How does Sg affect terminal performance?

[Western Living]

I've read some claims that Sg affects terminal performance. For example, Hammer Bullets claims:

"Each of our bullets has a required minimum twist rate that is calculated at sea level using the Miller Twist Formula. We use sea level as the standard to ensure that there is enough rpm’s for proper terminal performance. While the Miller Formula calculates stability for ballistic flight, we are most concerned with hunting performance and ensuring that there is enough stability for proper terminal performance. The required minimum twist rate is based on a gyroscopic stability factor (SG) of 1.5. An SG of 1.5 or higher is needed to achieve full bc value and expected terminal ballistics. In our impact testing for terminal performance, we have found that marginal SG (less than 1.5sg calculated at sea level) hinders a bullets ability to stay on track and open reliably for proper terminal performance."

In short, they're claiming their bullets may be stable in flight with a Miller-formula Sg of 1.5 at lower atmospheric pressures (high altitude, warmer temps), but unless the Sg would be 1.5 at sea-level, then the bullet may not have enough stability for terminal performance even at lower pressures.

I shoot at high altitude. The last mule deer was taken at 9200 feet. Achieving a Miller-formula Sg of 1.5 and in-flight stability is easier in these conditions, and I can use longer bullets than I could if I were at sea-level.

If the Miller formula defines what's required for in-flight stability, how can we quantify what's required for terminal performance? At sea-level, a bullet that requires 1:8 twist for a 1.5 Sg might only require a 1:10 twist at 9000 feet for the same Sg. But what will happen to terminal performance in game?

link to Miller calculator: https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
link to pressure calc: https://www.omnicalculator.com/physics/air-pressure-at-altitude

 

[DocUSMCRetired]

For hunting game you should be looking at the optimal game weight formula.
(V^3*W^2)*1.5*(10^-12) = Optimal Game Weight in lbs
V is velocity in FPS
W is weight of the bullet in grains.

Example. .308 175gr at 2550 fps

(2550^3*175^2)*1.5*(10^-12) = 762lbs

But you need to account for the velocity at the animal not at the muzzle. So to correct for this lets say we are now firing a .308 185gr Juggernaut @ 2550fps but the conditions are 20deg F, Pressure 29.41, Humidity 30% etc. and the Target is at 600 yards.

In this case the velocity would be 1652fps and KE is 1122ft-lbs. So: (1652^3*185^2)*1.5*(10^-12) = 231lbs.

So you need to account for the actual velocity at the target, not the muzzle.

---

As for why stability and an SG is important is because below 1.5 SG you lose performance. We can describe this loss as a known factor in BC which results in more drag. This in turn means the bullet is slowing faster than the shooter expects which will effect the impact kinetic energy, as well as the predicted firing solution (possible miss or wounding shot). So you want to keep the SG above 1.5 to ensure the best flight dynamics.

Lastly I will add that rotational degradation is far slower than velocity degradation. Meaning if you start at say SG 1.7 you should maintain Optimal performance as the spin rate won't be slowing as fast as the forward velocity.

 

[richthe1]

For hunting game you should be looking at the optimal game weight formula.
(V^3*W^2)*1.5*(10^-12) = Optimal Game Weight in lbs
V is velocity in FPS
W is weight of the bullet in grains.

Example. .308 175gr at 2550 fps

(2550^3*175^2)*1.5*(10^-12) = 762lbs

But you need to account for the velocity at the animal not at the muzzle. So to correct for this lets say we are now firing a .308 185gr Juggernaut @ 2550fps but the conditions are 20deg F, Pressure 29.41, Humidity 30% etc. and the Target is at 600 yards.

In this case the velocity would be 1652fps and KE is 1122ft-lbs. So: (1652^3*185^2)*1.5*(10^-12) = 231lbs.

So you need to account for the actual velocity at the target, not the muzzle.

