|
|
|
Discussion Forums
Forum Index
> Rifles general discussion
> Twist Rates and Projectile Weights
Twist Rates and Projectile Weights
@ 06:26 am (GMT) |
Eric FoustHi, I am currently a student with the Sonoran Desert Institute, and one of the assignments for our class is understanding how to determine the weight of a projectile to match it to the twist rate of our rifles. Currently we are reading your book “The Practical Guide to Bolt Action Rifle Accurizing and Maintenance” and so far I've only read to the fire lapping section in part one, but I feel a little overwhelmed because I’m more of a hands-on kind of guy. Could you point me in the right direction to understand this? Is there a set mathematical formula to determine this? Is barrel length also a factor in this as well? |
Replies
@ 09:28 am (GMT) |
Nathan FosterRe: Twist Rates and Projectile WeightsHi Eric, that is certainly a major topic once you start getting into it.Ok, to begin with, the Accurizing book primarily pertains to the bedding section of your gunsmithing course though other chapters are relevant. Information on selecting a suitable twist rate is provided in my book, the Practical Guide To Long Range Hunting Rifles. For the purposes of this discussion relative to your assignment: British mathematician Sir Alfred Greenhill developed the first formula for determining optimum twist rates. Don Miller created the more up to date Miller formula in later years and later still, the improved Miller formula. These days, shooters use online gyroscopic stability (SG) calculators based on the Miller formula. The Berger stability calculator is a primary example and is somewhat more advanced (more input options such as velocity temperature and altitude) than the original formula as presented here. One can approximate this math manually using the original Miller formula. As a reminder, the purpose of an SG calculator is to help with optimum twist rate selection. Alternatively, if one already has a rifle of a given twist, the calculator can be used to select a suitable bullet for that barrel. The SG model is based on a series of assumptions. An SG factor of less than 1 is considered unstable (bullet may tumble in flight), 1 to 1.4 is considered marginal, while a factor of 1.5 or higher is rated as being optimal. Please understand that there are contradictions / limitations / potential problems with such assumptions. For example, very high SG numbers fail to account for friction / possible over stabilizing / jacket core separation. The original Miller formula also utilizes a basic assumption. The formula assumes a muzzle velocity 2800fps (though the muzzle velocity does not need to be exactly 2800fps for the formula to work). Here is how you would use the original Miller formula, to determine the necessary twist needed to produce an SG of 1.5 with the .308 caliber Hornady 168gr ELD-M: For 168gr ELD-M, length 1.276”: Where: t = twist rate in calibers per turn (this is then multiplied by the bullet diameter d to get the twist in inches per turn). m = bullet mass/weight in grains. s = gyroscopic stability factor (e.g. 1.5). d = bullet diameter in inches (0.308 for .308 caliber). l = bullet length in calibers (bullet length divided by diameter i.e. l = 1.276 / 0.308 = 4.143 calibers). Steps: 1. Calculate l = 1.276 / 0.308 = 4.143 calibers. 2. Calculate the length factor: l × (1 + l²) = 75.25. 3. Calculate d³ = 0.308³ = 0.0292. 4. Multiply them together with s (let's say s = 1.5 for safe value): denominator = 1.5 × 0.0292 × 75.25 = 3.30. 5. Bullet weight for example is 168 grains. Numerator = 30 × m = 30 × 168 = 5040. 6. t² = 5040 / 3.30 = 1527. 7. t = √1527 (square root of 1527) = 39 calibers per turn. 8. Final twist in inches = t × d = 39 × 0.308 = 12 inches per turn. Those are the basic steps. In the improved Miller formula, the length of the plastic tip is omitted. Instead, only the metal part of the bullet is measured. OK, hopefully that covers the basics for you. All the best. Nathan. |
|
@ 02:00 pm (GMT) |
Eric FoustRe: Twist Rates and Projectile WeightsWow, thank you, Nathan.I appreciate how fast you got back to me. The answer you gave me is like drinking water from a fire hose, I have a lot more questions; but I will mull this over for a little bit in the meantime. I'll definitely check out the Practical Guide to Long Range Hunting Rifles, If not through my school I will get personally to develop on my professional goals. One question I do have though, I appreciate the answer from the projectile's perspective but if I already have a specific rifle in mind how do I determine the projectile best suited for the twist rate already defined by the rifle? [b] |
|
@ 07:07 am (GMT) |
Nathan FosterRe: Twist Rates and Projectile WeightsThank you for your kind words Eric.Keep in mind that as an SDI student, you are most welcome to contact me directly using the contact page as the initial contact point. I am here whenever you need me. Regarding selecting a suitable projectile, the common method is to rely on known / established information. For example, a .308 factory rifle will generally shoot all factory ammo weights from 125 (European ammo) to 180 grains. In other words, we can to begin the process by simply observing factory ammo weights. But we can of course take things a bit further. Probably the most important paragraph for you.... One must understand is that there is no substitute for test shooting. One of my readers for example, was told by his local ballistic enthusiasts that there was no way in hell his Contender 7-30 Waters would shoot the 162gr ELD-M accurately (9 twist at 2000 plus feet per second). As an upcoming gunsmith, you must try to avoid these theory traps. The reader put the question to me, I replied with enthusiasm and suggested he use double base ball powder to maximize speeds for stability without excessive pressure. The hand gun is currently shooting sub MOA at 2200fps. It is a wonderful load, ideal for his needs. So again, test shooting is the key. Regarding theoretical calculations, you can quite simply use the Berger stability calculator. Input your twist rate etc, then select a bullet and input its BC and length. The resulting calculation will tell you whether the bullet will be stable. Finally, should the internet go down long term, you can if you need to, use the basic miller formula and compare the outcome (the suggested twist) to your actual twist. Near enough is actually good enough provided you are not shooting through the 1100fps sound barrier. A few things to keep in mind. Marginal stability is not a bad thing. Provided the bullet is accurate at 100 yards in your rifle, it will not suddenly tumble downrange with ordinary or even long range killing usage. The BC will however be degraded somewhat. For long range killing, you must check actual drops and recalculate the BC for your rifle. The bullet will eventually yaw due to a major loss in stability as it passes through the transonic barrier. A marginal twist is therefore unsuitable for long range target shooting (particularly .308). But for long range killing, all bullets lose their ability to produce fast bleeding wounds / fast kills before reaching the transonic zone. Therefore, aside from the reduction in BC, issues at the transonic barrier are of no consequence to the long range hunter - unless we are talking pissing matches. Conversely, an excessive twist can place a projectile under considerable stress. Mid air blow ups with the .220 Swift are a primary example of this. That said, more subtle yet equally insidious problems have occurred in recent years as a result of increased twists rates. I have written about this in the book set but it has also been discussed recently within these forums e.g.,: https://www.ballisticstudies.com/Resources/Discussion+Forums/x_forum/17/thread/25819.html See also page 76 of the accurizing book (through to end of chapter) for a brief commentary. Just as an aside, The original M16 rifle was designed around an SG of 1. The purpose of this becomes clear if you read the .223 / 5.56 article in the Knowledge Base. Again, feel free to contact me directly if and when needed. |
Please wait
ABOUT USWe are a small, family run business, based out of Taranaki, New Zealand, who specialize in cartridge research and testing, and rifle accurizing.
|
KNOWLEDGEBASE |
KNOWLEDGEBASE |