The truth about 6.8 SPC SAAMI vs SPC-II

rsilvers · Jul 15, 2011

The actual difference between 6.8 SPC-II and 6.8 SPC SAAMI is at most about 30 fps.

A number of tests have been done in private by the industry, and I got permission to make one public. It backs up the other data from multiple carefully done studies.

----------------------------------

From: Robert Silvers
Sent: Thursday, July 14, 2011 5:06 PM
To: Johan Loubser
Subject: Re: Western Powders Ballistic lab

Johan,

Have you ever done a test where you take a 6.8 SPC SAAMI test barrel, measure velocity and pressure, then ream the same barrel to 6.8 SPC-II - and add more powder to match the pressure of the previous test, and then see how much extra velocity there is?

Robert.

----------------------------------

Robert,
Yes we did exactly that and the difference was negligible as to be ignored ca 20-30Fps and <1000Psi.
I know there is a lot of claims out there regarding this.
Some apply the effects of other calibers (such as the 223Rem) with completely different expansion ratios to the 6.8SPC.
The 6.98Spc is not sensitive to the slight changes in dimensions between the std and SPEC II.

Regards
Johan Loubser
Ballistician
Ramshot.Accurate.Powders

----------------------------------

Robert,
Example is that with SAAMI reference ammunition we got a difference of Pressure: -2.5%/-1200Psi and -19Fps
If one consider that is equal or less than one standard deviation I consider it negligible.
Regards
Johan Loubser
Ballistician
Ramshot.Accurate.Powders

lwrkeysfisher · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

For the record, I think you should post a disclaimer that you are the project manager for the 300 AAC BLK on each of these troll threads you create.

Move on, nothing to see here folks.

FNARSHOOTER · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

I think he makes it clear ,
rsilvers
ACC R&D
How is data a TROLL THREAD???

JSTARSZ · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

As an avid 6.8mm user/hunter this kind of information does not make me change my mind on the 6.8mm nor would preclude me from trying the 300 AAC Blackout. I am not a slave to a particular round nor do I chase the new and flashy either. If the 300 AAC turns out to be better I will give it a try. In the end of the day for me it comes down to what I can best hunt with.

I think Robert is being up front about who he is so lets get to discussing what this changes about the 6.8mm.

So Robert, what is the point of this thread and where do you want it to go?

In the mean time, my dealer just received my new 30-P1 Thunderbeast and my transfer is about to start. That will be added to my Noveske 10.5", 6.8mm SBR that will be my hunting rig. Once I get it I can start loading sub rounds for it and see how it works. Then I can properly add to this conversation as I am sure we are going to discuss suppressing it at some time?

lwrkeysfisher · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Scot</div><div class="ubbcode-body">I think he makes it clear ,
rsilvers
ACC R&D
How is data a TROLL THREAD??? </div></div>

AAC R&D does not necessarily translate to 300 BLK project manager, direct competitor of 6.8.

Second, this thread calls one piece of data "The Truth" even though other empirical data shows something different. IMO this is simply McDonald's opining about how bad the Whopper tastes.

FNARSHOOTER · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

Project manager, R&D staff what ever. Being as the 6.8 is super sonic and the 300 blackout is both sub and super sonic depending on loading I do not see a competitor, I see 2 different calibers with the 300 being more flexible. I 6.8 has been less then expected from the start .The 6.5 grendel kicked its ass from the start. I still say data is not TROLLING. If I had to pick I would say NONE of the above. 5.56 not enough?Go 308,260 7-08 ect.
Subsonic go 45acp nuff said

MLC · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

The 300 BLK is the Libertarian party of the two party 6.8 vs 6.5 political BS that's hogged bandwidth for the past 10 years.

rsilvers · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

This is a pet topic of mine, and here are some posts from me a year ago to prove I was on this before 300 BLK:

http://www.m4carbine.net/showthread.php?p=709029

People say twist plays a significant role in velocity and pressure, but it does not, and it is a gun myth that won't die. Twist is an accuracy and stability issue. The military has verified that going from 1:12 to 1:7 twist in 5.56mm does not significantly raise pressure or lower velocity. The difference is not even easily measurable. Another govt agency verified that 1:12 vs 1:8 twist in 308 makes no difference for velocity. So to say that if only 6.8 were 1:11 twist instead of 1:10 that ammo makers could sell hotter ammo - that is ludicrous.

