Forum Rules Firearms Safety Firearms Photos Links Library Lost Password Email Changes
 Register FAQ Calendar Search Today's Posts Mark Forums Read

 August 19, 2009, 07:07 PM #2 Marley09 Junior Member   Join Date: August 18, 2009 Posts: 1 In order for these calculations to help, you must have a known load of good accuracy to base the calculations. If one is starting from square one with no acquired loads for a certain caliber, he should start by finding the optimum twist for the bullet and compare it to the twist of the rifle he will be using. If the optimum twist of the bullet is slower than the rifle (ie. optimum twist for bullet = 1 in 12, barrel twist = 1 in 10) then one should start by selecting a lower velocity from the recommended loads. Rbb50, please correct me if i am wrong.
 August 19, 2009, 08:50 PM #3 Unclenick Staff   Join Date: March 4, 2005 Location: Ohio Posts: 9,243 Rbb50, Thanks for putting all that in one place. Marley09, Welcome to the forum. No, it's not so simple as you suggested. As the bullet goes slower the drag forces trying to turn if off course are also lessened so that less gyroscopic spin is required and the twist, as you hypothesized it, is still likely to be faster than required. The Greenhill forumla was developed by George Greenhill for 19th century artillery shells. It happens to work OK for a range of modern supersonic rifle bullets as well. However, you can do better with calculations based on the late Robert McCoy's book, Modern Exterior Ballistics. These are available in free online calculators into which you just plug the numbers. They are at the JBM calculator web site. One of McCoys colleagues, Don Miller, has an updated version of the Greenhill formula that adds velocity, weight, temperature and air pressure into the equation. This calculates the gyroscopic stability factor, s. s is a number that equals 1 when the bullet is on the ragged edge of instability. Under 1, and the bullet tumbles, spins off in arcs, hits the target sideways (keyholes) and otherwise makes general mayhem. Above 1, the bullet is stable, but if it is too close to 1 it won't settle well in its flight bath and its accuracy won't usually be best. Harold Vaughn thinks s=1.4 is about ideal. Don Miller thinks s=1.5 is about ideal. Pretty close to one another. If you spin the bullet still faster than is required for s=1.4—1.5, s gets larger. This exaggerates errors in the distribution of mass which cause wobble. But modern bullets are made pretty well, and you usually have to spin them awfully fast to get into trouble with so-called "overstabilization". The military usually sets twist so s=2 or a bit more at standard meteorological conditions. This ensures that in extremely dense atmosphere, such as antarctic temperatures, the s still won't drop below 1. Different conditions will change its gyroscopic stability factor, however. I have an Excel file I put together that uses Don Miller's formula that you can download free for use at my file repository. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Patron Member Last edited by Unclenick; August 19, 2009 at 08:56 PM.

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Hogan's Alley     Tactics and Training     Handguns: General Handgun Forum     Handguns: The Revolver Forum     Handguns: The Semi-automatic Forum The Hide     The Art of the Rifle: Bolt, Lever, and Pump Action     The Art of the Rifle: Semi-automatics     The Art of the Rifle: General     The Dave McCracken Memorial Shotgun Forum     NFA Guns and Gear     The Hunt The North Corral     Lock and Load: Live Fire Exercises     Competition Shooting     Curios and Relics     Black Powder and Cowboy Action Shooting The Skunkworks     Gear and Accessories     The Smithy     Handloading, Reloading, and Bullet Casting         Bullet Casting     The Harley Nolden Memorial Institute for Firearms Research The Conference Center     General Discussion Forum         TFL Photo Contest     Law and Civil Rights         Legal and Political     S.W.A.T. Magazine The Firing Line Gun Show     Retail Deals and Feedback Forum Support     Site Questions and Tech Support (NO FIREARMS QUESTIONS)     Software and Function Testing

All times are GMT -4. The time now is 02:17 AM.

 -- vBulletin 3 ---- vBulletin 3 - variable fonts ---- Low profile (unsupported) Forum Home Page - Archive - Top