The truth about 6.8 SPC SAAMI vs SPC-II

[silencertalk]

Not an attack, just a question; why did you post that data on the 6.8 forum? It was good reading, and the responses are psuedo-typical of the gun crowd --- a field where, frankly, not a lot of data is available for what happens in that microsecond in the chamber, or at the point of impact, for that matter. People end up with a "lucky-socks-that-won-the-game" mentality, and don't change from the .30-06 their great-grandpa used, because it worked then, and each subsequent time. I get the haze of voodoo that surrounds hyped and mythical cartridges (and try not to fall victim to it myself). Not attacking, and not saying you shouldn't have, but still, why did you post that there?

This is the data that has been missing from the 6.8 public - it was going to end the debate as to how many psi difference the two chambers were, and I wanted as many 6.8 people to see it as possible. In 308 most people bitch their chamber is too long. For some reason, the 6.8 world has this strange set of rules.

I would say only 25% of the people there believed the data I posted - though I think now, as of yesterday or so, it is sinking in and will be accepted by all but a small few.

Those few people who just can't follow science, a month from now, will cite a 2008 report which claims that a 1:11 twist barrel is 4000+ psi less pressure than a 1:10 twist - and they know this because they stuck a strain gage on two different rifles which differed in a bunch of ways, but they attribute the difference only to the twist. Let's not forget they did not know the true bore cross-sectional area of each. And let's not forget that you cannot even get reliable absolute data from a strain gage because you have to measure the barrel diameter and estimate the steel properties - and any attempt at calibration with reference ammo just normalizes the results.

What is most odd of all is that in the 5.56mm world, you are lame if you don't have 1:7 twist - and if you have 1:9 twist - you can't even sit at my table. In the 6.8 world, you are lame if you don't have 1:11 twist - which converted to 5.56mm is just like 1:9 twist. If I suggest 1:9 twist in 6.8 I am looked at like I have two heads - but that is like 1:7 in 5.56mm.

[TROOPER]

Someone tried to explain the downside of over-stabilizing a bullet on this forum before. What I got out of his explanation was that an 'over-stabilized', or 'over-spun' bullet would have a gyroscopic effect on the bullet that would make its nose keep a parallel angle to the bore. Which, if I understand correctly, would be a meaningless issue at shorter ranges, but would only begin to manifest itself at distances where there is a significant trajectory. At those longer distances, the bullet wouldn't be traveling nose-to-the-wind so much, and the larger drag coefficient would slow the bullet down faster than it would have normally had the nose been able to tilt gently down into the bullet's trajectory.

Now I imagine that by hitting a soft target at a gentle angle, the lethality would be theoretically improved, because tumbling would be initiated almost immediately.

In a way, I feel like this is kind of a non-issue for a 6.8 SPC, because -- that I know of -- no one is treating it like a long-range cartridge. Theoretically, over-spin couldn't even be identified (on a paper target) until beyond the point-blank range of the cartridge anyway, as any hole in the paper would appear perfectly round. At greater ranges, the hole would theoretically be more oval as the distance grows, due to the need to elevate the rifle upward -- like a mortar -- to gain that extra range. That said, the 'oval', while likely measurable, is practically irrelevant - since it is tied to the angle above the horizon for which the gun is fired; and no one is going to fire their rifle like its a mortar. However, the MPBR of the 6.8 SPC is somewhere around 250 yards (+/-).

Alright, so assuming all the above information is correct, then there wouldn't be any practical issue with over-spin on a 6.8, since I'll wager that the super-majority of shots are less than the 250 yard engagement. I'll also wager that the shots beyond 250 are on paper.

Any truth to this monologue? Anything to add?