---

As for why stability and an SG is important is because below 1.5 SG you lose performance. We can describe this loss as a known factor in BC which results in more drag. This in turn means the bullet is slowing faster than the shooter expects which will effect the impact kinetic energy, as well as the predicted firing solution (possible miss or wounding shot). So you want to keep the SG above 1.5 to ensure the best flight dynamics.

Lastly I will add that rotational degradation is far slower than velocity degradation. Meaning if you start at say SG 1.7 you should maintain Optimal performance as the spin rate won't be slowing as fast as the forward velocity.

Can you have a SG that’s “too high”?

For example, with Berger 300NM 230 Hybrid ammunition (26” barrel, 2803 fps, 1:8 twist, 59°F, 0ft Altitude) gives an SG of 2.15 and would have a bullet rpm of 252,270.

That same situation with a 1:9 twist would have an SG of 1.68 with an rpm of 224,240.

How does one make a decision between the two?

 

[XLR308]

Can you have a SG that’s “too high”?

For example, with Berger 300NM 230 Hybrid ammunition (26” barrel, 2803 fps, 1:8 twist, 59°F, 0ft Altitude) gives an SG of 2.15 and would have a bullet rpm of 252,270.

That same situation with a 1:9 twist would have an SG of 1.68 with an rpm of 224,240.

How does one make a decision between the two?

Yes, cup and core bullets have an RPM limit and will start to blow up shortly after leaving the barrel if pushed to or beyond that limit.
Other factors like a rough fire cracked throat on a worn barrel can add to that issue.

 

[DocUSMCRetired]

Can you have a SG that’s “too high”?

For example, with Berger 300NM 230 Hybrid ammunition (26” barrel, 2803 fps, 1:8 twist, 59°F, 0ft Altitude) gives an SG of 2.15 and would have a bullet rpm of 252,270.

That same situation with a 1:9 twist would have an SG of 1.68 with an rpm of 224,240.

How does one make a decision between the two?

Sort of but depends a lot on the bullet itself and the quality of the jacket. We often shoot bullets in 1 in 3 and 1 in 4 twists in testing with no problems. This is also why we have thicker or thinner jackets, and why the quality and smoothness of your rifling's matter.

 

[DocUSMCRetired]

Here you go. We did this with a 1 in 1 twist barrel, and the bullet didn't "explode".

 

[richthe1]

Interesting! Thanks for sharing. So you can’t really have an SG that’s too high as long as it doesn’t damage your bullet - got it. Do you have any specific bullets you’d recommend (looking mostly at 6.5CM, 300PRC, and 300NM). Do you have any rifling you’d recommend (I am looking to get my first custom barrel spun up)?

For example, I’m planning to shoot the following:

Berger 245 Long Range Hybrid Target
Or
Hornady 250 A-Tip

However, I have heard many stories of Hornady bullets “blowing up” midair. And should probably avoid any Berger Bullets (6.5 156gr EOL and 30 245gr Elite Hunter) that have the thinner J4 Precision Jacket due to it being slightly thinner, unless using specifically for hunting?

 

[richthe1]

Here you go. We did this with a 1 in 1 twist barrel, and the bullet didn't "explode".

That’s the video I saw a couple days ago that got me thinking!

 

[DocUSMCRetired]

Interesting! Thanks for sharing. So you can’t really have an SG that’s too high as long as it doesn’t damage your bullet - got it. Do you have any specific bullets you’d recommend (looking mostly at 6.5CM, 300PRC, and 300NM). Do you have any rifling you’d recommend (I am looking to get my first custom barrel spun up)?

For example, I’m planning to shoot the following:

Berger 245 Long Range Hybrid Target
Or
Hornady 250 A-Tip

However, I have heard many stories of Hornady bullets “blowing up” midair. And should probably avoid any Berger Bullets (6.5 156gr EOL and 30 245gr Elite Hunter) that have the thinner J4 Precision Jacket due to it being slightly thinner, unless using specifically for hunting?

You want Match bullets not hunting. Match bullets have thicker jackets. You also want smoother, not-sharp, non abrasive barrels/rifling's if possible.

