I am listening and open minded-------couple of counter points here on the theory you express. (and I certainly do not consider you to be the village idiot--you have gotten folks to look at tuners in a serious fashion and you have been open with your thoughts, conclusions and theories)
The bullet has blow by gases going by it as it starts its travel --that gas is going in excess of 4000 fps and it is blowing the atmosphere out of the barrel before the bullet has to push that air. If the rifle barrel was a closed tube--then yes lots of compression would be going on.
Have shot hundreds of tune curves with multiple rifles--the barrel vibrates in somewhat of a sine wave and the best tuning points are just past a peak or just before a valley. If you look at velocity versus barrel exit position as plotted by bullet impact change at 100 yards--that curve can be drawn.
temperature of ammunition and rifle effects velocity for given powder charge and water in the air effects the powder while we are loading it--which in turn effects velocity. the resultant velocity change then effects tune......
preloads have worked well for me as long as rifle is hooting well when I preload and then drastic temperature change is not experienced that would put gun out of tune (due to the velocity of bullet changing from that atmospheric temperature change)
Jim
Jim thanks for the kind words and civil manner in which you participate in these discussions; you are a true gentleman.
In your response above you said,
"I have shot hundreds of tune curves with multiple rifles--the barrel vibrates in somewhat of a sine wave and the best tuning points are just past a peak or just before a valley. If you look at velocity versus barrel exit position as plotted by bullet impact change at 100 yards--that curve can be drawn."
You are absolutely correct in everything you say. I am very familiar with the sine wave pattern you mentioned. Before studying Vaughn's book, I referred to this as "Runnin' a line." I would draw a horizontal line on the back of a target and place aim points along this line at about 1.5 inch intervals. I would then start at the lower end of the load window which with the 6ppc and N133 is 27 grains topping out at about 30.2 grains. I would load three rounds with 27.0 grains and fire these on the first aim point, the next three rounds with 27.3 on the second and so on all the way through 30.2. The groups would show when the rifle came into and went out of tune. There would be three distinct points at which the rifle shot well. The in tune positions were invariably 1.2 grains apart which equates to approximately 120 fps intervals. A sine wave pattern also appeared as the muzzle moved throughout its vibration pattern. I have often referred to this pattern as the barrel's 'signature.'
Jim, in your response above you also said,
"The bullet has blow by gases going by it as it starts its travel --that gas is going in excess of 4000 fps and it is blowing the atmosphere out of the barrel before the bullet has to push that air. If the rifle barrel was a closed tube--then yes lots of compression would be going on."
Of course, I realize that there is a certain amount of gas that blows by the bullet before the bore is completely sealed, but would that not contribute even more air/gas/pressure to that area between the nose of the bullet and the muzzle?
It is a well known fact that air cannot "get outta' the way" fast enough to prevent compression when penetrated by a high speed aircraft or projectile, even in open air. Although the barrel is not sealed at the muzzle, it still 'contains' the column of gas/air preventing it from dissapated laterally. The only place for this air/gas to go is out the muzzle and I asssure you that due to the weight of the atmosphere, there will be a great deal of compression created. I refer you to Vaughn's book, "Rifle Accuracy Facts" page 136. There you will find a shadowgraph photo of the spherical precursor shock wave that precedes the bullet's exit from the bore. Vaughn states, "The precursor is formed by the compressed air and gas ahead of the bullet."
In answer to your statement,
"Temperature of ammunition and rifle effects velocity for given powder charge and water in the air effects the powder while we are loading it--which in turn effects velocity. the resultant velocity change then effects tune......
Yes, I agree completely. Whether we preload in a climate controlled environment or load in ambient conditions at the range, we must be certain that all rounds were loaded in the same condition. If we control barrel temperature, (I use a wet towel) load and store our ammunition at ambient temperature, muzzle velocities should be fairly consistent. One of the biggest advantages of using a tuner is the fact that we can correct for any out of tune condition at the line. If the barrel is a little fouled or warm or if our ammunition is a little bit warm; anything that can change internal pressures/velocities, we can easily compensate with a tuner adjustment. There is no longer an excuse for being out of tune.
Later,
Gene Beggs
