the maximum strength it was designed for. Something "milspec" does not require.

The .290 was from LR2's posted drawing
Start wherever you want end wherever you want.
The 5.56 has the least bolt thrust followed by the 6.8 and then the 6.5G, 2 dont break bolts very often and one does.
Since you went and found every single broken 6.8 bolt you could, a whopping 3 I might add.
Heck I just found one rifle that broke 6 bolts, 6. 2 times what you could find for the 6.8 in one rifle.
Then lets add that in this forum alone their are twice as many 6.8 owners as 6.5 owners. Go find the poll from 6 months ago and see.
Would you like me to post every single Grendel broken bolt I can find?
It would be like looking for diamonds in a diamond store not needles in a haystack.
The G guys should stay off the bolt thing given its track record.

you just posted you reload for 5.56, but couldn't take the time to cut a case in half?  Ok...

one rifle that broke 6 bolts....  OK must be a cartridge issue, cause no way I would ever think that one rifle had a major issue.

one thing is for sure, given your post rate.  Your chances of breaking a bolt are super slim....

Seriously man, like everything else you guys would just say I'm blah blah blah blah blah..........like everything else.

I will not disagree with original development.

I said 'current loads'. Nothing to do with original development.

For reason for post: See: pg 5.
Posted: 5/18/2015 8:15:41 AM EDT

Fact # 556.68A:

SAAMI revision 1/11/13

Just looking at SAAMI for the .223 it states 55,000 MAP, 56,400 MPLM, 58,500 MPSM
The numbers are exactly the same for the SAAMI 6.8 55,000 MAP, 56,400 MPLM, 58,500 MPSM
Would this not then mean that anything deemed acceptable for the 5.56 by NATO with its leade change would then be acceptable with the SPCII and its changes.

SAAMI states that the 6.8 can handle exactly what the .223 can handle
Then we have the X855 that no one is sure why they raised it.

The beautiful thing about the 6.8SPCII is it is a refinement of the design that was built to get the absolute most out of the AR15 platform that could be reached.
Without being prone to breakage, fail to feed issues due to magazine stack angle issues and so forth.

When they come up with a better GPC, it wont be in the AR15.

Gee another class act

NO, it does not state that!  That is the dumbest most inaccurate misinformation I have ever heard any idiot say....You are really grasping at any straw you can see.

If the MAPs, Maximum Average Pressures, are the same, the other two will always be the same.  The value of the Maximum Probable Lot and Sample Mean Pressures are set by being 2 and 5 maximum standard deviations above the MAP.  SAAMI uses 700 psi as the maximum standard deviation for this pressure range, so (700 x 2) + 55,000 always equals 56,400 and (700 x 5) + 55,000 always equals 58,500.

But, to assume that because one organization (SAAMI) places the 5.56 and 6.8 cartridges at similar pressure limits, means that because another organization (US Military) allows a higher pressure limit on 5.56 automatically means 6.8 is also good to that limit is just plain dumb.  (The Military doesn't even use 6.8 So how would they know, or why would they care about what pressure it can handle.)

The other thing is the Military has much more control over weapons quality and maintenance than the civilian world, so it is taking a smaller risk by bumping up limits over civilian organizations, and individuals.

When FN or Colt gets a batch of bolts from a subcontractor, they also get a certification sheet that traces the steel back to its origin and establishes it as "per Print".  They get a copy of the heat treatment records to show that is was done "per print".  They can also get any other quality records they want.  They do this because when they sell a rifle to the Army, the Army wants proof that everything has been made from the correct  materials and according to the drawings.  Did you get a cert sheet with your rifle, or bolt?

Yes, but of those 150+ changes the military has made, none were changes to the bolt lug dimension, material or heat treat.*  The bolt of today is only as strong as it originally was when it was designed.  Over the years of bumping up the pressure and increasing the bolt thrust have just been eating away at the original safety factor.  


In fact, the only notable change done to the load bearing locking components was the changing of the feed ramp cuts on the back of the barrel extension, which removed material from the 6:00 o'clock locking buttress, if anything making it weaker.....

________________
*The civilian world has seen the addition of AISI 9310 steel, which is a comparable steel to Carpenter 158, neither substantially stronger, nor substantially weaker.