I believe 6.8 SPC-II is better than 6.8 SAAMI (I even chose the extra throat for 300 BLK) - but when I see people posting that there is a 200-300 fps difference, they are misleading others. The actual data is a 20-30 fps difference. This is not just from Western Powders, but, look at this quote from DocGKR:

"The information Mr. Silvers has posted is the factual truth. Every professional ballistics facility, including USG, foreign military, and Industry, that I am aware of that has tested 6.8 SPC vs. 6.8 SPCII using industry standard equipment has reported only a 1000-1500 psi change."

So how do I explain why some handloaders can get 200-300 fps more in their 6.8 SPC-II gun than their SAAMI gun? Probably their SAAMI gun was made out of spec (headspace too low, chamber below min, bore cross-sectional area below min) and their SPC-II gun has a generous bore and chamber. So these experiences are real, but not in conflict with the data from Western Powders. Moreoever, they are rifle to rifle variation, and could never be codified into an upgraded standard.

So what does this mean? To me it means rifle makers should chamber their rifles in 6.8 SPC-II (I would). BUT.... They should be free to do it in 1:10 twist if they want without being bashed! Faster twist is good for short barrels and also makes some types of ammo expand better.

If you like 1:11 twist for 6.8, you should like 1:9 twist for 5.56mm - as for the typical range of bullets used in each, that is how it maps out. Nothing wrong with that, but NOTHING is wrong with 1:10 twist or even 1:9 twist for 6.8 any more than 1:7 or 1:8 twist is bad for 5.56mm

And specialty ammo companies (Wilson, Double Tap, SSA, etc) should continue to make dedicated 6.8 SPC-II ammo while larger ammo companies should continue to make SAAMI ammo, but optimize their loads to be as fast as possible. If you can get 2700 fps in 6.8 SPC-II, you can get 2670 fps in 6.8 SAAMI - assuming both are test barrels made to minimum chamber and bore dimensions.

fdkay · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

That's nice.

While you invoke the .223/5.56 similarity you do NOT speak of the pressure differences that those two cartidges are loaded to, which is substantial.

The difference of the chambers allows them to be loaded to much higher pressures, which DOES have an effect on velocity. Duh.

Smoke and mirrors.

Just as a 38+p is capable of higher velocity than standard .38 special, so too is the AMMUNITION used in chambers specifically designed for higher pressures.

This is why it is NOT SAFE to fire the SSA combat loads in a SAAMI spec chamber.

But thanks very much for telling us that two identical cases, loaded to identical pressures fired in different chambers equals nearly the same velocity.

Although I will point out, that even in that instance, you get higher velocity out of the SPC II chamber.

By the way, you 6.5 grendel fans, chop that barrel to 14" and see how it compares to the 6.8 out of the same barrel length.

The 6.5 does not kick the 6.8's ass and never has. They are two distinctly different cartridges.
If you were to take the 6.8 and put it in a 20" barreled weapon firing the 110 grain accubond SSA combat loads and a 6.5 grendel shortened to 20", they even out quite nicely. Does the 6.5 still perform better at long ranges? Sure, but not by much.

Try feeding a 6.5 out of a full auto, doesn't work to well.

rsilvers · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fdkay</div><div class="ubbcode-body">This is why it is NOT SAFE to fire the SSA combat loads in a SAAMI spec chamber. </div></div>

Right. Likewise, it is not a good idea to shoot 5.56mm in a 223 chamber, or 300 AAC BLACKOUT in a Whisper(R) chamber.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fdkay</div><div class="ubbcode-body">But thanks very much for telling us that two identical cases, loaded to identical pressures fired in different chambers equals nearly the same velocity. </div></div>

Pressure and velocity have a direct relationship. The load will drop pressure in the SPC-II chamber, and so you need to add powder to it to get the pressure back up to where it was before. Once you do this, you have what is called 6.8 SPC-II ammo. This ammo will then be 20-30 fps faster than the previous ammo in the SAAMI chamber. This is the test which has been done multiple times with the same results.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fdkay</div><div class="ubbcode-body">Although I will point out, that even in that instance, you get higher velocity out of the SPC II chamber. </div></div>

The opposite is true - a given load will be faster in the tighter chamber because more pressure equals more velocity.

fdkay · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

You are correct, I mis-read to a certain extent.

