6.8 Spc, or 6.8 Spc II, or ?

K

KaiserNorton

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Minuteman
Aug 6, 2009
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Crete, ND to the right of The Duck Factory
#1
I have half an interest in swapping one of my AR's over to the 6.8 SPC cartridge, and in doing some research, note that there are three different chamber specs for this round (according to Wikipedia) - 6.8 SPC, 6.8 SPC II, and 6.8x43(DMR-C).

I have heard of the SPC II, but not the third variation. Can someone enlighten me on their differences to the SAAMI spec SPC? My understanding is that the "improvements" were necessary to accomodate the increased performance (and pressures) of new load developments. What of the bolt guns that were/are being chambered for this cartridge - are they unable to utilize this enhanced performance?

The Kaiser
 
#3
Re: 6.8 Spc, or 6.8 Spc II, or ?

Just quick- in late 05 there was some ammo on the market that was hot and faster than most, it caused issues with the SAAMI chamber because of a .050 leade/throat. Since some of us wanted to see the 6.8 adopted by the mil, the hot ammo increased the performance so we saw that as a good thing and designed chambers to work with the hot ammo. The SPC II and 6.8x43 (ARP) chambers allow loading to faster velocities to increase the performance by as much as 200fps with some bullets and loads. 11 and 12 twist barrels are better than the 10 and 9.5 twist since the 12 twist will stabilize any bullet capable of being loaded to mag length, the Sierra 130 SPBT being just about the limit to load to mag length.
 
#4
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">11 and 12 twist barrels are better than the 10 and 9.5 twist since the 12 twist will stabilize any bullet capable of being loaded to mag length, the Sierra 130 SPBT being just about the limit to load to mag length. </div></div>

That is benchrest school-of-thought... use the least twist that works under typical conditions. Combat rifles - a bit more twist than the minimal is popular. For example, in 5.56mm - most people accept that 1:9 is going to work for most people, but they still want 1:7.

Aside from the extra margin of stability, there is evidence that faster twist increases terminal ballistics with certain expanding bullets.

I see 1:9, 1:10, or 1:11 as all reasonable twists for 6.8, depending on your barrel length and the types of bullets you tend to shoot.

If you like 1:7 twist 5.56mm rifles, then get a 1:9 twist 6.8 SPC.
If you like 1:8 twist 5.56mm rifles, then get a 1:10 twist 6.8 SPC.
If you like 1:9 twist 5.56mm rifles, then get a 1:11 twist 6.8 SPC.

 
#5
Re: 6.8 Spc, or 6.8 Spc II, or ?

An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.

Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc.
 
#6
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.

Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc. </div></div>

Aren't you having enough trouble with having to modify 5.56 mags to get your Blackout to work and the bad accuracy with supersonic loads to worry about how we designed our chambers that have been working for over 4 years?

12 twist 6.8 barrels will stabilize a 130gr bullet, longest bullet usable in a 6.8 mag.
A 9 twist 5.56 will not stabilize 80 gr 5.56 bullets or some will not stabilize 75s and 77s so there is a reason to use 8 twist barrels in a 5.56, there is no reason to use a 8 twist or 9 or 10 twist in a 6.8 unless someone like you wants to try to shoot a 150gr bullet from a 6.8 which would be retarded. I hate to tell you just because you owned a suppressor company doesn't mean even 10% of the public is interested in shooting sub-sonic. The 6.8 is mostly a hunting rifle now and sub sonic loads don't work for shit compared to the terminal performance of super sonic loads.
 
#7
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.

Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc. </div></div>

With more freebore you can handload more powder in the case because you can load the bullet out longer(2.3" OAL in PRI mags) without jamming the lands which gives you more room in the case, when you add more powder in the case you get more velocity from it.
 
#8
Re: 6.8 Spc, or 6.8 Spc II, or ?

I don't know why there is a competition brewing between these two rounds, as they are completely different; and either will serve its purpose well. From the data I've seen, and my experience 200 fps/ even a little more is possible with handloads in 11 or 12 twist SPC II or DMR chamber over SPC w/1 in 10. The 30fps difference applies changing the twist only, though LWRC nearly matches the difference using their poly 1 in 10 twist barrels. IMO the SPCII leade is too long, and you give up too much accuracy for not much pressure gain over the 6.8X43 or Noveske Mod1 chamber--consider them the Wylde chambers of the 6.8 world.
 