Your wrong dood
"Our bolts are not just a normal AR15 bolt design made from 9310 like most of the others on the market.
Complete bolt redesigned for superior strength and machined from 9310 VAC ARC alloy.
Notice the thicker web between the recess and the lugs and the larger radius at the root of the lugs. The area of attachment and strength was increased by 24%.  The bolt is  larger in dia at the cam pin area increasing the thickness 17%. these improvements make them the most durable bolts on the market."

Your stating that the increased material in an AP Superbolt does nothing to increase bolt strength.

Laughable.
All my weapons are beyond milspec.

Yup, best thing in the world, US Govt quality control.

You seem to ignore the 6.8 is in allied military use.

Your still just hammering away at a personal opinion.
The 6.8 has been in use at the pressures stated in the OP for 7 years.
Even in commercial use.
Its been time tested extensively.
You cannot negate that with all your "but this", "but that".

If this was 2008 and the guys had just finished and released their report I could totally see your questioning the validity of my claims.
However this is 2015, 7 years later. 7 years of refinement, 7 years of testing loads, new powders, new materials.
7 Years of commercial use, some in high rate of fire 100* conditions.
We have guys working extensively on 130gr loads, 150gr loads for competition.
The 6.8 is in active duty in LWRC rifles.
We have the 6.8 in a SAW, in service in an allied militaries. I am not free to discuss what going on here but more is coming.
6.8 SAW
6.8 Linked ammo

You can post up all the old development specs and stats you want it does not change that fact that what I am saying is field tested.
Its already PROVEN that it can do it.
We are NOT seeing premature bolt wear, not seeing premature pitting, not seeing cracked bolt lugs prior to failure.

Yama, you're answering L13's technical points with rhetorical rebuttals. Apples and oranges.

Trying to say that AR15s running 6.8 at 58,500 is fine because "allied" militaries use it is like saying Obama is the best man for the job because more than half the country voted for him.

The "celebrity" endorsement from countries that have questionable loyalties in the global war on terror and who have more oil money than sense is not what I'd call a ringing endorsement. Equipping the Royal Palace Guard with some Six8s so they can protect the King while watching some men beheaded for owning a Bible or some women beheaded for not wearing their veils in public is not the same as equipping a whole army. You think they're gonna go to war with 90gr bonded soft points?

Perhaps the whole deal has more to do with Richard Bernstein's personal friendship with King Abdullah II of Jordan than the technical merits? Just something to think about. . . .

Anyway, between all your rhetoric and your 6.8 SPC "commercials," we actually having a good discussion and I'm weighing what Lucci and Beene have done technically, and it's not really bad work at all. It has its pros and cons, but for casual sporting use, I can see where they're coming from.

"Trying to say that AR15s running 6.8 at 58,500 is fine because "allied" militaries use it is like saying Obama is the best man for the job because more than half the country voted for him."
No sir, I'm pointing out that its field tested, in high heat environments.
Fully auto fire, desert temps. I'm point this out because powder temp variables has been brought up, a few times.

He states it was never designed for M855 pressures, but yet its approved for it, yet he cant say why.

Its being contested that WP says loads are good to 58,500 for the 6.8, but yet nothing about the 5.56 being approved to 62,350.

"The "celebrity" endorsement from countries that have questionable loyalties in the global war on terror and who have more oil money than sense is not what I'd call a ringing endorsement. Equipping the Royal Palace Guard with some Six8s so they can protect the King while watching some men beheaded for owning a Bible or some women beheaded for not wearing their veils in public is not the same as equipping a whole army. You think they're gonna go to war with 90gr bonded soft points?"
We did with 55gr FMJ's, which would you rather have?
The XM68GD has been tested to go thru a windshield and 2 pig carcasses.
Its a heck of a round actually.
Barrier blind, perfect expansion, over 95% weight retention, its hell on hogs.
I would prefer it 100 to 1 over a 62gr M855 or MK262 Mod 1
That was a darn silly statement sir, especially considering your self proclaimed jealousy over a cartridge the competes with and betters your favorite.

1/11/13 SAAMI report says both the .223 and 6.8 are capable of the same pressure handling abilities.
But that's not good enough either.

dang you sound like a couple  different  barrel  makers talking about puffy

You do see how that doesn't make sense, right? Considering a SAAMI report is not dealing with the "improvements" called the SPC II?