[silencertalk]

Someone tried to explain the downside of over-stabilizing a bullet on this forum before. What I got out of his explanation was that an 'over-stabilized', or 'over-spun' bullet would have a gyroscopic effect on the bullet that would make its nose keep a parallel angle to the bore. Which, if I understand correctly, would be a meaningless issue at shorter ranges, but would only begin to manifest itself at distances where there is a significant trajectory. At those longer distances, the bullet wouldn't be traveling nose-to-the-wind so much, and the larger drag coefficient would slow the bullet down faster than it would have normally had the nose been able to tilt gently down into the bullet's trajectory.

The effect is too small to consider.

There are (lots of) guys who won't buy a Ruger 6.8 rifle because it is 1:10 twist, or will buy it and toss the barrel in the trash, just to put in a 1:11 - because they read on the internet it makes much more velocity and that 1:10 has dangerous pressures. That is stunning to me.

[Crosshair]

They have to be loading to proof-round pressure for some of the numbers I saw.

As someone who tinkered with proof level loads in 22-250 years ago I can say with authority that this is a self-correcting problem.

[silencertalk]

Here is something interesting:

http://68forums.com/forums/showthread.p ... eske-Mod-1

The DMR chamber shrinks the freebore from 0.2781 to 0.2770 - which is BELOW max bullet diameter! Bullets can be up to 0.278 and still be in SAAMI spec.

A tighter throat can raise pressures thousands of psi over the 0.278 throat diameter used in SAAMI.

See how the pressure goes up for tight throats:

http://www.border-barrels.com/articles/ ... ortium.htm

It is imperative that if someone ever does submit a new chamber to SAAMI, that they use the original 0.2781 freebore diameter - not just to keep pressure down, but to make it easier to manufacture ammunition which must freely drop in and out of a chamber gauge without being forced.

[TROOPER]

Has anyone calculated the 'over-bore' value of a 300 BLK? Not so much a specific number, but what two calibers it falls between on the "over-bore" chart? It would have to be low, right? It is closer to being straight-walled than most other rifle cartridges except for some like the .375 H&H or a .22 Hornet.

[silencertalk]

http://bulletin.accurateshooter.com/201 ... efinition/

25.0/0.074506 = 336

[Crosshair]

The biggest problem I see with that chart is that it goes by case capacity when many of those cartridges have average powder charges that are much smaller. A good example is the 6.5x55 Swedish. Case capacity is listed as 57 grains, but my load books list most max powder charges in the 45-46 grain range. Going by average powder charge would be a far more accurate statistic.

[silencertalk]

50 BMG also has a lot of empty case.

[alleycat72]

LOL.. You went in to someone else’s house and told them their overweight wife was fat and they’re mad at you for pointing it out instead of her for eating too much. Then they took their ball and went home

[Crosshair]

50 BMG also has a lot of empty case.

Yea, now that I look at that I see that too, I just happened to know 6.5x55 charge weights off the top of my head.

[silencertalk]

Evidently there is no test one needs to pass before being able to post on the internet:

http://68forums.com/forums/showthread.p ... 6.8-bullet

I did not think the concepts were complex - maybe 8th grade level. Maybe I am just out of touch with what I can expect people to comprehend.

Thankfully Todd -K- helps out and backs me up on one of the last pages, but the forum members don't seem able to read charts or drawings.

[chrismartin]

I think most of the issue is a problem with protecting their monetary investment in a particular thing.
You stated very plainly that in some cases a tighter chamber is desired, but for proper reliability maybe not.
This was similar to the first thread you posted about. Lots of arguing about "real world" versus the published data that was provided and claims that you are only there to promote 300BLK and detract from 6.8whatever. It's a little funny really.

I had a similar issue on a home town forum with an "operator". I published data, he came with "experience", which in this case was incorrect. He proceeded to make claims about me, personally, because he could not produce any data to refute the data I linked to. It really made no sense to me why he spent so much time arguing against published data with nothing but personal attacks. He just didn't understand that it wouldn't matter what I had shot before or my personal experience, I was just relaying the data.