Are you suggesting then that the SSA combat loads deliver the same velocity as the standard pressure loads?

The revised chamber allows a higher pressure, which allowa higher velocity.

Who cares is equal pressure loads deliver similar velocities.
That is not the point of the SPC-II chamber.

The REAL truth is that there are at least two distincly different chamber dimensions, the SAAMI spec and the SPC-II, much like the .223 vs 5.56 chambers.

This statement is at the least, very mis-leading, at worst, patently FALSE.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">The actual difference between 6.8 SPC-II and 6.8 SPC SAAMI is at most about 30 fps.
</div></div>

Ammunition designed to be used in the improved chamber at HIGHER than SAAMI spec pressures will have a significantly higher velocity.

rsilvers · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Are you suggesting then that the SSA combat loads deliver the same velocity as the standard pressure loads?</div></div>

No, Combat loads are faster. I am not sure why you asked though.

Ok, sorry - I will be more clear on the other point. The actual difference between 6.8 SPC-II and 6.8 SPC SAAMI, when each are loaded to 55,000 psi, is at most about 30 fps.

That does not change my point - a handloader can load either one to above 55,000 psi if they choose.

fdkay · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

I was pointing out that in your initial post, you insinuste that there is no real difference in velocity.

So, we agree.

1) The combat loads ARE loaded to higher pressure.

2) The combat loads ARE faster.

3) Firing the combat loads in a SAAMI spec chamber is unsafe.

4) Firing the combat loads in s SPC II chamber is perfectly safe.

5) The design of the SPC II chamber allows loads the use of higher pressure safely.

So this leads me to the conclusion that they are NOT equal.
Just as the .223 and 5.56 are not equal.

You, sir, are obfuscating.

No one here would expect to shoot a .223 pressure load out of a 5.56 chamber and expect 5.56 velocities, just as no one expects to shoot .38 special loads in a .357 magnum revolver and get .357 velocities.

jamesb74 · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

As stated in the other thread you started in another forum. Your data is correct for the ammunition used. The point is ammunition can be made to better standards and higher velocity with better results with an spc 2 chamber then with a SAAMMI chamber.

Yes the 115 fmj super uber ammo not available to the public will perform the same in both chambers. But what if you added another grain to it or another grain and a half. It would still perform beautifully in the spc 2 with better statistics and would blow primers in the SAAMMI chamber.

try shooting out side of a laboratory sometime it might help a little. IMHO.

rcw3 · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

rsilvers

I am not sure I am entirely clear what the differences are between the 6.8 SPC and the 6.8 SPC II and would appreciate your commentary or help on this (or others as well).

I have a SAAMI reamer drawing of the 6.8 SPC and one purported to be the 6.8 SPC II. It looks like the II has a .030" longer free bore and the same throat angle of one and a half degrees. The II has a minutely tighter and tapered free bore, and the II also has a little tighter neck and a 35 degree chamfer at the end of the case mouth chamber area vs a 45 degree chamfer of the SAAMI. On the reamer drawings I have that seems to be it.

Do I have things correct or is my II drawing not actually a II?

As a commentary, if these are the only differences in the chambers, there is no way a major velocity difference would be logical with one chamber vs the other and your 30 fps suggestion seems more in order.

rsilvers · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fdkay</div><div class="ubbcode-body">I was pointing out that in your initial post, you insinuste that there is no real difference in velocity.

So, we agree.

1) The combat loads ARE loaded to higher pressure.

2) The combat loads ARE faster.

3) Firing the combat loads in a SAAMI spec chamber is unsafe.

4) Firing the combat loads in s SPC II chamber is perfectly safe.

5) The design of the SPC II chamber allows loads the use of higher pressure safely.

So this leads me to the conclusion that they are NOT equal.
Just as the .223 and 5.56 are not equal.

You, sir, are obfuscating.

