07/16/16

! READ ME FIRST ! Read the Summary below and the original paper before trying this technique Barrel lengths are measured from the bolt face to the muzzle crown For best accuracy with QuickLoad, measure your fired case volume in grains of H2O and use this value in QuickLoad For best accuracy with QuickLoad, use the actual load round overall length in QuickLoad Calibrate QuickLoad to match your range measured velocity data by changing the burning rate factor (Ba) and the bullet weight Adjust load until QuickLoad velocity for desired OBT is reached Remember: ! Never Exceed Safe Charge Weights or Pressures ! ! The Computer Can't Think For You ! ! Safety is Your Responsibility! ! QUICK LINKS ! Original Optimal Barrel Time Paper Tools so you can try this method for yourself! : Download an Excel version of my OBT calculator here This is the new and improved version Mar 2015. Allows for an arbitrary starting node number. Requires MS Excel with macros enabled. Enable Editing and Content in Excel if you have security settings set to medium or higher. Thanks to Johann Yssel at Xpert Bullets in South Africa for pointing out a bug for odd negative node numbers For those that don't want to deal with Excel's macro settings, you can download Johann Yssel's excellent version of the Macro Free OBT calculator here. Many thanks to Johann for making this available to the shooting community View a comprehensive table of OBT times here (PDF) A whitepaper on how to best tune QuickLoad and get near perfect correspondence between prediction and measured load chronograph data Rifle Physics and Accuracy: Optimal Barrel Time Hypothesis and Theory Summary: This page documents some of the thinking and research that I have been undertaking in order to more fully understand the physics of a rifle during the actual shot event. I started thinking about why some rifles shoot better than others, why small changes in load parameters can make such a large difference in accuracy, and why "good" loads seem to work pretty well in a large number of different rifles, even rifles with very different barrel profiles and stock configurations. The theory presented here, what I have been calling Acoustic Shock Wave Theory, leading to the concept of Optimal Barrel Time (OBT), is my attempt at answering these questions. The concept is simply that there is a very deterministic physical process that causes a rifle bullet to diverge from the statistical mean by a statistically greater or lesser amount, depending on the time of bullet barrel exit in relation to the time of initiation of the propellant burn event. This hypothesis differs from most vibratory barrel models in that it is based on the proposed existence of longitudinal shock or acoustic waves in the barrel steel, and not simple harmonic bending vibration modes. Prediction of these modes is relatively simple compared to the prediction of the bending modes, and is based exclusively on the length of the barrel, and not the contour or thickness. The rifle barrel, stock, bullet, and propellant is considered as a system, and some of the possible interactions are explored. Computer simulations were performed to model the longitudinal wave shapes. These wave shapes led to the discovery of the presence of optimal exit times for the bullet: times where the muzzle is the least disturbed in a statistical manner, and therefore the least likely to perturb the bullet at launch. Tables of these times were generated, and a simple linear system of equations developed to allow easy computation of these times. Extensive range testing using QuickLoad, a chronograph, and a PressureTrace instrument was performed, validating the presence and location of these exit times. I have, since the paper was first made public, received many hundreds of independent confirmations of the OBT values by shooters at large. The physics may not be explained in a rigorous manner, but there is now far too much empirical evidence to doubt that there exists some process that has the periodic patterns that my original research has predicted. As a practicing engineer, I can accept this fact without having formal proof. If it works, use it... The paper presented here is my first writing of the concept, and has not been significantly edited to correct tortured syntax, typographical or grammatical errors, or to make it more readable. You are seeing my thoughts as I wrote the paper one long afternoon. I make no apologies for the less-than-polished format. I encourage you to read the material presented here, and try this out for yourself. If you have a few pet loads, and access to QuickLoad , use QuickLoad to determine the barrel exit time for these loads, and compare these times to those predicted by this theory. And, if you have the time, please drop me a line to let me know what you observe. Good Shooting! Disclaimer and Copyright Notice: The information contained herein is to be used for your own personal edification and enjoyment, not for commercial purposes. Unless explicitly stated, all material herein is under copyright protection; any use of this material for sale or profit without express permission of the author is prohibited. You use any or all of this data at your own risk. Reloading can be dangerous, so approach all new combinations with caution, and work up slowly. Home | Internal Ballistics | External Ballistics | Reloading | Hunting Trips | Site Map | My Rifles | Favorite Links | Projects This site was last updated 03/02/15