#9
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">From the data I've seen, and my experience 200 fps/ even a little more is possible with handloads in 11 or 12 twist SPC II or DMR chamber over SPC w/1 in 10. The 30fps difference applies changing the twist only, though LWRC nearly matches the difference using their poly 1 in 10 twist barrels. IMO the SPCII leade is too long, and you give up too much accuracy for not much pressure gain over the 6.8X43 or Noveske Mod1 chamber--consider them the Wylde chambers of the 6.8 world. </div></div>

No, twist will make about a 3 fps difference, which is not significant at all. The concept that changing from 1:10 to 1:11 makes a noticeable difference in velocity potential is a myth perpetuated by misleading pseudo-science tests.
 
#10
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.

Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc. </div></div>

With more freebore you can handload more powder in the case because you can load the bullet out longer(2.3" OAL in PRI mags) without jamming the lands which gives you more room in the case, when you add more powder in the case you get more velocity from it. </div></div>

I agree, but not 200 fps if everything else stays the same (for example, if you match bore cross-sectional area, chamber body diameter, and freebore diameter). Much less.
 
#11
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.

Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc. </div></div>

Aren't you having enough trouble with having to modify 5.56 mags to get your Blackout to work and the bad accuracy with supersonic loads to worry about how we designed our chambers that have been working for over 4 years?</div></div>

The only round which I have developed that requires the mag to be modified is the 175 grain 330 Major PF 3-gun load. Otherwise, I like the 110-125 grain bullets - as they are both higher velocity and are proving the most accurate.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">12 twist 6.8 barrels will stabilize a 130gr bullet, longest bullet usable in a 6.8 mag.</div></div>

The neck on a 6.8 SPC case is 0.273 long. The Sierra 150 #1840 (1.250 long), when loaded to 2.283 OAL, is seated 0.657 deep. Which means it is protruding 0.384 into the case body.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">A 9 twist 5.56 will not stabilize 80 gr 5.56 bullets or some will not stabilize 75s and 77s so there is a reason to use 8 twist barrels in a 5.56, there is no reason to use a 8 twist or 9 or 10 twist in a 6.8 unless someone like you wants to try to shoot a 150gr bullet from a 6.8 which would be retarded. </div></div>

A 223 caliber Sierra 80 grain bullet is 1.090 long. When loaded in 5.56mm to 2.260 OAL, it protrudes 0.589 into the case. Since the neck is 0.203 long, the bullet is going past the neck into the body by 0.386 inches - MORE than the 150 grain 6.8 bullet!

So that raises the question - why do you consider it 'retarded' to go 0.384" into a 6.8 SPC body but important to be able to go 0.386" into a 5.56mm case?

An 80 grain 0.224 bullet is as long as one would ever want to use, and makes about as much sense as a 150 grain 0.277 bullet - also the longest one would ever want to use. They both have the same limiting disadvantage of starting to protrude into the case body by pretty much exactly the same amount.

Now let's calculate the Miller stability for the 80 grain in 5.56mm and 1:7 twist as the baseline:

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

80 grain Sierra 0.224: 1.090 OAL.
Lowest velocity it would be expected to go: 2400 fps.
Cold temp 0 degrees F.
Twist: 1:7

Result: Stability is 1.65.

Now we ask.... what twist in 6.8 SPC would give that same stability factor with a Sierra 150?

150 grain Sierra 0.277: 1.250 OAL.
Lowest velocity it would be expected to go: 2100 fps.
Cold temp 0 degrees F.

Twist: 1:7 - stability 2.77
Twist: 1:8 - stability 2.12
**Twist: 1:9 - stability 1.68 **
Twist: 1:10 - stability 1.36
Twist: 1:11 - stability 1.12
Twist: 1:12 - stability 0.94

Bingo - we have a match. As you can see, if you like 1:7 twist for 5.56mm, then the closest to that in 6.8 SPC is 1:9 twist.

