HARPERC
Well-Known Member
Thank you!
Need some rifle twist experts to help me out...
- Thread starter MudRunner2005
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Thank you!
Gerard Schultz
Well-Known Member
You are welcome.
For general information it must also be considered that all stability calculators are not intended for all bullets. Any calculator based on the Miller formula for instance, cannot be used to calculate anything other than lead or jacketed lead core bullets. For a different specific gravity (SG) such as copper or brass, one would need a calculator based on the method developed by McCoy.
A typical Miller formula calculator would ask for caliber, weight, length, twist, speed, temperature and altitude. The McCoy method, in addition to the parameters of the Miller formula, also asks for ogive length, meplat diameter, material density, boat tail length, boat tail angle and the shape of the ogive.
Take a solid copper bullet and enter the following data in a Miller based formula for gyroscopic stability (Sg).
Caliber: .300 (Humor me)
Weight: 150gr
Length: 1.25"
Twist: 10"
Speed: 3000fps
Temperature: 59
Altitude: 0
The Sg calculated would be about 2.1
Enter the above data into a McCoy calculator as well as:
Ogive Length: 0.669"
Meplat Diameter: 0.087"
Material Density (SG): 8.95
Boat Tail Length: 0.244"
Base Diameter: 0.256"
Head Shape: 1 (for a typical slender bullet forebody)
The calculated Sg would be around 1.66 which is correct for solid copper.
Finding a McCoy calculator: Search for "McCoy Twist Rate Formula"
For general information it must also be considered that all stability calculators are not intended for all bullets. Any calculator based on the Miller formula for instance, cannot be used to calculate anything other than lead or jacketed lead core bullets. For a different specific gravity (SG) such as copper or brass, one would need a calculator based on the method developed by McCoy.
A typical Miller formula calculator would ask for caliber, weight, length, twist, speed, temperature and altitude. The McCoy method, in addition to the parameters of the Miller formula, also asks for ogive length, meplat diameter, material density, boat tail length, boat tail angle and the shape of the ogive.
Take a solid copper bullet and enter the following data in a Miller based formula for gyroscopic stability (Sg).
Caliber: .300 (Humor me)
Weight: 150gr
Length: 1.25"
Twist: 10"
Speed: 3000fps
Temperature: 59
Altitude: 0
The Sg calculated would be about 2.1
Enter the above data into a McCoy calculator as well as:
Ogive Length: 0.669"
Meplat Diameter: 0.087"
Material Density (SG): 8.95
Boat Tail Length: 0.244"
Base Diameter: 0.256"
Head Shape: 1 (for a typical slender bullet forebody)
The calculated Sg would be around 1.66 which is correct for solid copper.
Finding a McCoy calculator: Search for "McCoy Twist Rate Formula"
Michael Eichele
Well-Known Member
I maybe wrong but I believe what causes bullets to tumble is the destabilizing effects of the transonic zone. I don't think loosing velocity in and of itself causes tumbling other than the loss causing them to go subsonic. Some combinations can survive the transonic wall but most combos cannot. It's very difficult to calculate or predict what will survive and continue to fly straight. While supersonic though, it is mathematically provable that stability can and does increase over time. All bets are off when they hit the transonic wall.
If you ever watch slow motion videos of fighter jets coming down from supersonic speeds, they get knocked around a bit when they cross that threshold.
Michael Eichele
Well-Known Member
I'm not a mathematician or physicist and I'm not as credible as Gerard so please don't take this as an argument. Just my experiences and subsequent opinions based on those.
I prefer to use the Miller method even for 100% copper bullets. My reasoning is because even though miller doesn't account for material density (accepted nominal values are 8.89 for copper and 10.7 for jacketed lead) it does account for weight, caliber and length not just length and caliber like early equations used. This allows for proportionate values to be used. For example, using the Miller method, if you compare equal caliber, air density, velocity and length but change the weight, the stability factor increases as weight goes up. Using a density factor here would also work similarly but to simplify things, it's using proportion. Obviously a 190 grain 30 caliber bullet 1.38" long is denser than another 190 of equal nose and boat tail profile that is greater in length. The Miller method does reflect this. Whether you have two 190s of equal density but having different lengths (due to form factors) or having two 190s of equal form factors but different lengths (due to different densities) the Miller method reflects these both. IMHO it is a very simple yet also very accurate method for both the leaded and unleaded bullets.
A 30 caliber 177 grain all copper GS bullet with a length of 1.540" is considerably longer than the 30 caliber 178 BTHP Hornady jacketed lead bullet at 1.340". Granted, the 177 is sleeker, there is only so much streamlining you can do be it jacketed lead or all copper. My point is that with the limitations it should be obvious by looking at the GS 177 (knowing it weighs 177 grains) that it is not jacketed lead. A bullet with the dimensions of a 208 Amax either weighs 208 grains if made of jacketed lead or 177 grains if made of copper. The Miller shows that even though the 177 and 208 are of very similar dimensions that the 208 is much more stable for a given twist at a given velocity reflecting how density plays into bullet stability, it just doesn't use an official density factor. It uses caliber, length and weight as a reference.
To validate my theory is a couple of experiments I did a few years back. I was trying to validate the accuracy of Miller method. One test was the 208 Amax in a 12x barrel. The other was the 177 GS bullet in an 11x barrel. Since we're talking about the Miller versus all copper, I'll go with the 177.