Try this thought experiment: If you read a SAAMI report saying both the .223 and the 6.5 Grendel are capable of the same pressure handling abilities, would THAT make sense to you? I hoping putting it that way might help you think outside the box.

Yes.

And, the maximum average pressure limit is 55,000 psi, not 58,500 psi.

58,500 psi should only come up occasionally, 10 to 20 rounds per 10,000.

1) These are one company's bolt, not the 'standard bolt' as stated.  

2) Are all 6.8 mm Remington SPC bolts made by this company, or to this design?

3) The fact that some people feel the need to redesign the bolt should tell you something about bolt loading....

Until all bolt are made to a higher standard, no increase in MAP should be advised.  And the standard is still the basic M16 design.  This is why 7mm Mauser is still held to such a low pressure.  7mm Mauser should be capable of almost* anything 7mm-08 can do, but it is hampered by all those old weak Spanish rifles that weren't made to handle such high pressure.
_______________________
*There are limitations  due to internal case geometry.

You guys have been quoting SAAMI for years saying that the 6.8 chamber cannot handle the pressures the 5.56 can run at.
This whole thing about the pressures we are running at was focused on the chamber and ringing the chamber and claimed and never proven failed chambers.
SAAMI and WP both say that the difference in size between a 6.8 and a 5.56 chamber does NOT lower the pressure vessel containment capabilities like you guys have been eluding to for years.
Now your focusing on the bolt. Also part of the containment, probably what you should have focused on all along.

It has been said here pressure spikes associated with the SAAMI chamber are not a reason to lower abilities of a cartridge when this is not true.
Late pressure spikes associated with the .050 leade and 50:50 land grove ratio can create a very dangerous situation, calling for a reduced initial loading to make sure the pressure spikes do not happen.
Secondary Pressure Spikes

Yama has more red herrings than a fish factory, but the following is the crux of the whole discussion. This is the technical basis for his claims that the 6.8 SCP II of 2015 can handle a Maximum Average Pressure of 58,500. Other than the tweaks to the standard system listed below, his claims are also founded on the assumption that those fussy old fuddy-duddies can afford to have some of their safety margin encroached upon, because, c'mon, they're waayyy too conservative.

SUPER SECRETS OF SOUPIN' THE 6.8 SPC II

1. CORRECTED CHAMBER DRAWING. Cool. Got no problem with that. Might as well make it the best it can be. Strike that balance between cartridge feeding, pressure management, and accuracy. Can't fault anyone for trying to optimize a chamber.

This applies to any cartridge, and is not specific to the 6.8 SPC.

2. UPGRADED AR15 BOLT. Attempt to strengthen bolt by making design dimension tweaks, as well as improved steel and heat treat specification. This is fair, as long as it works within the confines of the standard AR15 envelope. (Even if it's no longer a standard AR15, such as the LWRC Six8 platform, if guys want to buy a proprietary design, it's their money.)

This applies to any variant cartridge in the AR15 system, and, in fact, predates any work on the 6.8 SPC by several years.

3. CUSTOM AR15 RECEIVER. Gunsmith trued receiver for balanced bolt-lugs to extension-teeth contact. This is cool, although, again, it takes it from the realm of being able to work with standard parts and puts it to needing custom or higher-quality parts.

This technique is not unique to the 6.8 SPC for any higher quality AR15-style rifle to get best accuracy. That the 6.8 SPC requires it to avoid overpressure problems is not ideal.

4. LOAD LONG. Top performance requires special magazines or loading long, relative to SAAMI standards. I actually consider this fair game — as long as it fits in the standard AR15 magazine well. (Personal opinion: Cutting magazines and — yikes! — magazine wells is ridiculous. But, hey, it's a free country.) That the 6.8 SPC requires it to avoid overpressure problems is not ideal, and the advantages gained therein are not limited to the 6.8 SPC, but can also be applied by other cartridges.

I have zero problem with an intermediate case trying to load high BC bullets; removes the fundamental flaw of the 6.8 SPC. Murray admits this; Beene, in his heart of hearts, knows this and though he's far too hyper-competitive and making too much money on 6.8s to ever admit it publicly, he'll admit it privately. Loading a 6.8 beyond the AR15 mag well means it's no longer a 6.8 SPC or a 6.8 SPC II, but is a totally new cartridge. And that's a whole 'nuther discussion.