Likewise, 1:11 twist for 6.8 SPC is about like a 1:9 twist 223 AR. Not that there is anything wrong with that for most people/most uses... just less versatile.
 
#13
Re: 6.8 Spc, or 6.8 Spc II, or ?

<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: bustin</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.

Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc. </div></div>

Aren't you having enough trouble with having to modify 5.56 mags to get your Blackout to work and the bad accuracy with supersonic loads to worry about how we designed our chambers that have been working for over 4 years?</div></div>

The only round which I have developed that requires the mag to be modified is the 175 grain 330 Major PF 3-gun load. Otherwise, I like the 110-125 grain bullets - as they are both higher velocity and are proving the most accurate.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">12 twist 6.8 barrels will stabilize a 130gr bullet, longest bullet usable in a 6.8 mag.</div></div>

The neck on a 6.8 SPC case is 0.273 long. The Sierra 150 #1840 (1.250 long), when loaded to 2.283 OAL, is seated 0.657 deep. Which means it is protruding 0.384 into the case body.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">A 9 twist 5.56 will not stabilize 80 gr 5.56 bullets or some will not stabilize 75s and 77s so there is a reason to use 8 twist barrels in a 5.56, there is no reason to use a 8 twist or 9 or 10 twist in a 6.8 unless someone like you wants to try to shoot a 150gr bullet from a 6.8 which would be retarded. </div></div>

A 223 caliber Sierra 80 grain bullet is 1.090 long. When loaded in 5.56mm to 2.260 OAL, it protrudes 0.589 into the case. Since the neck is 0.203 long, the bullet is going past the neck into the body by 0.386 inches - MORE than the 150 grain 6.8 bullet!

So that raises the question - why do you consider it 'retarded' to go 0.384" into a 6.8 SPC body but important to be able to go 0.386" into a 5.56mm case?

An 80 grain 0.224 bullet is as long as one would ever want to use, and makes about as much sense as a 150 grain 0.277 bullet - also the longest one would ever want to use. They both have the same limiting disadvantage of starting to protrude into the case body by pretty much exactly the same amount.

Now let's calculate the Miller stability for the 80 grain in 5.56mm and 1:7 twist as the baseline:

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

80 grain Sierra 0.224: 1.090 OAL.
Lowest velocity it would be expected to go: 2400 fps.
Cold temp 0 degrees F.
Twist: 1:7

Result: Stability is 1.65.

Now we ask.... what twist in 6.8 SPC would give that same stability factor with a Sierra 150?

150 grain Sierra 0.277: 1.250 OAL.
Lowest velocity it would be expected to go: 2100 fps.
Cold temp 0 degrees F.

Twist: 1:7 - stability 2.77
Twist: 1:8 - stability 2.12
**Twist: 1:9 - stability 1.68 **
Twist: 1:10 - stability 1.36
Twist: 1:11 - stability 1.12
Twist: 1:12 - stability 0.94

Bingo - we have a match. As you can see, if you like 1:7 twist for 5.56mm, then the closest to that in 6.8 SPC is 1:9 twist.

Likewise, 1:11 twist for 6.8 SPC is about like a 1:9 twist 223 AR. Not that there is anything wrong with that for most people/most uses... just less versatile.
</div></div>
Do you know what an ogive is? Do you typically stuff a bullet in the case with the PT back behind the case mouth? Well most don't, if you want to shoot 150s in a 6.8 at 2000fps go right ahead it will have better terminal performance than the 300 does but no one out of the 12,000 members on 6.8forums shoots 150s in a 6.8 except Chris and another guy trying to get sub loads to cycle, none of them think subsonic loads are the right thing to use to hunt hogs or deer when super sonic loads work so much better. A 150gr .270 bullet was designed for the 270, designed to expand at much higher velocities than a 6.8 can push a 150 so only an idiot would use a 150gr bullet in a 6.8. As far as your perfect twist thoughts, if you think a 9 twist SAAMi chambered 6.8 will handle 30.5gr of 10X with a 110gr bullet with no pressure signs and it is usable as a normal loads then just let me know I'd like to be there when you pull the trigger.

