[rob_s]

I'm sure it's been done before, but I'd like to talk about it in the current market conditions in terms of ammo and barrel/chamber/twist selections available. I'd like to talk about it more in terms of generalities than the "my 123 load with xyz powder and 987 bullet generates 60k psi", but if it has to go there then it has to go there.

I've seen it posted that the limits set by the originators of the 6.8 were around 52k. I've also read the SAAMI spec on .223 Remington and IIRC they list it at around 55k. I've seen mention that the 5.56 is upwards of 62k.

What do you think is the best cap for 6.8? Why do you think so?
What are the downsides of increased pressure?
Are there longevity concerns with shooting 62k 6.8 rounds for thousands of rounds? Can it be quantified if there is (i.e. you can shoot 5k rounds at 55k but only 2.5k rounds at 60k)?
What are the pressure limits for SAAMI and SPCII and DMR? How is this determined?
What role does the burn rate of powder play in pressure?

and, since I don't see anyone putting a gauge on their gas tube, is there a way to calculate pressure, or at least guestimate it, based on known quantities like velocity and projectile weight?

Remember to speak slowly and use small words

In terms of examples, if you can point to pressure numbers of currently available factory loads like those from Hornady, Remington, and SSA that would be great.

 

[ron169]

Have you checked the performance test where they tested different loads with the pressure trace system?

 

[paulosantos]

Art from SSA indicated that his 115 GR SMK Tactical Loads produce 54K from a 1:10"/6-groove/SAAMI Chamber and 49K from a 1:10"/6-Groove/SPCII Chambered Barrel. So it will be even lower from a 1:11"/4-Groove/SPCII Chambered barrel. The SSA 115 GR SMK is the highest pressure round available for the 6.8SPC. As you can see, it is safe to shoot it in a SAAMI Chambered barrel.

55K is the pressure that most companies use for the 6.8SPC.

 

[Equalizer_2]

No numbers to back anything up but it is simple physics that higher pressures create more stress thus causing parts to wear or weaken sooner. So, the lower the pressures the longer the lifecycle.

 

[ArtFWTx]

Many if not most of the topics in your post are covered in the following.

Here is the pressure testing report conducted by HTR, Constructor, and Tim_W using Pressure Trace measuring equipment. If you haven't read this, you need to. It was a sticky here before we switched forums, and is posted on ARFCOM somewhere. A variety of factory loads and barrel specifications were used. You can see the difference between SAAMI/1:10, and SPC II 1:11, and other combinations. Note the pressure and velocity differences. This pretty much explains the performance capabilities of the 6.8 using the improved specifications.

http://m.b5z.net/i/u/6132121/i/6.8_20SPC_20Performance_20Testing_20Report_1_.pdf

 

[DocGKR]

For a match gun that is only going to be shot for one season, feel free to run your pressure at 55-60k; however, for a durable duty rifle, you want to go less. Regarding 6.8 mm pressures, Cris Murray wrote:

"...as I told folks when we were developing the 6.8 and they always wanted more speed, more speed means more pressure, and more pressure means more heat. This is why in my final version of the chamber I used a 0.100 length lead, which is not the same lead that Remington used. Pressures must be kept below 52K or during a fight your rifle will be usless after the second magazine. True, during slow fire you can push the pressures up, the Army's long range 5.56 match load is a 90 gr projectile over 60K, but its slow fire-single loaded for 1000yd matches only. The old 1000yd, 180 gr 7.62 match load would literally melt down a M14 if you could've loaded them in a magazine and shot them for rapid fire. This is the one reason I restarted worked on the 7x46UAC, because it allows excellent ballistics without 52K+ pressures that would render it useless in a real fight."

Note that Remington my have gotten a bad rap on the 6.8 mm SAAMI chamber issue--in recently reviewing some of the old AMU Mike Rock 1/10 5R 6.8 mm barrels from 2002, it turns out the 6.8 mm chamber Remington was given and what they used to develop the SAAMI specs was an earlier AMU experimental "tight" chamber and not Murray's final design. Thus, the Remington engineers designed their SAAMI chamber specs on what they thought AMU wanted based on the earlier experimental barrel they were mistakenly sent--not on the final "Murray"/SPCII design...a seemingly small failure to attend to details that resulted in a lot of long term unanticipated consequences.

