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Old September 2, 2009, 02:38 AM   #1
Field
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bullet seating

if your die is set up and you are seating 124g 9mm bullets you technically wouldnt need to adjust it when seating 147g or 115g bullets because the length of the bullet vs the travel into the die on your press cancels out.
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Old September 2, 2009, 02:45 AM   #2
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another thing i was wondering was is the reason for powder charge amount decreasing as bullet weight increases is due to the fact you need a certain air/powder ratio for a proper explosion to launch the bullet and with a longer bullet seating into the same case you have less room for air inside and therefore are required to use less powder..?
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Old September 2, 2009, 02:46 AM   #3
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It does if the noses have the same exact profile. Two different profiles will usually touch the taper of the seater punch at a slightly different place, so often it will work out that even two differenc same-weight bullets need a different seater adjustment.

The reason for the charge difference is the space under the bullet base. Less space means higher pressure for a given charge and bullet weight and construction (jacketed, lead, solid, etcetera).
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Old September 2, 2009, 03:30 AM   #4
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Adding on to Unclenick's post, the propellants used in firearms do not require air. The oxidizer is part of the chemical mix.

Sometimes you get lucky and don't have to change the seating die (much) when you change bullets. Be careful not to seat deeper than minimum listed for the load you are using.

Heavier bullets take less powder because the chamber can only take so much pressure. If X grains of powder develops 20,000psi of chamber pressure under a 147gr bullet it stands to reason that it will develop less pressure under a 115gr bullet which is easier to get moving. Therefore the 115 grain bullet can take a larger powdr charge without exceeding maximum chamber pressure.

As an aside, don't try to use PSI and CUP pressure ratings interchangibly. PSI is a peak measured pressure while CUP is a deformation measurement which is affected by pressure applied over time.
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Old September 2, 2009, 09:03 AM   #5
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To be a little clearer about charge weight verses bullet weight: there are TWO factors at play:

1. When you seat a heavier bullet to the same overall cartridge length in a case, it takes-up more of the internal space. (The amount of AIR doesn't matter, because the air is not part of the chemical reaction when the powder "burns". Gunpowder "burns" by rearranging the chemical elements within itself to produce a lot of small gaseous molecules from some much larger solid molecules.) What matters in how much space there is for those gases to occupy before the bullet gets to move very far. If you reduce the amount of space available for the gases, you increase the pressue.

2. But, you can't just decrease the amount of powder in proportion to the decrease in the amount of space, like you might expect from the "Perfect Gas Law" that we were taught in high school chemisty class. The reason for that is the additional weight of the bullet and the fact that it moves some before the burning powder gets the gases to the maximum pressure. The heavier the bullet, the slower it will accelerate due to the gas pressure on its base, so the less room there will be in the case by the time the pressure peak is reached. This can be compensated for to some extent by using a different powder that burns more slowly. So, you will see in the data that slower powders have higher charge weights in a cartridge with every bullet used, and often see that the slower powders give higher velocities with heavier bullets while powders that are a little faster give higher velocities with lighter bullets. The key is that all of the powder/bullet combinations are limited to the same peak pressure by the strength of the gun.

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Old September 2, 2009, 09:51 AM   #6
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The seating die establishes the COL, or cartridge overall length. Changing bullets will not change the OAL. What will happen is that a shorter bullet will increase the case volume, and a longer bullet will decrease it. The amount of case-to-bullet friction also changes since you will have either more or less case holding the bullet by friction fit.

For any given powder charge and case, pressure will INCREASE with the weight of the bullet (more energy required to propel the bullet) and with the depth of the seating (deeper seating = more case friction = more energy to propel).

Follow precisely the load manuals, without exception. Every load in a manual has the exact bullet type and weight, the powder type and charge, the primer type, and the COL, which by the way is a "minimum" length not a maximum.
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