Your 300 may be a fun gun for tacticool guys to go out and play with but anyone that knows anything about terminal performance is not going to use subsonic loads too hunt with. Will it kill? sure but so will a 22LR.
 
#14
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">
A 223 caliber Sierra 80 grain bullet is 1.090 long. When loaded in 5.56mm to 2.260 OAL, it protrudes 0.589 into the case. Since the neck is 0.203 long, the bullet is going past the neck into the body by 0.386 inches - MORE than the 150 grain 6.8 bullet!

So that raises the question - why do you consider it 'retarded' to go 0.384" into a 6.8 SPC body but important to be able to go 0.386" into a 5.56mm case?

An 80 grain 0.224 bullet is as long as one would ever want to use, and makes about as much sense as a 150 grain 0.277 bullet - also the longest one would ever want to use. They both have the same limiting disadvantage of starting to protrude into the case body by pretty much exactly the same amount.

</div></div>

Only you would think of it like that, I don't load bullets with the PT/Ogive back behind the case mouth, you know the same reason you can't shoot the 75 Amax or 90 VLDs from the mag. Do you know why the BRs and PPCs are so accurate? because they use short fat case bodies with the bullets loaded out long with the boattail up above the donut in the case, the bullet body in the leade of the chamber aligns the bullet better in the bore. Of course Lapua brass and small primers help but that came after the parent cartridges were developed.
You on the other hand like to load 220gr bullets with half of the length of the bullet stuffed down in the case taking up half of the case capacity.
 
#15
Re: 6.8 Spc, or 6.8 Spc II, or ?

Kaiser,

The 6.8 is a good/accurate round that very performs well out of short barrel and is very reliable. If I was looking at adding another to my fold I would never consider the original SPS chamber and I wouldn't be looking at the DMR either . I would focus on the SPC-II or 6.8x43 chamber in 11 or better twist. The 10 twist would only be an option for something is the less than 12" barrel length for me.
 
#16
Re: 6.8 Spc, or 6.8 Spc II, or ?

Even if the Sierra 150 is not optimal, someone who wants to make the 6.8 compete against the 6.5 for long range shooting might want a high-BC heavy bullet.

How many 223 owners do you think shoot 80 grain bullets? You should rally against 1:7 twist for 5.56mm rifles.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">if you think a 9 twist SAAMi chambered 6.8 will handle 30.5gr of 10X with a 110gr bullet with no pressure signs and it is usable as a normal loads then just let me know I'd like to be there when you pull the trigger.
bustin said:
</div></div>

Strawman argument.

http://en.wikipedia.org/wiki/Straw_man

I never said a SAMMI chamber could handle as much pressure as an SPC-II chamber. What I said was that going from 1:11 to 1:10 did not play a significant role in pressure.
Click to expand...
 
#17
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tbag</div><div class="ubbcode-body"> in 11 or better twist. The 10 twist would only be an option for something is the less than 12" barrel length for me. </div></div>

Why, what don't you like about 1:10 twist?

Even for say, a 110 Accubond:

110 Nosler AccuBond at 2400 fps and 0 degrees F. 1.105 long.

1:10 twist 1.49 stability. Nice.
1:11 twist 1.23 stability. Marginal.
1:12 twist 1.03 stability. Not ok.
 
#18
Re: 6.8 Spc, or 6.8 Spc II, or ?

I think what has happened is that long-range shooters have migrated to the 6.5 Grendel, so the 6.8 people just want the short/light bullets for hunting.

Meaning, there is less demand for long range wind-bucking bullets in 6.8 than in other calibers.

That being said, slow twist does give up the potential advantage of helping with terminal effects with certain expanding bullets.
 
#20
Re: 6.8 Spc, or 6.8 Spc II, or ?

No, the GS 105 SP, 120 Barnes for target and 130 Sierra are used. The 110TTSX is longer but not many use it for hunting because it doesn't fit in the mag.
Anything over 130 is just too slow for hunting or target.

We were pushing the 6.8 for mil adoption, that's over so now it has become a short maneuverable rifle for deer and hog hunters and maybe some who welcome the Zombie invasion.
It has never been a long range cartridge although it will hold it's own out to around 400yds only having a few inches difference in trajectory from the Grendel/264 LBC because it will push same weight bullets faster than the G can out of the same length barrel.
The little 85gr Barnes TSX at over 3100fps will pass through hogs end to end, same with the 80gr GS HV which we have had upto 3380fps out of a 20" barrel, that is as fast as 243 velocities.