No one here would expect to shoot a .223 pressure load out of a 5.56 chamber and expect 5.56 velocities, just as no one expects to shoot .38 special loads in a .357 magnum revolver and get .357 velocities.</div></div>

1. Yes.

2. Yes.

3. I don't know. I would expect 1500 extra psi.

4. I don't know.

5. Yes, by 1500 psi.

You are assuming SSA Combat loads are what I would consider normal 6.8 SPC-II ammo. I can't determine that without knowing the OAL and pressure and I know neither.

rsilvers · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rcw3</div><div class="ubbcode-body">rsilvers
I am not sure I am entirely clear what the differences are between the 6.8 SPC and the 6.8 SPC II and would appreciate your commentary or help on this (or others as well). </div></div>

PTG can supply you with reamer drawings.

rcw3 · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

rsilvers

If you could comment on my posting above I would be most interested in anything you can provide info wise.

rcw3 · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rcw3</div><div class="ubbcode-body">rsilvers
I am not sure I am entirely clear what the differences are between the 6.8 SPC and the 6.8 SPC II and would appreciate your commentary or help on this (or others as well). </div></div>

PTG can supply you with reamer drawings. </div></div>

That's an easy way to side step things.

branson1369 · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

Damn, I had to use the dictonary on this one "obfuscating". I am a shooter these big words albiet very good one are tough

rsilvers · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jamesb74</div><div class="ubbcode-body">Yes the 115 fmj super uber ammo not available to the public will perform the same in both chambers. But what if you added another grain to it or another grain and a half. It would still perform beautifully in the spc 2 with better statistics and would blow primers in the SAAMMI chamber.

try shooting out side of a laboratory sometime it might help a little. IMHO. </div></div>

The problem with shooting outside of a laboratory is that with any given rifle, you don't know if the chamber and bore cross-sectional area is at min or max (or even below min or above max).

With a pressure test barrel, they are blueprinted. When a reamer comes in, it is put on a giant optical comparator and verified to the drawing.

I don't doubt your real world experiences at all, but there is no way to codify that into a standard. Maybe you have a barrel with a tapered bore from max to min cross-sectional area, but do you really want the 6.8 SPC-II standard to require that? If you do then there is no accounting for tool wear during production.

Do you want the 6.8 SPC-II formal standard to allow for 60,000 psi? That will raise the proof round pressure to 78,500 psi. Can everyone's brass handle that? I don't think so.

Do you want the OAL changed from 2.26 to 2.29? If so, you will never see aluminum or plastic mags in the future. Do you want that? For velocity yes, but I don't think it would be a good idea in terms of platform flexibility.

To fdkay's point - I was not accounting for if some people are loading 6.8 SPC-II to over 55,000 psi. I was only comparing chamber differences. If you load 6.8 SPC-II to 60,000 psi or more, then yes - for sure - the velocity will be more than 30 fps difference.

rcw3 · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

Open question to all posters on the difference between the performance of the 6.8 SPC and the 6.8 SPC II - - -

Can any of you explain the exact chamber differences between the chambers you tested?

If not, how is anyone to accept the supposed test results as being indicative of anything?

I posed the query of chamber differences in my first posting on this thread and so far no response.

Frankly if you are going to do performance testing on chamber differences and have meaningful results, you really should have certified reamers for your chambers and know exactly what you have for chambers, so it should not be a problem to discuss them.

fdkay · Jul 16, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rcw3</div><div class="ubbcode-body">Open question to all posters on the difference between the performance of the 6.8 SPC and the 6.8 SPC II - - -

Can any of you explain the exact chamber differences between the chambers you tested?

If not, how is anyone to accept the supposed test results as being indicative of anything?

I posed the query of chamber differences in my first posting on this thread and so far no response.

Frankly if you are going to do performance testing on chamber differences and have meaningful results, you really should have certified reamers for your chambers and know exactly what you have for chambers, so it should not be a problem to discuss them.
</div></div>

I can not speak to the accuracy or validity of these figures or statements:
http://www.frfrogspad.com/68chambers.htm

bribri · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">For the record, I think you should post a disclaimer that you are the project manager for the 300 AAC BLK on each of these troll threads you create.

Move on, nothing to see here folks. </div></div>

He also posted this exact same thread on 68forums.com and it really stirred shit up, and the thread is now locked.

RUTGERS95 · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bribri</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">For the record, I think you should post a disclaimer that you are the project manager for the 300 AAC BLK on each of these troll threads you create.