5. TRICK BARREL. Custom barrel having "micro" lands with a land-groove ratio of 25:75, the lands being of 5R-type profile, using a smaller land depth. This, along with the improved chamber, is the basis for the claims the 6.8 SPC II can handle a Maximum Average Pressure of 58,500. It's actually a nice piece of work (and nitriding the barrel is probably necessary), but the fact that the 6.8 SPC requires a custom barrel profile to avoid overpressure problems is not ideal.

The downside to the "micro" land trick is that the rifling wears down sooner than standard, more robust rifling. Imagine a stack of six 8-foot 2x4s. This is standard rifling. If I give you some sand paper and say, "Sand all these 2x4s down to sawdust," it will take you X amount of time. This is shooting standard pressure rounds until the barrel is shot out.

Now, the analogy to a "micro" land barrel: Imagine a stack of three 1x2s. Now I give you a handheld power sander and say, "Sand these three 1x2s to sawdust." This is higher pressure loads on "micro" lands.

Which will sand down faster?

So the "micro" land barrel for the 6.8 SPC II is a clever trick for those shooters who don't shoot enough to prematurely shoot out their barrels. Those who sell these barrels are making a perfectly reasonable bet that they'll make their money long before their buyers ever notice the reduced barrel life. But this technique is a non-starter for serious military use.

That about sums it up. Did I miss anything?

I have to admit your trying very hard.
1. The Murray chamber was good from the beginning, Remington was the problem. By the way he stated it would run 2800fps from a 24in barrel not a 16in.
2. We have no breakage with C158 bolts. LR2 just stated their have been NO improvements to design. A whole lot of guys are running Stag and DMPS C158 bolts with no issues.
3. Custom receiver? Really? a $59 Anderson receiver works fine, nobody's breaking anything when they dont true the receiver face. I am just pointing out there are many things guys can do to "blueprint" their build, taking it beyond milspec even with standard lower priced parts. You want bolt life in any AR, true the receiver, your bolt will last longer.
4. Your not reading very well. WP's load of the 110gr with LT30 is at 2.26 at over 58,000psi. My 130 VLD's are not running in cut mags, 140's either, only the 130 CH requires a cut mag because of the ogive. All the other WP loads are a 2.26 OAL. There are a boatload of loads that run 2.27 to 2.295 OAL up to 58,500 with no issues. Tell you what, run the number of a 130 VLD .452 BC at 2500 fps, heck even 2450. I am right there with the G all the way out. What we can do and what we have to do are 2 different things.
5. There are no "micro" lands. Nor are we getting short barrel life. You whole dissertation at the end has no bearing. ARP's melonited barrels will outlast chrome lined barrels. No one is reporting short barrel life. I have easily over 5,000 rounds down the pipe, probably more.  Do I have an accuracy problem with my Bison Stainless barrel?


Your missing everything.

No, I have not.  If you go back and carefully re-read (or just read) my posts, one thing will be clear - You are spreading ill-advised and possibly dangerous information, especially since SAAMI recommends a maximum average pressure of 55,000 psi

Quoted: If this was so unsafe they would not have data published for pressures to that amount.

Quoted: Westerns data shows they are confident any 6.8SPCII barrel can be loaded to 58,500 psi.

Quoted: 58,500 sir, by major manufacturers.
.
.
.
Et cetera, et cetera...

Also, you have made several inaccurate statements, which I have try to correct.  But honestly I have missed a few.

Since one is already up ("58,500 sir, by major manufacturers.") I will address it, now as well, so far, you have only listed one powder manufacturer as listing loads to the MPSM, I have not seen any evidence of wide spread loading and recommendation of loading to the MPSM pressure.


Never addressed this piece if misinformation, either, but - False. AISI 9310 is about the same as Carpenter 158 in the heat treats used in bolts.

The original AR-10 design as designed and produced in 1958 or thereabouts weighed 7.5 pounds, and was considered a very accurate weapon, even when compared to non-military automatic rifles.  The weight is about the same as a modern M16A2, you can if you are prudent make a modern .308 AR that weight, even lighter with care, (mine weighs in a 8.5 with a medium-heavy barrel, so I know it can be made lighter if you tried), and keep the the accuracy.