Strawman argument.

http://en.wikipedia.org/wiki/Straw_man

BTW I don't pay much attention to formulas made by some lab rat, I shoot bullets for accuracy in the field to determine if the combination of twist, rifling design and loads is capable of sub-moa, if it isn't we don't use it.
 
#21
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">BTW I don't pay much attention to formulas made by some lab rat, I shoot bullets for accuracy in the field to determine if the combination of twist, rifling design and loads is capable of sub-moa, if it isn't we don't use it. </div></div>

To do real-world testing it should be (I feel for a combat weapon) in a worst-case environment - it both has to be at a very cold temp (-40F, -20F, 0F - or whatever the coldest that it may be shot at) and with the slowest velocity any individual bullet may go - and use Doppler radar to track the bullet orientations near the muzzle (as bullets get more stable down-range). And if you use a sound suppressor, it is wise to not go with too slow a twist as the bullets are just leaving the bore where they are least stable.

To find the slowest velocity a bullet may go, you can shoot the weakest load for commercial ammo and subtract 3 times the standard deviation of velocity from the average velocity.
 
#22
Re: 6.8 Spc, or 6.8 Spc II, or ?

<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: bustin</div><div class="ubbcode-body">BTW I don't pay much attention to formulas made by some lab rat, I shoot bullets for accuracy in the field to determine if the combination of twist, rifling design and loads is capable of sub-moa, if it isn't we don't use it. </div></div>

To do real-world testing it should be (I feel for a combat weapon) in a worst-case environment - it both has to be at a very cold temp (-40F, -20F, 0F - or whatever the coldest that it may be shot at) and with the slowest velocity any individual bullet may go - and use Doppler radar to track the bullet orientations near the muzzle (as bullets get more stable down-range). And if you use a sound suppressor, it is wise to not go with too slow a twist as the bullets are just leaving the bore where they are least stable.

To find the slowest velocity a bullet may go, you can shoot the weakest load for commercial ammo and subtract 3 times the standard deviation of velocity from the average velocity.
</div></div>

I'm not in combat I've never had a deer or hog attack me in the 35 years I have been hunting.
It is also illegal to use suppressors to hunt with in Colorado. I don't shoot subsonic unless my 9mm light loads are just slow.
 
#23
Re: 6.8 Spc, or 6.8 Spc II, or ?

I forgot, you are in Colorado. The higher altitude is going to make it easier to stabilize bullets.

Standard pressure at 0 feet is 29.921
Standard pressure at 5000 feet is 24.896

A 1.105 inch long 110 grain 0.277 bullet at 2400 fps and at 0 degrees F in a 1:12 twist barrel at 5000 feet altitude is a stability factor of 1.24 - which is not great, but it would appear stable on paper.

Move to sea level, and it is 1.03 stability - a sideways bullet is suddenly a real risk.

You have to change to a 1:10.95 twist (basically 1:11) to have the same stability at seal level that you enjoy at 5000 feet.

So when you recommend a 1:12 twist barrel, people at sea level should be thinking 1:11. And if you recommend a 1:11 twist barrel, that is closer to 1:10 for others.
 
#24
Re: 6.8 Spc, or 6.8 Spc II, or ?

Funny, here in TX my 16" 1 in 12 Noveske or my 18" WOA 1 in 11 barrel has never given me any problems with stability with the 110 pro-hunters at 50 or 400. I know, I know, the math doesn't lie... Is this the plan, sell your product by debating whether or not the competitor should use a 1 in 10.95 twist or a 1 in 12 twist based upon their elevation. I've really been considering the 300 AAC for home defense (once you have subs), but each time I look at your responses to 6.8 threads on Arfcom, 68forums, or here I like it less. Maybe it's a personality thing---but I still don't remember this approach being considered effective in marketing. In fact remember something about talking about your products merits or providing relationship between the product and positive personal emotions being the effective approach. Nonetheless, each to their own. I'm not taking H's side here, as I don't listen to everything people who have something to sell say, just calling it as I see it.
 
