question about the lyman manual
- Thread starter halfded
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Read the front portion of the Lyman Manual it has a great intro external ballistics....then for more info.....http://en.wikipedia.org/wiki/External_ballistics
David Wile
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Hey Halfded,
One of the great things about reloading is that you don't have to learn anything about BC or SD if you do not want to do so. If you are just reloading your hunting ammo and happy with the results, you don't have to worry about the BD and SD of a particular bullet. If your reloading path moves along to the really esoteric areas, however, you may find that BC and SD information may become very important in your choices for certain purposes. For the average reloader, however, BC and SD are not all that important to their more mundane usage.
Best wishes,
Dave Wile
One of the great things about reloading is that you don't have to learn anything about BC or SD if you do not want to do so. If you are just reloading your hunting ammo and happy with the results, you don't have to worry about the BD and SD of a particular bullet. If your reloading path moves along to the really esoteric areas, however, you may find that BC and SD information may become very important in your choices for certain purposes. For the average reloader, however, BC and SD are not all that important to their more mundane usage.
Best wishes,
Dave Wile
Shoney
Member
Everyone has a little different twist on these definitions. The first set is from SAAMI, and the second is from Sniper Country. Notice the particular difference in thr SD deinition:
BALLISTIC COEFFICIENT An index of the manner in which a particular projectile decelerates in free flight expressed mathematically as:
c = w/id2 where: c = ballistic coefficient, w = mass, in pounds, i = coefficient of form (a.k.a. form factor), d = bullet diameter, in inches. Represents the bullet’s ability to overcome the air resistance in flight.
SECTIONAL DENSITY The ratio of bullet weight to its diameter.
Ballistic Coefficient ( BC ) - A mathematical factor representing the ratio of the sectional density of a bullet to its coefficient of form. Simply put, BC expresses a bullet’s length ( relative to diameter ) and aerodynamic shape, thus indicating its ability to overcome air resistance in flight. The higher its BC factor, the better a bullet retains its velocity and energy, and the flatter its trajectory. Most bullets have BCs between .100 and .700. Higher BCs are required for long-range shooting.
Sectional Density ( SD ) - A mathematical factor expressing the ratio of a bullet’s mass ( weight ) to its cross sectional area. SD relates a bullet’s diameter to its length ( in a given calibre, the heavier a bullet is, the longer it is ). All other factors being equal, bullets that are longer in relation to their diameter retain their velocity better, hence have flatter trajectories, hit with higher energy, and penetrate deeper, than bullets that are shorted relative to their diameter. SD does not take into account the aerodynamic shape of a bullet, which also influences velocity retention, trajectory, etc. Useful only for comparing bullets of similar shape, or when incorporating into Ballistic Coefficient factors.
BALLISTIC COEFFICIENT An index of the manner in which a particular projectile decelerates in free flight expressed mathematically as:
c = w/id2 where: c = ballistic coefficient, w = mass, in pounds, i = coefficient of form (a.k.a. form factor), d = bullet diameter, in inches. Represents the bullet’s ability to overcome the air resistance in flight.
SECTIONAL DENSITY The ratio of bullet weight to its diameter.
Ballistic Coefficient ( BC ) - A mathematical factor representing the ratio of the sectional density of a bullet to its coefficient of form. Simply put, BC expresses a bullet’s length ( relative to diameter ) and aerodynamic shape, thus indicating its ability to overcome air resistance in flight. The higher its BC factor, the better a bullet retains its velocity and energy, and the flatter its trajectory. Most bullets have BCs between .100 and .700. Higher BCs are required for long-range shooting.
Sectional Density ( SD ) - A mathematical factor expressing the ratio of a bullet’s mass ( weight ) to its cross sectional area. SD relates a bullet’s diameter to its length ( in a given calibre, the heavier a bullet is, the longer it is ). All other factors being equal, bullets that are longer in relation to their diameter retain their velocity better, hence have flatter trajectories, hit with higher energy, and penetrate deeper, than bullets that are shorted relative to their diameter. SD does not take into account the aerodynamic shape of a bullet, which also influences velocity retention, trajectory, etc. Useful only for comparing bullets of similar shape, or when incorporating into Ballistic Coefficient factors.
Shoney said:
Yes ... and the one from Sniper's Hide is wrong and misleading. Mass is Weight/g and SD uses bullet diameter explicitly in the calculation and not cross sectional area (a function of bullet diameter obviously). So why confuse the perfectly simple, perfectly accurate definition given to us by SAAMI?
Makes me wonder how the folks at Sniper's Hide calculate bullet energy given that they think mass and weight are the same thing.
Maj Dad
Member
A superb example of what sectional density means to the target is the behavior of 2x4s and pine needles in tornados: they drive through brick walls and wooden walls like shaped charges because all of their weight (mass) bears on their cross-sectional area (diameter per the formula, but you get the idea). That's why 185 gr 45 slugs don't penetrate kevlar vests up close and personal, and why 62 gr 5.56s penetrate a kevlar helmet at 600 meters. Many deer hunters using 30 calibers use a 150 bullet for whitetails (small bodies) and 165 or 180 gr for mulies and larger critters, where you want the bullet to drive farther into the body.
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