 

[hlee]

You want Match bullets not hunting. Match bullets have thicker jackets. You also want smoother, not-sharp, non abrasive barrels/rifling's if possible.

Are you sure? This is counter to what I have read. For example, Hornady’s ELD-M bullets have a very thin jacket while the ELD-X has roughly 2x the jacket thickness. The former is a match bullet, the latter a hunting bullet. My understanding is that thinner jackets are easier to manufacture to the tolerances necessary for match bullets, while thicker jackets help with uniform expansion and weight retention of the bullet when it impacts an animal…

 

[DocUSMCRetired]

Are you sure? This is counter to what I have read. For example, Hornady’s ELD-M bullets have a very thin jacket while the ELD-X has roughly 2x the jacket thickness. The former is a match bullet, the latter a hunting bullet. My understanding is that thinner jackets are easier to manufacture to the tolerances necessary for match bullets, while thicker jackets help with uniform expansion and weight retention of the bullet when it impacts an animal…

I am certain on this. Match bullets are expected to be placed into hot chambers (long in both time and volume strings of fire) where as hunting bullets are expected to be colder chambers and to expand. Wanting a bullet to expand properly at high and low velocity means a thinner jacket is better. Once you have melted the lead core, that's it. Your bullets journey ends there. So the thicker jacket helps with this when your chamber is in the hundreds of degrees.

You want to protect the lead core from heat as best you can. Hunting rifles often don't encounter this issue where as competition rifles do.

 

[bulletbluesky]

Soft tissue is 800-900 times denser than air so I guess no reliable extrapolation can be made. But terminal ballistics is wonder of its own. 6.5x52 mm FMJ Carcano bullet that struck JFK in rear of his head, at impact speed 1850-1900 ft/s, fragmented like crazy. Of course, nobody, except dr. Oliver, could repeat such performance of the FMJ bullet in tests on heads of goats and swines

 

[chutinlead]

For hunting game you should be looking at the optimal game weight formula.
(V^3*W^2)*1.5*(10^-12) = Optimal Game Weight in lbs
V is velocity in FPS
W is weight of the bullet in grains.

Example. .308 175gr at 2550 fps

(2550^3*175^2)*1.5*(10^-12) = 762lbs

But you need to account for the velocity at the animal not at the muzzle. So to correct for this lets say we are now firing a .308 185gr Juggernaut @ 2550fps but the conditions are 20deg F, Pressure 29.41, Humidity 30% etc. and the Target is at 600 yards.

In this case the velocity would be 1652fps and KE is 1122ft-lbs. So: (1652^3*185^2)*1.5*(10^-12) = 231lbs.

So you need to account for the actual velocity at the target, not the muzzle.

---

As for why stability and an SG is important is because below 1.5 SG you lose performance. We can describe this loss as a known factor in BC which results in more drag. This in turn means the bullet is slowing faster than the shooter expects which will effect the impact kinetic energy, as well as the predicted firing solution (possible miss or wounding shot). So you want to keep the SG above 1.5 to ensure the best flight dynamics.

Lastly I will add that rotational degradation is far slower than velocity degradation. Meaning if you start at say SG 1.7 you should maintain Optimal performance as the spin rate won't be slowing as fast as the forward velocity.

I'm curious as to where this formula came from. Is the 1.5 a constant or should that change based on the SG calculated for your bullet at your elevation?

 

[hlee]

I'm curious as to where this formula came from. Is the 1.5 a constant or should that change based on the SG calculated for your bullet at your elevation?

Some fudd article in a legacy gun/hunting rag….

 

[DocUSMCRetired]

I'm curious as to where this formula came from. Is the 1.5 a constant or should that change based on the SG calculated for your bullet at your elevation?

It is a constant. Edward Matunas, Big Game Cartridges and Rifles. Also you will find it Lyman's and possibly other reloading manuals which were authored in part by Edward Matunas. https://www.amazon.com/Books-Edward-Matunas/s?rh=n:283155,p_27:Edward+A.+Matunas