#25
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">Funny, here in TX my 16" 1 in 12 Noveske or my 18" WOA 1 in 11 barrel has never given me any problems with stability with the 110 pro-hunters at 50 or 400. </div></div>

Nor does the Miller Twist formula expect it would. You would have to shoot in cold temps for it to predict a problem. I don't think it gets very cold in Texas. Further, you did not demonstrate that you shot at a muzzle velocity at or below the threshold I listed in your testing.

You are doing what is called an Ad hominem attack - rather than argue facts, you attempt to discredit me as a person. That is a logical fallacy.

http://en.wikipedia.org/wiki/Ad_hominem

I am not selling any product. I am just pointing out that going from a 1:11 to 1:10 twist does not chamber pressure in a significant and probably not even measurable way.
 
#26
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">I forgot, you are in Colorado. The higher altitude is going to make it easier to stabilize bullets.

Standard pressure at 0 feet is 29.921
Standard pressure at 5000 feet is 24.896

A 1.105 inch long 110 grain 0.277 bullet at 2400 fps and at 0 degrees F in a 1:12 twist barrel at 5000 feet altitude is a stability factor of 1.24 - which is not great, but it would appear stable on paper.

Move to sea level, and it is 1.03 stability - a sideways bullet is suddenly a real risk.

You have to change to a 1:10.95 twist (basically 1:11) to have the same stability at seal level that you enjoy at 5000 feet.

So when you recommend a 1:12 twist barrel, people at sea level should be thinking 1:11. And if you recommend a 1:11 twist barrel, that is closer to 1:10 for others.





</div></div>
Does it mean if I get attacked by an elk here in Colorado the bullets will be stabilized good enough to put them down but if I'm in Texas,(just above Rockport on the coast) it won't? I'll have to tell those hogs to stop playing dead but I can't give the tenderloins back. Poor Chris and his clients have been shooting those 400+ hogs over the last 3 years with bullets that aren't properly stabilized according to you from a 12 twist 6.8, shame on him.

The difference between you and I, you sit in a lab and formulate what might work or try to get others to do the testing for you, I go out and shoot stuff and see that it works.
 
#27
Re: 6.8 Spc, or 6.8 Spc II, or ?

You have nothing to sell???

And thanks for the Wikipedia link, I wouldn't have had a clue what an ad hominem was if you didn't point me in the right direction, that Latin stuff is just so confusing. Maybe your condescending tone, and desire to bash others to help prove your superiority is where my trouble lies... I'm not the first to point this out, so I'll just assume you found the ad hominem stuff while self-reflecting.

Oh and I get it now---where it is exceedingly cold at sea level, be careful because you MAY have a stability issue with this one bullet. Good thing were not splitting hairs...

BTW---given your overwhelming expertise on the matter, do you even own a 6.8?
 
#28
Re: 6.8 Spc, or 6.8 Spc II, or ?

I shoot a Noveske Recon 16" in 6.8 using the SSA 110gr TSX's and have found it to be a great combo for hogs here as well as getting right at 1 moa accuracy if not a shade less out to 300 yards. This is by far my favorite set up for those swiney bastards. Putting my AAC 7.62 can on it with NV makes it even more fun.
 
#29
Re: 6.8 Spc, or 6.8 Spc II, or ?

I have a 6.8 SPC II chamber with 1:11 twist in a 12" barrel.
What would you recommend as the best bullet for hunting hogs (I have been planning to load Barnes 85 grain TSX)?
My shots will be 150 yards and less...
 
#30
Re: 6.8 Spc, or 6.8 Spc II, or ?

I've shot several with the 85gr TSX and a few with the 110gr TSX; I couldn't see a difference, as most of them were DRT unless I was off target. When I hunted his ranch, Bill Wilson swore by the 110gr TSX and the 95gr TTSX, and he has killed hundreds of hogs with an 11.5" 6.8.

Here is one I took with an 85gr:
[img:left]
186.jpg
[/img]
 
#32
Re: 6.8 Spc, or 6.8 Spc II, or ?