If I were desirous of sending 7mm bullets down range at high velocity, I would want this type performance:

100 grain bullets at  3,000 fps, 10% faster than SPC.
140 grain bullets at 2,800 fps, 20% faster than SPC
or
168 grain bullets at 2450 fps, something SPC is not capable of.

AND do all this while keeping my maximum chamber pressure comfortably 5% below the SAAMI recommended MAP for the cartridge in question.  With 7mm-08 Remington, I can and still get to have a 25 round magazine, and a lightweight, handy rifle.

Pushing the pressure up to the MPSM to get the performance you want is ill-advised, get a bigger cartridge if you want that type performance.

Not really relevant to 2015, but you got a link to prove it?



Your argument from history is somewhat fair, except that you have no way of actually knowing round counts. It's a Wild Ass Guess on your part whether 6.8 owners are even coming close to fatigue.

Further, it's already been pointed out that saying, "We ain't crashed yet," is not a substitute for engineering and testing rigor.



Your custom reloads for your personal gun are largely irrelevant to what MAP the industry ought to set for the 6.8 SPC II. You know what you're doing and are willing to take the risk, no different from any other reloader, so it proves nothing about industry standards for a weapon series.



"It's the land-groove ratio! It's the land-groove ratio! IT'S THE LAND-GROOVE RATIO!"

Now it's not? <sigh>

Anyway, I want to thank all who've contributed here: Yama, Pavlov, LR, L13. I know it's been a long slog and not for the faint of heart. I know that in the back-and-forth of debate I've learned pretty much what I came here to learn. Got a clearer understanding of today's 6.8 SPC II program, and have some food for thought.

Everybody have a great Memorial Day weekend!

Your proof

Your right 7 years is to small a sample of time. egads

With the way the G breaks bolts perhaps we could run 65,000 psi and get to the same level of stress of the components.
Because we would likely have to to break bolts as often.

Your perception of the land to groove ratio is a bit off.

Have a great Memorial Day as well!

In engineering, design and manufacturing the end result is product testing and then the final is how does it work once released and its out in the field.
The 6.8 SPCII loads are out in the field, the rifles are not failing, the bolts are not failing, chambers are not ringing, the throats are not ringing.
The bolts are not wearing prematurely, the barrels are not wearing out prematurely.
Rifles are not blowing up, people are not getting hurt.
None of the crap you guys have thrown for 10 pages is happening.
You can talk about it all you want, but none of it is happening after 7 years.
You can argue till your blue in the face, you can say I'm dangerous till your blue in the face.
I did not come up with the progress of the 6.8, I'm just enjoying the benefits of it.
I admit it requires thinking outside the box.
The track record of it stands on its own.
There is no 6.8 Voodoo, it just works. Its not breaking.

Happy Memorial Day everyone, thank you all for you service.......

This thread started another thread over at the 68forums.
Harrison from AR Performance started a bunch of pressure load tests on a variety of powders and popular 6.8 projectiles.
Also his thoughts on pressure and the 6.8
Good info for those interested.
If your reloading for the 6.8 you should check out his pressure test findings for a reference point.
Most are right where I expected them to be.
Reloading and pressure with the 6.8 SPCII
There is a lot of good info here. Stuff all 3 Tim W., Dr Lucci and Harrison agree on.

Some 6.8 Hog action from Lone Start Boars - he is sponsored now by LWRC and tests ammo for Hornady

Monster Hog Down

Best Of 2014

They recommend it?

Where?  Not being argumentative, I have not seen that anyone that recommends it.  Published reloading data from the other powder manufacturers keep the loads below 55K.

I am talking about running loads that are on the warm side up the the 58500 range.
Nobody in their right mind would take loads to the max every time if they could run a powder and get the same speed with the same accuracy at a lower pressure.
As well if your barrel/bullet/case/powder/primer exhibits pressure signs then stop and back off half a grain and find your sweet spot.
I have said before with all the powder and bullets combinations I have tried there  seems to be a moderate and a warm accuracy node.
If your wrecking brass, swipes not related to timing or over gassed, primers flattening or cratering then don't load to the upper one.
If your not doing the aforementioned then load to the upper node.
Since this thread started and Harrison started running pressure tests it has confirmed what I was getting with reading brass and running loads over the crono.

