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Rifles, Reloading, Optics, Equipment
Reloading
Purpose of bullet jump
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<blockquote data-quote="BallisticsGuy" data-source="post: 1794739" data-attributes="member: 96226"><p>I go in ~30-40thou increments just looking for a place that's better than the others in group size. In 4 increments set .030 apart I'll see a trend in much in the way that Rhovee describes but I will likely see only a continuous increase or decrease in group size because I don't try to find the point of absolute mininmum group size. I do not get bent around a rod trying things out at single digit thou increments and I don't jam unless it's really necessary but if I was doing bench rest competition I almost certainly would do at least one of those things. For 90% of people going to that small of an interval is just getting into the weeds.</p><p></p><p>What is actually the "reason"? Let me be 100% clear: I don't "know". So, SPECULATION BELOW.</p><p></p><p>Here's some speculation which seems backed up well by experimental data but I don't have enough data to say my observations are more than total coincidence. It's seems to be to do with having enough velocity built up between the time the bullet defeats the case mouth friction and the time it reaches the rifling origin to defeat engraving pressure without pressures having to climb excessively high. There are several events that happen when you fire a rifle. Primer detonation increase case pressure sharply which should dislodge the bullet from case neck clamping. While the powder conflagrates and pressures further build the bullet is jammed into the rifling by some amount (probably not a lot) until pressures build enough to finish the engraving process and overcome bore friction to send the pill the rest of the way down.</p><p></p><p>Long leades of especially low angle would probably want jammed bullets. They don't need a lot of inertia because the rifling comes up very gently so each unit of distance the bullet moves requires less force to get it there than a sharper rifling angle might net.</p><p></p><p>"Normal" heavy bullet optimized leades of average steepness should probably show a tendency to like a relatively relatively short but definite jump. Abrupt rifling angles would probably show a tendency to like much longer jumps so that enough inertia is built to engrave the bullet. Tangent vs. Secant ogive shapes probably play a rather significant role in that system too. Classic VLD's with the sharpish transition from shank to nose I can see making the whole works more sensitive to all kinds of little things based on the above reasoning.</p></blockquote><p></p>
[QUOTE="BallisticsGuy, post: 1794739, member: 96226"] I go in ~30-40thou increments just looking for a place that's better than the others in group size. In 4 increments set .030 apart I'll see a trend in much in the way that Rhovee describes but I will likely see only a continuous increase or decrease in group size because I don't try to find the point of absolute mininmum group size. I do not get bent around a rod trying things out at single digit thou increments and I don't jam unless it's really necessary but if I was doing bench rest competition I almost certainly would do at least one of those things. For 90% of people going to that small of an interval is just getting into the weeds. What is actually the "reason"? Let me be 100% clear: I don't "know". So, SPECULATION BELOW. Here's some speculation which seems backed up well by experimental data but I don't have enough data to say my observations are more than total coincidence. It's seems to be to do with having enough velocity built up between the time the bullet defeats the case mouth friction and the time it reaches the rifling origin to defeat engraving pressure without pressures having to climb excessively high. There are several events that happen when you fire a rifle. Primer detonation increase case pressure sharply which should dislodge the bullet from case neck clamping. While the powder conflagrates and pressures further build the bullet is jammed into the rifling by some amount (probably not a lot) until pressures build enough to finish the engraving process and overcome bore friction to send the pill the rest of the way down. Long leades of especially low angle would probably want jammed bullets. They don't need a lot of inertia because the rifling comes up very gently so each unit of distance the bullet moves requires less force to get it there than a sharper rifling angle might net. "Normal" heavy bullet optimized leades of average steepness should probably show a tendency to like a relatively relatively short but definite jump. Abrupt rifling angles would probably show a tendency to like much longer jumps so that enough inertia is built to engrave the bullet. Tangent vs. Secant ogive shapes probably play a rather significant role in that system too. Classic VLD's with the sharpish transition from shank to nose I can see making the whole works more sensitive to all kinds of little things based on the above reasoning. [/QUOTE]
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Purpose of bullet jump
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