A 1:12" twist will stabilize all of the bullets that are used in the 6.8 SPC, which are the 85-130 GR range. The 1:10" twist is the fastest I would go. Even the .270 uses the 1:10", so why would the 6.8 use the 1:9". Makes no sense. The 1:11" twist will stabilize every bullet in any weather condition.
 
#33
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: paulosantos</div><div class="ubbcode-body">A 1:12" twist will stabilize all of the bullets that are used in the 6.8 SPC, which are the 85-130 GR range. </div></div>

Only if the muzzle velocity is above a certain threshold.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: paulosantos</div><div class="ubbcode-body">The 1:10" twist is the fastest I would go. Even the .270 uses the 1:10", so why would the 6.8 use the 1:9". Makes no sense. The 1:11" twist will stabilize every bullet in any weather condition. </div></div>

Stability is a function of bullet RPM. RPM has to do with both twist and velocity. The 270 has higher muzzle velocities and so does not need as fast a twist for the same bullet.
 
#34
Re: 6.8 Spc, or 6.8 Spc II, or ?

<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: paulosantos</div><div class="ubbcode-body">A 1:12" twist will stabilize all of the bullets that are used in the 6.8 SPC, which are the 85-130 GR range. </div></div>

Only if the muzzle velocity is above a certain threshold.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: paulosantos</div><div class="ubbcode-body">The 1:10" twist is the fastest I would go. Even the .270 uses the 1:10", so why would the 6.8 use the 1:9". Makes no sense. The 1:11" twist will stabilize every bullet in any weather condition. </div></div>

Stability is a function of bullet RPM. RPM has to do with both twist and velocity. The 270 has higher muzzle velocities and so does not need as fast a twist for the same bullet. </div></div>

The guys that created the 6.8 wanted a 1:12" twist. I'm sure they tested it more than we have.
 
#35
Re: 6.8 Spc, or 6.8 Spc II, or ?

So an 8 twist will stabilize a 30 cal bullet, 220gr 1.483" long going 1150fps but you think it takes a 9 twist to stabilize a .270 bullet 130gr 1.114" long going 2500fps?

So what is the difference in the stability factor between those 2? I'll take a SWAG the 6.8 bullet has a higher stability rate.

SWAG-
sceintific wild ass guess.
meaning I guessed how/what to do.
 
#36
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">So an 8 twist will stabilize a 30 cal bullet, 220gr 1.483" long going 1150fps but you think it takes a 9 twist to stabilize a .270 bullet 130gr 1.114" long going 2500fps?

So what is the difference in the stability factor between those 2? I'll take a SWAG the 6.8 bullet has a higher stability rate.
</div></div>

I think you meant to ask about 1050 fps. At 0 degrees F, the 30 cal is a 1.84 stability factor.

The 130 grain 0.277 example you gave would be 2.15 stability in a 1:9 twist. In a 1:10 twist and at 2400 fps, it would be 1.72. In a 1:11, it would be 1.44 - so that bullet should be fine in a 1:11 twist. In a 1:12 twist at 2500 fps, it would be 1.2 - which should work also.

The Barnes 130 is 1.325 long. At 2400 fps and a 1:9 twist it would be a 1.28 factor - which seems fine. This bullet at 2200 fps in a 1:12 would be a stability factor of 0.70.
 
#37
Re: 6.8 Spc, or 6.8 Spc II, or ?

I don't shoot anything sub sonic so I missed by 24 fps

The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s). This is 1,236 kilometres per hour (768 mph), or about one kilometer in three seconds or approximately one mile in five seconds.
 
#38
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">I don't shoot anything sub sonic so I missed by 24 fps

The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s). This is 1,236 kilometres per hour (768 mph), or about one kilometer in three seconds or approximately one mile in five seconds.</div></div>

I assumed it was a typo from 1050 fps as it is common to load to 1050. In reality, it is best to load to about 980 fps as the bullet flight noise jumps up a lot at around 300 meters/sec:

bulnoise.GIF
 
#40
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: lwrkeysfisher</div><div class="ubbcode-body">I don't know why there is a competition brewing between these two rounds, as they are completely different; and either will serve its purpose well. From the data I've seen, and my experience 200 fps/ even a little more is possible with handloads in 11 or 12 twist SPC II or DMR chamber over SPC w/1 in 10. The 30fps difference applies changing the twist only, though LWRC nearly matches the difference using their poly 1 in 10 twist barrels. IMO the SPCII leade is too long, and you give up too much accuracy for not much pressure gain over the 6.8X43 or Noveske Mod1 chamber--consider them the Wylde chambers of the 6.8 world. </div></div>

What contributes to the increased accuracy of the 6.8x43/noveske over the spcII?
 
