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Rifles, Reloading, Optics, Equipment
Reloading
Getting the Best Precision and Accuracy from Berger VLD bullets in Your Rifle
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<blockquote data-quote="Unclenick" data-source="post: 2382018" data-attributes="member: 106107"><p>A couple of points. One is that the Berger paper is about the original secant ogive VLDs. The newer hybrid ogive bullets start out tangent and switch to secant. This provides the more forgiving tangent contact point, which is less choosy about seating depth, and the higher BC secant ogive over most of the rest. It should be expected that your tests may be harder to get a good indication from with the hybrids.</p><p></p><p>Second, if you look at the <a href="http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5" target="_blank"><span style="color: rgb(41, 105, 176)">1965 published research by Dr. Lloyd Brownell</span></a> and funded by DuPont at the U of M, you see his measurements on the effect of bullet seating depth on pressure has a peak when the bullet contacts the lands and that this backs off as you seat deeper, then starts to rise again as you go deeper still. The pressure value is dominated by the amount of gas bypass around the bullet until it is so deep no further space between the bullet and throat allows significantly more bypass. Seating deeper starts raising pressure again by taking away from the powder space in the case. So when you seat bullets to different depths, you are not only changing the bullet's experience getting down the bore and how it aligns and how much of a gas cushion it has around it, but you are also changing peak pressure, and with it, barrel time.</p><p></p><p>That all means, when you adjust seating depth, you are not only adjusting pressure and jump, which affects where a velocity ladder flat spot might lie, but you are also are adjusting the timing of the bullet exit which will affect whether the bullet exits at a favorable phase in the whip of the muzzle from recoil moment and pressure distortion effects (see<span style="color: rgb(41, 105, 176)"> <a href="http://varmintal.com/aeste.htm" target="_blank">Varmint Al's analysis work on Esten's barrel tuner</a>. </span></p><p></p><p>The bottom line is, by changing seating depth you are changing two and possibly three variables at a time, hoping to run into a place where you achieve a coincidence of velocity and muzzle whip flat spots as well as of what tends to make the bullet feed into the throat with best alignment and consistency. The trick to sorting that out requires firing a matrix of at least two variables, and possibly three. The bottom line is, shoot a powder charge ladder at each of the seating depths suggested by Berger (closer spacing for shorter ogives, greater for longer ones, as the long ones have to move more to yield the same change in the size of the annular gap between bullet and bore that determines gas bypass), looking for a velocity flat spot. Then shoot groups from the middle of the velocity flat spots for each loading depth. See which one shoots the smallest groups. It will be the one closest to the optimum muzzle whip barrel time. Then you can look at tweaking either side of the best point or between the two best if they match and are adjacent. And good luck. It's a lot of work.</p></blockquote><p></p>
[QUOTE="Unclenick, post: 2382018, member: 106107"] A couple of points. One is that the Berger paper is about the original secant ogive VLDs. The newer hybrid ogive bullets start out tangent and switch to secant. This provides the more forgiving tangent contact point, which is less choosy about seating depth, and the higher BC secant ogive over most of the rest. It should be expected that your tests may be harder to get a good indication from with the hybrids. Second, if you look at the [URL='http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5'][COLOR=rgb(41, 105, 176)]1965 published research by Dr. Lloyd Brownell[/COLOR][/URL] and funded by DuPont at the U of M, you see his measurements on the effect of bullet seating depth on pressure has a peak when the bullet contacts the lands and that this backs off as you seat deeper, then starts to rise again as you go deeper still. The pressure value is dominated by the amount of gas bypass around the bullet until it is so deep no further space between the bullet and throat allows significantly more bypass. Seating deeper starts raising pressure again by taking away from the powder space in the case. So when you seat bullets to different depths, you are not only changing the bullet's experience getting down the bore and how it aligns and how much of a gas cushion it has around it, but you are also changing peak pressure, and with it, barrel time. That all means, when you adjust seating depth, you are not only adjusting pressure and jump, which affects where a velocity ladder flat spot might lie, but you are also are adjusting the timing of the bullet exit which will affect whether the bullet exits at a favorable phase in the whip of the muzzle from recoil moment and pressure distortion effects (see[COLOR=rgb(41, 105, 176)] [URL='http://varmintal.com/aeste.htm']Varmint Al's analysis work on Esten's barrel tuner[/URL]. [/COLOR] The bottom line is, by changing seating depth you are changing two and possibly three variables at a time, hoping to run into a place where you achieve a coincidence of velocity and muzzle whip flat spots as well as of what tends to make the bullet feed into the throat with best alignment and consistency. The trick to sorting that out requires firing a matrix of at least two variables, and possibly three. The bottom line is, shoot a powder charge ladder at each of the seating depths suggested by Berger (closer spacing for shorter ogives, greater for longer ones, as the long ones have to move more to yield the same change in the size of the annular gap between bullet and bore that determines gas bypass), looking for a velocity flat spot. Then shoot groups from the middle of the velocity flat spots for each loading depth. See which one shoots the smallest groups. It will be the one closest to the optimum muzzle whip barrel time. Then you can look at tweaking either side of the best point or between the two best if they match and are adjacent. And good luck. It's a lot of work. [/QUOTE]
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Getting the Best Precision and Accuracy from Berger VLD bullets in Your Rifle
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