The point all along has been if you need to run up to 58,500 with a particular powder and a particular bullet in YOUR barrel/cases/primers, to get the speed and accuracy to get the results you want at the distance required for the task you can run up to those pressures.
From the new pressure report testing my loads look to be running between 55,000 and 58,000+, exactly where I thought they are.

The 140 VLD from SSA and 95 TTSX from Wilson were 2 such loadings. SSA sold to Nosler, Wilson is having trouble getting the needed powder whatever they were using.
Neither stopped because of pressure or bolt issues.

One of our issues is you keep referring to SAAMI load data, I am talking about SPCII loading.

To anyone loading warm never, I mean NEVER use a powder drop or automated equipment.
Make sure every load is + or - no more than .1 of a grain from your desired charge.
Consistency is key.

Edit: Also bear in mind if you are hand loading you do NOT get the same variance in pressures you do from production ammo.
You wont see the stated 4000 to 6000 psi variances claimed by someone earlier here.
You will see 1000 to 1500 psi with good reloading techniques.

I am going to attempt to clear one of the reasons SAAMI limited the original wrongly submitted chamber.
It has been stated in this thread that pressure spikes dont mean anything, cant limit the pressures SAAMI approves for a caliber.
Ly3 this is more for LR than you.

"Despite the incredible amount of knowledge on these handloading sites, pressure spikes are rarely written about--but yet it is allegedly one of the most important factors when SAAMI determines max pressure for a cartridge."

"In the mid-90's, a couple friends and I had access to a pair of Oehler Ballistic Labs, and we glued strain gauges to a bunch of rifles (we bought gauges by the carton!). We weren't entirely aware of the whole pressure spike thing, and when we saw spikes on our graphs, we initially chalked up to our handloading techniques. Eventualy we learned more about pressure spikes, and some cartridges certainly showed more spikes. Of course, some loads undoubtedly were probably more prone to spikes than others.
For example, a couple of 280R's we had hooked up to the M43 showed a fair amount of pressure "spiking" with a variety of loads--which is why I kinda' cringe when the 280 fans want to load to 65k psi.....I think there is good reason for the SAAMI max pressure."

"The real problem is that to really understand the phenomenon one would need to film at very high speed the burn process like they do in engine combustion chambers. The pressure in the chamber makes that almost impossible. There has been a lot of burn work done in engine combustion chamber/fuel system design that would be applicable here if the pressures weren't so high."

"Some cartridges exhibit a wider range of pressures than others.
The question remains why some cartridges exhibit larger spikes than other cartridges based on the same case.
But it does explain why SAAMI limits are different for different cartridges based on the same case."

"That "pressure" spike is a "repeat" of chamber pressure - only several inches down the tube."

"I think it has something to do the throat length of different manufacturers. There doesn't seem to be a definite throat length set for the 7 MMmag.
A rifle with a short or no throat will give much higher pressures than a rifle that is throated properly."

These are a few examples of people talking about pressure spikes.
This is what was happening with a short leade of .050 and to much engraving friction in the barrel.
Internal ballistics are not just in the chamber but down the full length of the barrel until the bullet exits.
Anyone mocking this is just showing how little they know about internal ballistics.

ALL cartridges exhibit spikes to varying degrees usually these are minor spikes that really do little more than degrade accuracy.
Sometimes they even happen at lesser pressures with a powder that is to slow burning for the application. This kind has been catastrophic in other calibers before and lead to complete barrel failure.

Some exhibit these spikes when reaching higher pressures, as was the case of the fubar SAMMI chamber.

The bottom line is SAAMI WILL limit cartridges that exhibit pressure spikes. They will limit calibers with land/groove ratios that exhibit these characteristics.

For a "new" guy I have seen some pretty lame statements from guys that claim more knowledge they they purvey.

Why not just let the trolls run?  Why the need to "fix" them?

If the pressures are unsafe, eventually someone will blow their face off.  Perhaps they will sue arfcom and the persons posting data for spreading misinformation.

But this thread is like the special Olympics; even if you win, you're still retarded.

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

Greg