why do velocities at max load significantly differ by powder used?

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why do velocities at max load significantly differ by powder used?

Post by Aprilian on 2/23/2017, 1:43 pm

I was looking at the load data for Nosler 185 gr bullets https://load-data.nosler.com/load-data/45-auto-acp/ and saw that the velocity was dramatically different at each powder's max load.  For example, Titegroup's min load's FPS of 852 is faster than the max load FPS of 840 for Bullseye.   

I know max load is related to a safe max chamber pressure, but I don't understand why each powder's max pressure yields such a variety in bullet velocities since it is the pressure which pushes the bullet out of the barrel.   

Can someone point me to something to read that would explain this?   

Thanks
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Aprilian

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Re: why do velocities at max load significantly differ by powder used?

Post by dronning on 2/23/2017, 1:59 pm

Same pressure readings with different powders will not get you the same velocity.  Burn rate impacts speed too.

- Dave
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Re: why do velocities at max load significantly differ by powder used?

Post by Aprilian on 2/23/2017, 2:12 pm

Assuming the two different bullets release at the same internal case pressure... does that mean that the bullet from the case with the slower burning powder looses less pressure as it travels to the end of the barrel?  And does that higher pressure at barrel exit show up as higher velocity?
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Re: why do velocities at max load significantly differ by powder used?

Post by desben on 2/23/2017, 2:25 pm

https://en.wikipedia.org/wiki/Internal_ballistics#Pressure-velocity_relationships

A few things to keep in mind. How fast the pressure builds is related to the powder burn rate. Pressure is related to how much gas is produced by the powder, divided by the volume available. As the bullet starts moving, the volume increases, and the powder is still burning. Determining the final velocity is an integration problem (calculus!). Thankfully, there's software that can do it for you.
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Pressure versus Time Curve

Post by Richard Ashmore on 2/23/2017, 6:28 pm

Aprilian wrote:I was looking at the load data for Nosler 185 gr bullets https://load-data.nosler.com/load-data/45-auto-acp/ and saw that the velocity was dramatically different at each powder's max load.  For example, Titegroup's min load's FPS of 852 is faster than the max load FPS of 840 for Bullseye.   

I know max load is related to a safe max chamber pressure, but I don't understand why each powder's max pressure yields such a variety in bullet velocities since it is the pressure which pushes the bullet out of the barrel.   

Can someone point me to something to read that would explain this?   

Thanks

  The burn rate of the powder is related to the pressure versus time curve, which affects maximum pressure and muzzle velocity.  Slower powders have a flatter P-T curve, which is why they can yield higher muzzle velocities at lower peak pressures.
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Re: why do velocities at max load significantly differ by powder used?

Post by Wobbley on 2/23/2017, 7:27 pm

Slower powders allow more gas to be generated because more powder is burned.  The specific energy of powder is pretty much the same for any given chemistry of powder.  Same percentages of nitrocellulose, nitroglycerin and coatings will generate the same specific energy.  The difference is in the speed.  Slower powder allows for more gas being generated.
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Re: why do velocities at max load significantly differ by powder used?

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