Right before I was planning a trip to Arizona, I decided to use that opportunity to test the Miller method. I had some 177s on hand as well as an 11x barrel. The Miller formula showed for my location and velocity I should be just under 1.0. Between .98 and .99. 11 degrees F. and 29.98" mercury. At 100 yards none of the bullets were sideways but severely key holed. Some looked like they were dang near at 45 degrees when the entered. Velocity was 2752 avg. It was obvious that these were less than stable. I never fired them at 200 yards but I'm sure they would have gotten worse. Going to Arizona and shooting in 24.95" Hg., 60 degrees F. showed textbook round holes at 100-300 yards. The Miller method showed a 1.3 stability factor which is in line with the results.
My results were similar with the 208 in a 12x. They stabilized in warm air but not cold air as predicted by Miller. When Miller said .99-1.0, they were key holing and by 700 yards they were getting worse. Any less than that such as .97-.99 and lower, they key holed worse early on. I got identical results using the Miller method using both all copper bullets and jacketed lead.
I'm not saying Miller is better or that McCoy is not right, I just think the miller is pretty good and more usable than many think. Including for all copper bullets.
Will the Miller method work every time? Maybe not. But I think if you're just looking to see if you have a chance of being stable or not with a given combo, the Miller method can show you whether or not its worth your time to try or a waste of your time to try.
I had a 22-284 shots lots of 80 and 90 grain bullets. I used a 32 in barrel. Muzzle velocity was 3900 to 4000. Bullets stablized all the way out to 1000yd. Shot 5 or 6 1000yd matches with no issues. The only issue I had was barrel life. 400 rounds and both barrels were toast. I switched to the 6-284 shooting the 105-107 bullets and was much happier shooting them out of a 32 inch 1-8.5 twist barrel. I was getting just over 3500 fps. Problem is my barrel life was almost 800 rounds. I would never build another 22-284 but I would build another 6-284. Hope this helps your decision.
Mikecr
Well-Known Member
Michael Eichele, keep in mind the Miller math represents a rule of thumb for modern bullets.
It is better by far than Greenhill, but nowhere near McCoy's actual science.
In other words, when you account for more details in a bullet's design, as Gerard mentioned, McCoy's math goes where rules of thumb can't.
It is better by far than Greenhill, but nowhere near McCoy's actual science.
In other words, when you account for more details in a bullet's design, as Gerard mentioned, McCoy's math goes where rules of thumb can't.
Michael Eichele
Well-Known Member
That's pretty much what I was getting at. Miller will help you figure out if a combination is worth a try or not with minimal effort.
Gerard Schultz
Well-Known Member
The problem comes when dimensions become marginal. To illustrate what I have encountered it is probably best to use an actual example. A reloader has a new Sako 300 Win Mag. All new Sako and Tikka rifles in .30 caliber have 11" twist hammer forged barrels.
He wanted to use our 177gr HV for plains game and our 180gr FN for the small 5 to save cape damage and in case he walks into a cape buffalo.
The Miller method gave the Sg of the 177gr HV as 1.12 and the 180gr FN as 2.19 and both are numbers in line with what he wants. The minimum required for an HV is Sg 1.1 and for an FN it is over Sg 2.0.
McCoy, on the other hand gives the 177gr HV as Sg 0.95 and the 180gr FN as 1.24. Both fall far short of what is required.
When he started working up his load, the 177gr HV was sideways on a 25 m target. Fortunately he did not try the 180gr FN. The 180gr FN will give stable flight but fail to penetrate in a stable manner giving massive meat damage and unreliable penetration.
We changed him (with great difficulty) to a 150gr HV at Sg 1.4 and a 145gr FN at Sg 2.8 and he is hunting with exactly the results he wants.
Every user is free to experiment and spend his money as he wishes. The problem starts when a user is not as knowledgeable as Michael and does not understand why he is getting a result he did not expect. He then perceives his actions as a waste of his money and time and blames the product, thinking that he has acted correctly. We try to avoid that.
He wanted to use our 177gr HV for plains game and our 180gr FN for the small 5 to save cape damage and in case he walks into a cape buffalo.
The Miller method gave the Sg of the 177gr HV as 1.12 and the 180gr FN as 2.19 and both are numbers in line with what he wants. The minimum required for an HV is Sg 1.1 and for an FN it is over Sg 2.0.
McCoy, on the other hand gives the 177gr HV as Sg 0.95 and the 180gr FN as 1.24. Both fall far short of what is required.
When he started working up his load, the 177gr HV was sideways on a 25 m target. Fortunately he did not try the 180gr FN. The 180gr FN will give stable flight but fail to penetrate in a stable manner giving massive meat damage and unreliable penetration.
We changed him (with great difficulty) to a 150gr HV at Sg 1.4 and a 145gr FN at Sg 2.8 and he is hunting with exactly the results he wants.
Every user is free to experiment and spend his money as he wishes. The problem starts when a user is not as knowledgeable as Michael and does not understand why he is getting a result he did not expect. He then perceives his actions as a waste of his money and time and blames the product, thinking that he has acted correctly. We try to avoid that.
Michael Eichele
Well-Known Member
Very true. 11x barrels are not always exactly 11x. Or any other twist.
Gerard, it is obvious that you guys get quite technical in your business. It's good to see and we appreciate it.
M