Move on, nothing to see here folks. </div></div>

He also posted this exact same thread on 68forums.com and it really stirred shit up, and the thread is now locked.

He's just upset, because the 6.8spcII outperforms the 300 blackout in accuracy and ballistics. </div></div>

That thread was locked but it was continued in another thread. It's at 13 pages now

346ci · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RUTGERS95</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bribri</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">For the record, I think you should post a disclaimer that you are the project manager for the 300 AAC BLK on each of these troll threads you create.

Move on, nothing to see here folks. </div></div>

He also posted this exact same thread on 68forums.com and it really stirred shit up, and the thread is now locked.

He's just upset, because the 6.8spcII outperforms the 300 blackout in accuracy and ballistics. </div></div>

That thread was locked but it was continued in another thread. It's at 13 pages now </div></div>

13 pages full of corrected info, unlike the OP, the guys on 6.8F know the round.

rsilvers · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

My post was not my data, but from Western Powders ballistics lab.

Vindex · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

Not sure what the point of the post is.

Did you say you were using SAMMI pressures for shots out of a SAMMI and a SPC II chamber? If so, it is not surprising the velocities aren't much different. And you are actually providing support for the need for chamber differences.

The SAMMI chamber will almost always show higher pressure readings than the same round (same powder charge, primers, case) than the same round in a SPC II chamber. That is why is is not safe to shoot "combat loads" in a SAMMI chamber, but they are perfectly safe to shoot in a SPC II or DMR chamber.

And let's not loose sight of how and why the 6.8 was developed. The goal was to use a design that allowed optimal use of existing M16 parts. It was developed by the 5th Special Forces Group on a shoestring budget. And its performance is VERY close to the round that the 5.56 beat out for the new NATO round - the .280 British. It was designed to improve on the terminal performance of a combat round over the 5.56 in combat conditions. The optimal round for kills on man sized targets. Velocity of around 2700 or more fps, with a .277 size bullet weighing between 85 and 120 grains appears to be that sweet spot (see Doc GKR's research).

The 300 blackout, at 7.62 x 40, is basically a tweaked and improved AK 7.62 x 39 cartridge. Add another 11 mm to length and you have a .308.

Are we trying to compare the 300 BO to the 6.8?

RUTGERS95 · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 346ci</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RUTGERS95</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bribri</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">For the record, I think you should post a disclaimer that you are the project manager for the 300 AAC BLK on each of these troll threads you create.

Move on, nothing to see here folks. </div></div>

He also posted this exact same thread on 68forums.com and it really stirred shit up, and the thread is now locked.

He's just upset, because the 6.8spcII outperforms the 300 blackout in accuracy and ballistics. </div></div>

That thread was locked but it was continued in another thread. It's at 13 pages now </div></div>

13 pages full of corrected info, unlike the OP, the guys on 6.8F know the round.

</div></div>

amen brother.....

rsilvers · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

I got an email from Art of SSA after he read the locked thread on 68forums.

Art said that he has a lot of 6.8 rifles, and he has noticed some of them cannot handle 50,000 psi, while others can handle 55,000+ psi. This is consistant with what the 'real world results' people are saying - that when you look at rifles in the wild from different makers, there is a difference of up to 5,000 psi.

He also said that he cannot disagree with my statement that there is only a 1000-1500 psi difference and less than a 30 fps increase comparing SAAMI vs Spec-II chambers in a 24 inch test barrel.

bustin · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">This is a pet topic of mine, and here are some posts from me a year ago to prove I was on this before 300 BLK:

http://www.m4carbine.net/showthread.php?p=709029

People say twist plays a significant role in velocity and pressure, but it does not, and it is a gun myth that won't die. Twist is an accuracy and stability issue. The military has verified that going from 1:12 to 1:7 twist in 5.56mm does not significantly raise pressure or lower velocity. The difference is not even easily measurable. Another govt agency verified that 1:12 vs 1:8 twist in 308 makes no difference for velocity. So to say that if only 6.8 were 1:11 twist instead of 1:10 that ammo makers could sell hotter ammo - that is ludicrous.

