The 155 grain Lapua will work in 1 in 10 twist

[helotaxi]

Bryan Litz says the desired SG is "1.4" is the Army says "1.5" and I just used a spread.....

No, he said the "minimum" was 1.4. You need to go back an read that chapter. He plainly stated that once the minimum was reached, barring a bullet failure, there is no maximum.

[FUBAR]

Not in the context of what SG we are trying to achieve:

...."In Figure 3, we found that the benchmark requires a 1:8.6 twist to achieve the desired Sg of 1.4 at 2950 fps. Table 1 shows what happens to Sg if we keep the 1:8.6 twist and change other conditions. Notice that for higher speeds and temperatures, the Sg goes up. For lower speeds and temperatures, the Sg goes down. Decreasing barometric pressure and increasing humidity also make the air less dense. These things have the same effect as increasing temperature....."

You are thinking of minimum twist?
"Minimum twist required for an Sg=1.4. You may get away with slightly slower twist, but itís not recommended. Higher twist rates are generally ok."

[acemisser]

GS *is* the Miller number or Miller index. The idea that anything outside 1.3 (which is considered marginally stable, btw) and 1.5 isn't "correct" is silly. In fact, the stability index increases as the bullet travels downrange and its velocity decays and the bullet doesn't go crazy as a result.

The 168SMK shoots very well from a .308 with a 1:10 twist. That bullet has a stability index somewhere north of 2.1 from such a rifle. My 1:9 .243 is way sub MOA out to 375yds shooting 58gn Vmax over 4k fps with a stability index of 3.3. By your assertion, those are both "incorrect". On target results would seriously disagree with your assertion and my results are normal.

I disagree with you on the 168 SMK shooting so well in the 1 in 10 twist...I know of 2 other guys plus my own rifle will NOT shoot this bullet worth a hill of beans..Different powders tried,and OAL.....nothing...

[FUBAR]

Acemisser:

That's the point, and I'm sure your example is correct, but Helo Taxi's example may also be correct, it's just anecdotal back and forth without understanding the why....it's not just bullet length or bullet weight, not one thing at all...

The combo might shoot to some level of accuracy in some situations, it's totally dependent on many variables...and with you shooting Benchrest it's good to understand why.

[Savage13]

he 168 SMK and 168 NCC and 168 H A-Max all shoot well in my rifle out to about 700 yards after that the 155s and 175s all shoot better. What other 168s have your tried? its highly possible that your gun just doesn't like that one bullet. Can't imagine as to why but it could. If you are talking about the SMK past 600 yards that's the reason right there. Its boat tail has given it issues at atime when it starts to go threw the trans sonic barrier it just get wonky. Depending on velocites that barrier is usually between 600-800 yards.

[Elkbane]

You guys are missing one factor about why some bullets shoot well in your rifle and some don't. Sometimes it's not the length, it's the diameter - both absolute and variability.......Think all of them are the same diameter? Measure some for yourself...........From what I can tell about the Lapua bullets (at least the 6.5mm 123's and 139's) the have the least variability of any I've measured/weighed. And I think they are slightly smaller diameter than most other manufacturers... you might want to mike some yourself.
Elkbane

[helotaxi]

Not in the context of what SG we are trying to achieve:

...."In Figure 3, we found that the benchmark requires a 1:8.6 twist to achieve the desired Sg of 1.4 at 2950 fps. Table 1 shows what happens to Sg if we keep the 1:8.6 twist and change other conditions. Notice that for higher speeds and temperatures, the Sg goes up. For lower speeds and temperatures, the Sg goes down. Decreasing barometric pressure and increasing humidity also make the air less dense. These things have the same effect as increasing temperature....."

You are thinking of minimum twist?
"Minimum twist required for an Sg=1.4. You may get away with slightly slower twist, but itís not recommended. Higher twist rates are generally ok."
The "desired" being discussed is the minimum required to stabilize the bullet. "Correct" is your word and is completely misapplied here. Again read the chapter in its entirety rather than taking certain little snippets out of context. At no point does Litz state, hint at, or imply that there is an "ideal" stability index number and in fact he states pretty much exactly the opposite. There is a minimum and that's it.

His conclusions on that chapter:

"The combination of barrel twist and muzzle velocity needs to produce a stability factor of at least 1.4 in order in insure [sic] adequate gyroscopic stability for a particular bullet."

"When using high quality bullets, there is little relation between spin rate and precision, assuming adequate stability."

His number one "take away":

"Insure [sic] that your barrel twist produces a gyroscopic stability factor (Sg) of at least 1.4 for the bullets that you're shooting."

[FUBAR]

Wow, Helotxax you need to re-read what was said and keep it in context....


This started because you were/are obfuscating your statement of "there is no such thing as overspin" I said yes there is overspin....and there is also underspin.

Avoiding the topic of overspin you asked me what was the correct Miller number. I used the same numbers that Litz and the Army uses as the desired number using Millers formula, never said that anything higher was incorrect, you just inferred it. I already stated that I did not believe a bullet could be overstabilized.

My argument is and always will remain: no one variable defines what bullet will shoot well in a particular twist rate barrel..

As for the 168smk in a 308 Win, the SG can get below the desired 1.4 and it depends on temp, humidity, velocity, and spin rate, and the Miller formula can provide the why, anyone can test it with the JBM tool http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[Savage13]

should i grab some popcorn for this?

[FUBAR]

No pop corn, I think Helotaxi is trying to point out that some list the acceptable Sg spread as 1.3 to 2, but the bullet starsts flying right, if the stabilizing force is greater than an Sg of 1.

Here is a direct quote from Bryan Litz "Shooters often ask about the ‘optimal’ twist rate, or the ‘optimal’ RPM range, but the fundamental measure of stability is the stability factor, Sg. You could say that whatever twist rate or RPM generates an Sg of 1.4 is the 'optimal' twist rate or RPM."

Serious shooters should take what we "talking Parrots" repeat with skepticism and research the answers for themselves, that way they will understand which bullets will shoot to what level of accuracy, and at what range given a specific caliber&twist rate.

The 168SMKs are known to tumble when shot somewhere in the 600 - 1000 yard range in a 1:10 twist 308 Win. Millers formula will reveal the why mystery if correct info is imputed. The combination is so close to the "Sg cliff" that a change in any of the varibles like range, temp, and/or MV can change the way the bullet flys....

[helotaxi]

The 168SMKs are known to tumble when shot somewhere in the 600 - 1000 yard range in a 1:10 twist 308 Win. Millers formula will reveal the why mystery if correct info is imputed. The combination is so close to the "Sg cliff" that a change in any of the varibles like range, temp, and/or MV can change the way the bullet flys....

Litz talks about that bullet in particular when he disucsses dynamic stability, which is different from gyroscopic stability. The bulle has issues when it enters the transonic region (around 1300fps) regardless of barrel twist and MV. It's problem has to do with the aerodynamic forces acting on it as the shockwave starts to change shape with the bullet approaching Mach. No amount of spin will correct this and it's limited to that particular bullet.