 

[ArtFWTx]

So by reviewing the pressure testing report and the 52K Mr. Murray recommended, the SPC II or DMR type chamber and 1:11 or 1:12 twist is very important to reaching those numbers with some performance maintained. The SAAMI chambered 1:10 is over that level significantly with the 115 OTM commercial loads, and the SPC II and other improved chamber and twist barrels are under the 52K for conversations sake.

 

[HTR]

For a match gun that is only going to be shot for one season, feel free to run your pressure at 55-60k; however, for a durable duty rifle, you want to go less. Regarding 6.8 mm pressures, Cris Murray wrote:

Note that Remington my have gotten a bad rap on the 6.8 mm SAAMI chamber issue--in recently reviewing some of the old AMU Mike Rock 1/10 5R 6.8 mm barrels from 2002, it turns out the 6.8 mm chamber Remington was given and what they used to develop the SAAMI specs was an earlier AMU experimental "tight" chamber and not Murray's final design. Thus, the Remington engineers designed their SAAMI chamber specs on what they thought AMU wanted based on the earlier experimental barrel they were mistakenly sent--not on the final "Murray"/SPCII design...a seemingly small failure to attend to details that resulted in a lot of long term unanticipated consequences.

Thanks Doc, for helping keep some clarity in these discussions. We need your input. I did not know that Remington received a "tight" version of the AMU design.

1) Why then, didn't anyone go back and change the SAAMI spec under revision, once this was discovered? It defnitely appears that the SAAMI spec should have had the .100" freebore, does it not? Would Cris Murray and Remington help us to get that changed, if this information were brought to light? You would thnk that Remington would be happy to be absolved of the blame for the mstake!

2) What Doc is saying must be parsed in several ways. If this round ends up a military caliber and one were to shoot rapid fire(semi or auto...burst) , with multiple mag dumps, then the 52K becomes important, so a military load, say the 85 grainTSX, might then be limited to a load that gets 3000 FPS from a 16" barrel rather than 3150, if that is what is necessary to reduce pressure from 55 to 52k.

3) if you are a hunter, you can easily sustain 58K, which, with the best rifles (12 twist, 3 groove or 11.25 poly rifling and proper chamber) will generate the highest velocities we have ever seen in the 6.8

These are important distinctions which tell us that, even if there are pressure limits like those noted above, the barrel's twist, rifling design, chamber and the bullets chosen become that much more important, but still allow for the performance at which we are now peaking.

Which is exactly why we have been saying all along that we wanted the highest velocity achieveable with the lowest possible pressure....period.

 

[ArtFWTx]

Thanks Doc, for helping keep some clarity in these discussions. We need your input. I did not know that Remington received a "tight" version of the AMU design.

1) Why then, didn't anyone go back and change the SAAMI spec under revision, once this was discovered? It defnitely appears that the SAAMI spec should have had the .100" freebore, does it not? Would Cris Murray and Remington help us to get that changed, if this information were brought to light? You would thnk that Remington would be happy to be absolved of the blame for the mstake!

2) What Doc is saying must be parsed in several ways. If this round ends up a military caliber and one were to shoot rapid fire(semi or auto...burst) , with multiple mag dumps, then the 52K becomes important, so a military load, say the 85 grainTSX, might then be limited to a load that gets 3000 FPS from a 16" barrel rather than 3150, if that is what is necessary to reduce pressure from 55 to 52k.

3) if you are a hunter, you can easily sustain 58K, which, with the best rifles (12 twist, 3 groove or 11.25 poly rifling and proper chamber) will generate the highest velocities we have ever seen in the 6.8

These are important distinctions which tell us that, even if there are pressure limits like those noted above, the barrel's twist, rifling design, chamber and the bullets chosen become that much more important, but still allow for the performance at which we are now peaking.

Which is exactly why we have been saying all along that we wanted the highest velocity achieveable with the lowest possible pressure....period.

Good post. Now you have me going back and studying some of these topics again.

 

[DocGKR]

For a match or hunting rifle, then pressure--as long as it remains in a safe zone--is less critical, because the number of rounds fired is substantially less than for a carbine that will be shooting 300-500 rounds a day for many days a year during intense CQB training.

It would be ideal if Remington would go back and fix the SAAMI issue. Unfortunately, in the immediate period after it was first discovered to be a problem, internal politics at Remington precluded a timely solution.

 

[constructor]

Sure is nice to see everyone on the same page.
lower pressure, higher velocities, increase in range of terminal performance.