#41
Re: 6.8 Spc, or 6.8 Spc II, or ?

From the long dead...
No one knows exactly what the Noveske chamber is other than he says it has a .100 freebore, same as the SPCII.
The ARP chamber is .015 shorter in freebore and .0005 tighter in the throat to align the bullet better.
With the 6.8 and factory Hornady 110gr HPBTs it is easy to get sub moa
 
#42
Re: 6.8 Spc, or 6.8 Spc II, or ?

Resurrected due to a goggle search.

The 6.8 looks like a really good mid-range answer to the problem of an underpowered AR platform. The low BC and limited case capacity would be a problem for long range shooting but for deer sized game at modest distances it has definite possibilities. Looks like it performs reasonablly well in carbine lenght platforms. Not an '06 or 270 Win but adequate for someone wanting to hunt with an AR.

If I throw down the hundreds of dollars to get set up with an upper and all the fixins to start reloading I don't want to invest in something that is already obsolete because of the "latest and greatest" mod that accompanies any new cartridge.
 
#43
Re: 6.8 Spc, or 6.8 Spc II, or ?

The performance was maxed out in 2008.
The 6.8 works well for hunting out to about 300 yds.
Factory 110gr projos hit apx 2600 from a 16" barrel and can be handloaded up to about 2750fps. the little 85gr Barnes TSX will hit 3100+ with handloads and go through hogs end to end.
Just stick with 11, 11.25 or 12 twist barrels with 4 or 5R rifling those will give the best performance.
 
#44
Re: 6.8 Spc, or 6.8 Spc II, or ?

I got a 16" barrel from ar15performance in 6.8x43. You can shoot all the spcII ammo through it, and supposedly something with the chamber helps with accuracy. I've been very happy with it.
 
#45
Re: 6.8 Spc, or 6.8 Spc II, or ?

http://bisonarmory.com/16-recon-subsonic-6-8mm-stainless/

Bison has their 1/7 twist barrels out now. They are SPC-II chambers, so they can shoot the hottest ammo. The faster twist allows them to shoot any bullet right up to and including heavy subsonics, as well as it will help normal expanding bullets open up at lower velocities - so that should give it an edge in terminal performance.
 
#47
Re: 6.8 Spc, or 6.8 Spc II, or ?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">http://bisonarmory.com/16-recon-subsonic-6-8mm-stainless/

Bison has their 1/7 twist barrels out now. They are SPC-II chambers, so they can shoot the hottest ammo. The faster twist allows them to shoot any bullet right up to and including heavy subsonics, as well as it will help normal expanding bullets open up at lower velocities - so that should give it an edge in terminal performance. </div></div>

Yep, it should be alot better than the 300BLK for terminal performance.
 
#48
Re: 6.8 Spc, or 6.8 Spc II, or ?

We know 6.8 has less drop at 300 yards - advantage 6.8 for long range, but can 6.8 really top this? Maybe, maybe not. I don't really know.

300blk300small.jpg


This is all from a 9 inch!

300aacblkbarriersummary.jpg
 
#49
Re: 6.8 Spc, or 6.8 Spc II, or ?

Yep, I have no interest in the 300BLK for anything.
The 5.56 is the best choice for a training/SD/combat round, ammo is cheaper, more plentiful, will never go away when FG stockholders decide to drop it, more reliable.
The 6.8 is a better hunting round, faster, flatter trajectory, more terminal performance, more choices for hunting ammo.
 
#50
Re: 6.8 Spc, or 6.8 Spc II, or ?

I know you don't. You built your company around 6.8. It is what you do.

5.56mm - I would not be without one - I like having what the military has.
 
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