I believe 6.8 SPC-II is better than 6.8 SAAMI (I even chose the extra throat for 300 BLK) - but when I see people posting that there is a 200-300 fps difference, they are misleading others. The actual data is a 20-30 fps difference. This is not just from Western Powders, but, look at this quote from DocGKR:

</div></div>
Slivers, you still don't get it. It isn't just the twist, it is everything together that makes as much as 400fps difference. Chamber difference, twist, bore to groove ratio which changes bore area, the rifling design, using small rifle primers in SSA or Hornady brass instead of large primers in Remington brass and loading the cartridge longer all plays a part in increasing performance. At one point in time 6 years ago Remington ammo was producing 2350-2400fps and blowing primers in some rifles especially when subjected to extreme heat like in IRAQ. Now with all of the changes to the barrel and chamber designs and using SSA or Hornady brass handloaders can get 2800fps out of 16" barrels and not blow primers.
The mil isn't going to do anything with the 6.8 but hunters can load their own and use the extra performance if that is what they choose to do.

rsilvers · Jul 17, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

Bustin - I agree with all that - chamber, bore cross sectional area, rifling design, brass, and OAL combined can add 200 or more fps.

What I think is going on is that there are a bunch of 1:10 twist barrels out there with cross-sectional areas below SAAMI min, causing high pressure, and therefore not doing as well as other barrels in other twists. It does not mean 1:10 is the problem, but people associate it with that. But if you made a barrel of a larger cross sectional area in 1:10 twist, it would not have higher pressure. It would not effect a rifle from Ruger, for example, who hammer forges their own barrels.

I actually saw a guy post on 68forums that he got a new Ruger in 6.8 and plans to scrap the barrel because it has the wrong specs. I suggested he ream it to SPC-II.

RUTGERS95 · Jul 18, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">Bustin - I agree with all that - chamber, bore cross sectional area, rifling design, brass, and OAL combined can add 200 or more fps.

What I think is going on is that there are a bunch of 1:10 twist barrels out there with cross-sectional areas below SAAMI min, causing high pressure, and therefore not doing as well as other barrels in other twists. It does not mean 1:10 is the problem, but people associate it with that. But if you made a barrel of a larger cross sectional area in 1:10 twist, it would not have higher pressure. It would not effect a rifle from Ruger, for example, who hammer forges their own barrels.

I actually saw a guy post on 68forums that he got a new Ruger in 6.8 and plans to scrap the barrel because it has the wrong specs. I suggested he ream it to SPC-II.

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this I agree with 100%. the problem with the twist argument is that it's been propagated for several years now and you have people babbling about it without understanding rifling, materials in barrel construction, chamber specs, barrel length (in 6.8 this is very important as testing shows sub 16" you are bette off with 1/10 vs 1/11+) etc.... so much goes into the pressure issue that one variable is hardly the cause or remedy.

bustin · Jul 18, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

exactly, it is people only remembering the part they understand and repeating it without the rest of the specifics.

rsilvers · Jul 18, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

One can also radius the edges or not, and it all has some effect. The effect of engraving force is probably greater for solid copper bullets.

Production rifle bores need to be allowed to vary from max to min - and a max rifle made on a Monday will differ from a min rifle made on a Friday - even for the same model rifle. Typically, a hammer forging mandrel will be designed to start making barrels at SAAMI max and then they would discard it when the barrels reach SAAMI min.

Because barrels from any given company can range from min to max on the bore and chamber, big ammo companies just develop ammo for the smallest bore cross-sectional area. In other words, their test barrels have minimum chambers and minimum bores, as then they allow for worst case.

So factory ammo, even to a new spec, will never be as hot as handloads just because of the rifle-to-rifle variation.

As for adopting a higher psi as standard, with 55,000 psi max average pressure, the proof rounds are already up to 78,500 psi. If 60,000 psi were adopted, 16 SAAMI companies would have to vote on if they wanted to allow proof rounds up to 85,500 psi. I can't see that happening. 270 Win is high pressure, but that is a bolt-action round.

Whichever company sponsors it is responsible for providing reference, proof, and dummy rounds to everyone else.

rsilvers · Jul 25, 2011

Re: The truth about 6.8 SPC SAAMI vs SPC-II

Here is something interesting:

http://68forums.com/forums/showthread.php?18458-6.8-chambers-SAAMI-SPCII-DMR-6.8x43-Noveske-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/Pressure_Trials_Consortium.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.

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The truth about 6.8 SPC SAAMI vs SPC-II

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