 

[kipkipkip]

Paul, I understand the pressure, and the 1/10, 1/11 twist. I don't understand the "Grooves"

Kip

 

[ron169]

First of all, do your know what lands and grooves are? IF not, the grooves are the raised section inside of the barrel that actually comes in contact with the bullet. The rifled grooves cause the the bullet to rifle down the barrel. With the 6.8, the fewer the number of grooves, the lower the pressure. Most of the manufactures have went with a 4 groove 11 twist, or a 3 groove 12 twist.

 

[constructor]

to expand a little, the ratio of groove to land is what is important, most rifle specs say 60/40-70/30 with the 60% being the groove. The less area the lands take up or higher the ratio the more bore area you have, the more bore area you have the less resistance you have, the lower pressure you have. 70/30 is better than 60/40 in the application you speak of.
This is getting too close to trade secrets so I don't want to go too far into it.

 

[HTR]

Paul, I understand the pressure, and the 1/10, 1/11 twist. I don't understand the "Grooves"

Kip

It is really rather simple:

But the definition of groove is the space that is cut out, when a button is passed through the blank, and the resulting ridges are called lands. These are what actually engrave the bullet.

The bullet encounters the rifling when it enters the throat. At this point, centrifugal force, turns the bullet, because it is engraved, and caught by the lands, which are spiraling down the bore.

If these lands are very tall and fat, there is less bore area, and the bullet jacket is scored very deeply and over more of its surface. This is called "bearing."

"Gentler" lands means a little smaller, i.e thinner and shallower. They engrave less deeply and thus impart less friction because they gouge into the copper jacket less. So, there is less friction and less pressure created in the travel of the bullet.

So, if you use a 1 in 10 inch twist, this increases the number of turns through which the bullet must encounter spiraling lands and maintain contact with rifling. Contrarily, if you use 1 in 12 inch twist with much gentler, and fewer lands (like the 12 twist 3, groove versus the 10 twist, 6 groove) then, the bullet is afforded less contact with engraving lands, thus less bearing and thus lower pressure. As long as the rifling can stabilize, then it makes no sense to expose the bullet to more lands/ grooves than necessary to get the job done.

 

[Jamesb74]

It is really rather simple:

But the definition of groove is the space that is cut out, when a button is passed through the blank, and the resulting ridges are called lands. These are what actually engrave the bullet.

The bullet encounters the rifling when it enters the throat. At this point, centrifugal force, turns the bullet, because it is engraved, and caught by the lands, which are spiraling down the bore.

If these lands are very tall and fat, there is less bore area, and the bullet jacket is scored very deeply and over more of its surface. This is called "bearing."

"Gentler" lands means a little smaller, i.e thinner and shallower. They engrave less deeply and thus impart less friction because they gouge into the copper jacket less. So, there is less friction and less pressure created in the travel of the bullet.

So, if you use a 1 in 10 inch twist, this increases the number of turns through which the bullet must encounter spiraling lands and maintain contact with rifling. Contrarily, if you use 1 in 12 inch twist with much gentler, and fewer lands (like the 12 twist 3, groove versus the 10 twist, 6 groove) then, the bullet is afforded less contact with engraving lands, thus less bearing and thus lower pressure. As long as the rifling can stabilize, then it makes no sense to expose the bullet to more lands/ grooves than necessary to get the job done.

If I come up with some tough term papers I want you to write them.

 

[Eliminator]

What was the original projectile weight(s) used in development of the 6.8? Was an optimal weight determined?

 

[HTR]

What was the original projectile weight(s) used in development of the 6.8? Was an optimal weight determined?

the original weight was 115 grains.

There are many weights which could be said to be "optimal" for different purposes.

For example, if you want versatile terminal performance, which includes barriers, the 110 grain OTM now under testing seems "optimal."

If you want maximum penetration on tough animals, the Barnes TSX in 85 grains is hard to beat.

If you are using a 6.8 SBR for HD, and are concerned about over-penetration, then the Speer 90 grain TNT is near optimal.

If you want great performance on soft skinned game like deer then the 110 SPH or the 100 grain Accubond are near perfect.

Lots of good choices

 

[Eliminator]

the original weight was 115 grains.

Thanks HTR. So if I understand correctly, the original 6.8SPC designers envisioned a hard use weapon having a chamber with a .100" leade, an 11 to 12 twist barrel, shooting a 115gr projectile, all the while keeping pressures below 52K psi(and higher pressures if used in slow fire situations). Is this essentially correct? What velocities were they getting?

 

[rcrandall]

Initial 6.8 testing had the 115 grain Hornady OTM at 2650 to 2700 fps from a 16" barrel.