Oh, he also claimed that he was firing an AAC 5.56 silencer on a 10" M16 and it filled up with unburned powder and exploded, sending shrapnel into his face. I wouldn't dispute the explosion part, but I have trouble believing enough gunpowder could accumulate AND that the silencer could hold the pressure enough to explode like a pipe bomb. I mean, it's got a hole in the end to vent pressure. I just have trouble imagining that his claims of gunpowder in the silencer caused the explosion.

[Crosshair]

I think most of the issue is a problem with protecting their monetary investment in a particular thing.

"Progress means getting nearer to the place you want to be. And if you have taken a wrong turning, then to go forward does not get you any nearer.
If you are on the wrong road, progress means doing an about-turn and walking back to the right road; and in that case the man who turns back soonest is the most progressive man."
— C.S. Lewis

It's not always easy to admit that you screwed up. It may suck in the short run, but in the long term you find yourself that much better off.

It's fun to watch these debates while I'm busy on the sidelines falling in love with the 22 Hornet. Is it wrong to want to egg them on until they start cracking bolt lugs and blowing primers?

[silencertalk]

These guys will take off a new Ruger barrel and throw it away because it is 1:10 twist and that 'may raise pressure' (it does not in any measurable way) but then buy a barrel which has a 0.0775 throat which *does* raise pressure by about 2000 psi.

[silencertalk]

http://68forums.com/forums/showthread.p ... ger-throat

Constructor (Harrison) believes he invented the concept of using a larger bore area to drop pressure and revolutionized 6.8 performance.

He also seems to believe that the only way to do this is to change the number of grooves and land/groove ratio and does not seem to know you can use any land/groove combination with a larger diameter and end up with the same area. So there is no reason to put down 6 groove barrels in favor of 4 without knowing the area of both.

[Crosshair]

These guys will take off a new Ruger barrel and throw it away because it is 1:10 twist and that 'may raise pressure' (it does not in any measurable way) but then buy a barrel which has a 0.0775 throat which *does* raise pressure by about 2000 psi.

Why would anyone not believe it when you say that twist rate does not affect pressure to any great degree? It's kinda obvious given that in 223 you have everything from 1-7" to 1-14" twist and no manufacture warns not to use a 55 grain FMJ load, designed in a 1-14" barrel, in a 1-7" twist barrel because of high pressure.

These 6.8 people are an interesting bunch.

[silencertalk]

Look back there for what I just posted. There is no way to disagree with it, but they will.

I have a question:

I agree it is well accepted that a tighter throat makes it easier to have an accurate barrel, as it forces the bullet more straight in the cartridge.

I agree it is also well accepted that a shorter throat makes it easier to have an accurate rifle because there is less bullet jump.

Additionally, it is a fact that a shorter throat, as in the SAAMI chamber, raises pressure - 1500 psi seems to be the expected amount.

It is also a fact that a tighter throat raises pressure. We are not sure by how much, but 2000 psi seems likely based on existing data.

Both of these chamber techniques - shorter throat and tighter throat, are done by benchrest and Palma shooters to help them win matches. So it is reasonable to make use of them if you want to optimize for accuracy.

Since either one will raise pressure a similar amount, how come the SAAMI short-throat method is demonized but the 6.8x43 tight-throat method is celebrated by certain pockets of the 6.8 community? I would call it hypocrisy, but I don't think it is - I think it is more of just not realizing either method of trying to gain accuracy has a similar effect on pressure.

If the 6.8x43 0.2775 throat were changed to 0.2781, you could load to about an extra 30-40 fps and have the same pressure.

[mstennes]

Evidently there is no test one needs to pass before being able to post on the internet:

http://68forums.com/forums/showthread.p ... 6.8-bullet

I did not think the concepts were complex - maybe 8th grade level. Maybe I am just out of touch with what I can expect people to comprehend.

Thankfully Todd -K- helps out and backs me up on one of the last pages, but the forum members don't seem able to read charts or drawings.

Maybe you should try a pie chart with colors?