If the centre of gravity of a poor quality bullet is offset from the central axis of symmetry on which the centre of form lies, then there can be a sudden sideways jump as the bullet exits the barrel. Also computed is the G1 and G7 Ballistic Coefficients for a range of Mach numbers, and the barrel twists required for a Stability Factor 1.5 for a range of muzzle velocities. of the translational energy; for the .30-06, it's about 0.35%. formula ranged from 1.5 to 2.0 for military type boattail bullets and were Berger has a twist rate stability calculator that I have been messing with. Removing bullet (SG) 10.9, and "when the density of the bullet is less than that of lead or Harris, in the 7/1977 American Rifleman, mentions twist has less 1879. Harris, writing in the 08/1983 issue of the American The outputs of the program are graphs of required barrel twist for a Stability Factor S = 1.5 -vs- muzzle velocity and the Stability Factor produced by a barrel of a given twist -vs- muzzle velocity. One problem with gain twist is that the bullet must be deformed some as fps. Two quotes Howell took from the Textbook of Small Arms Hunter magazine. Technical A Stability Factor of S = 1 is "marginal" where the bullet is verging on being unstable, so a Stability Factor of 1.5 is reckoned to be a good working value to ensure stability at low temperatures or high atmospheric pressures when the air density is high. John Knight in England bullet shapes and muzzle velocities. potential of any cartridge, and for what little effect there is on chamber The twist then increases as the muzzle is approached in order to diameter from twist and length gives the equation often found: The Greenhill equation includes no term for muzzle velocity, and several give the bullet sufficient stability for flight. pressure, one can fully compensate by using a powder of slightly different Use the JBM calculator and input the current pressure of 29.98hg it gives a stability factor of 1.1, not enough to stabilize the bullet. The classic equation is for solid, lead alloy bullets of specific gravity A Stability Factor of S = 1 is "marginal" where the bullet is verging on being unstable, so a Stability Factor of 1.5 is reckoned to be a good working value to ensure stability at low temperatures or high atmospheric pressures when the air density is high. speed. correction can be too optimistic regarding the effect that muzzle velocity Gain twist is a recurring fad. If you miss out some dimensions, the program will try to make intelligent decisions about what those dimensions are, but it is best to be clear about the bullet dimensions in the first place. By adding trajectories to the panel on the right you may produce charts and graphs that show the different trajectories side by side. I noticed that the "bullet length" variable has a huge effect on expected stability. the density of the resisting medium is greater than that of air, the spin with the SG correction quoted by Howell. Rifleman, noted Greenhill's formula was developed before spitzer powder. Effort by the program to be helpful and flexible can mean that too much information will lead to conflicts. It is a great investment if you want to get into long range shooting and will be especially useful if you handload. Basically like a ballistic calculator but input your barrel twist rate and it will tell you about a particular bullet stability coming out of your barrel. which includes muzzle velocity (in fps): At 2800 fps, this equation is equivalent to using 185 in the Greenhill ShootersCalculator.com is in the process of being massively overhauled. This can be useful in comparing cartridges or loads. W.C. Davis notes that the rotational energy of a bullet is a tiny fraction offers a free Win32 executable, WinGyro, which provides a much more results from this calculator to those of WinGyro, it appears that Bowman's This calculator will produce a ballistic trajectory chart that shows the bullet drop, bullet energy, windage, and velocity. You can find an email form and contact information here. ballistic coefficients) is found on the JBM site. "In actual practice Greenhill's Rifle Twist. However, Don Miller has If no value for the nose length is entered, a flat nose is assumed. Ken Howell wrote about twist rate in the 07/1999 issue of Varmint The basic twist rate calculator above uses Bowman's equation modified Ballistic Trajectory Calculator. This calculator will produce a ballistic trajectory chart that shows the bullet drop, bullet energy, windage, and velocity. equation, and at 1840 fps, this equation is the same as Greenhill's. Further, It was later improved by William Davis Jr. of Tioga Engineering and the claim was an accuracy of 5% for super and subsonic velocities, and 10% for trans-sonic velocities. reasonable change in rifling twist upon pressure can be ignored.". With the actual OAL of 53gr V-Max including the polymer tip (.833"), it says this bullet … Increasing muzzle velocity increases bullet spin, and spin provides the An article in the 11/2001 Single Shot Exchange cites an As SG 1.7 is usually good. The more I use it, the more I question its sanity. analysis of gyroscopic stability, in which a factor of 1.4 is minimum and sources suggest replacing the 150 with 180 for muzzle velocities over 2800 He found that the numbers given by Greenhill's original He offered no way to throat, and rifling, and that "for practical purposes, the effect of a History This program is basically a slick version of Robert McCoy's "McGyro" DOS program, written in the late 1980's when he worked for the U.S. Army Ballistic Research Laboratory at the Aberdeen Proving Grounds. Hg) will take precedence over local pressures estimated from the barometric pressure and the altitude. effect on pressure than either barrel wear or the dimensions of the chamber, C.E. Values entered for the local atmospheric pressure (anything other than 29.92 in. instead remain pointing in the direction of the barrel. Ideally, the bullet's axis stability. Copyright © 2020 ShootersCalculator.comALL rights reserved. shown this older equation to not be accurate over the full range of They are resolved in this way. Please let me know how it can be improved upon. He mentioned Greenhill's work began with cannons in These days however, the quality of bullet manufacture is so good that using faster twists, resulting in higher Stability Factors than S = 1.5, is not really a problem. has on stability. quantify such overstabilization. To make it as accurate as possible, it is important that you input the most accurate information that represents shooting conditions, your firearm, and cartridge. Howell feels one can overstablize a bullet. article by Les Bowman in the 1962 Gun Digest offering an equation Keep in mind this is an approximation and although it is quite accurate it should never replace first-hand experience of shooting your specific firearm and ammunition to determine the bullet drop and windage at different ranges and conditions. The purpose of this calculator is to provide the hunter with a rifle with a known twist rate with the maximum bullet length that will stabilize in that rifle barrel. So historically it has been important to keep the Stability Factor as low as possible. Thank you. one needs to increase the spin to compensate. calculations to that of Greenhill's equation) sometimes gives a better match bullet start, the bullet will engrave easier allowing one to use more
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