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Old May 19, 2020, 09:27 AM   #1
Gawp
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What is the "Shot start pressure" in QuickLOAD?

Hi,

I've just bought QuickLOAD and I'm doing simulations for a .30-30 Win Subsonic load.

In the "Charge" window, there is a setting named "Shot Start (Initiation) Pressure".

From what I understand, this is the minimum needed pressure to push the bullet into the barrel and engrave the bullet into the rifling. So it depends on the material of the bullet (some materials may be softer or harder). Is this correct?

Then, why does QuickLOAD suggest to "try to lower shot start pressure" when we don't set a high enough charge of powder in the settings? I mean, it seems to me this "Shot Start Pressure" is a constant we cannot change in real life, so why even bother trying to tweak it in QuickLOAD?

Also, what could go wrong (in real life) if the "Shot Start Pressure" is too low? The bullet will just get stuck into the barrel?

Last edited by Gawp; May 19, 2020 at 09:57 AM.
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Old May 19, 2020, 03:31 PM   #2
Unclenick
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Start pressure in QuickLOAD is the pressure in the case when the bullet starts to move in the model. The default number is one that tends to make the model work as accurately as it can with the bullet hardness and friction level expected. It is not a real start pressure number as you will find in your gun because of dynamic complications. Real start pressures for starting the bullet moving vary with how fast pressure builds in the case and then how far the bullet jumps to the throat where it meets added resistance and so its acceleration is mitigated and then it starts accelerating more again as the pressure continues to increase. Real start pressure in high power rifles is typically higher than the QuickLOAD model uses. In some low pressure cartridges the real start pressure is lower.

The whole interaction with the throat is not directly modeled in either program, though you can mimic the effect of, for example, loading a bullet that jams into the throat by raising the start pressure by adding 7200 psi to it. Then you get a correct rise in pressure of about 20%. If you get very close to the throat (short jump) you need to gradually raise the start pressure for realistic modeling. It's not precise, but starting at about 0.020" of bullet jump, I add 360 psi to start pressure for every 0.001" closer to the lands the bullet gets to try to compensate for the pressure rise occurring from the reduction of gas bypass around the bullet.
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Old May 19, 2020, 05:25 PM   #3
Bart B.
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https://www.google.com/search?q=shot...obile&ie=UTF-8

If a 30 caliber bullet needs 20 pounds of force to push it out of the case neck, it'll take about 260 psi in the case to move it.

Formula is (1/bullet .0765 cross section area in inches) X bullet's 20 pound push force needed.

About 13 X 20 = 260 psi.

It'll take a few times that to fully push the bullet into the rifling.
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Old May 20, 2020, 08:56 AM   #4
Gawp
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Quote:
Start pressure in QuickLOAD is the pressure in the case when the bullet starts to move in the model. The default number is one that tends to make the model work as accurately as it can with the bullet hardness and friction level expected. It is not a real start pressure number as you will find in your gun because of dynamic complications. Real start pressures for starting the bullet moving vary with how fast pressure builds in the case and then how far the bullet jumps to the throat where it meets added resistance and so its acceleration is mitigated and then it starts accelerating more again as the pressure continues to increase.
Thank you Unclenick! Could you explain why QuickLOAD sets a limit for the Shot Start Pressure number? For example, the default Shot Start Pressure for a Standard Jacketed Rifle Bullet is 3625 psi. However, if I set a low charge (e.g. 5.09 gr of Vectan A1 for a 170 gr bullet), QuickLOAD will show a popup: "Shot start pressure for this data too high" and I will need to lower the Shot Start Pressure from 3625 psi to about 2769 psi.

What does this translate to in real life?

Quote:
If a 30 caliber bullet needs 20 pounds of force to push it out of the case neck, it'll take about 260 psi in the case to move it.

Formula is (1/bullet .0765 cross section area in inches) X bullet's 20 pound push force needed.

About 13 X 20 = 260 psi.

It'll take a few times that to fully push the bullet into the rifling.
Thanks for your answer. Do you know where this formula comes from?
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Old May 20, 2020, 02:34 PM   #5
Bart B.
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Formula is one of the basic hydraulic ones: pressure per square inch = force in pounds/area in square inches.

If an inflated car bare tire supports 1000 pounds of weight with 31.623 psi air pressure, its contact patch is 31.623 square inches.

Last edited by Bart B.; May 20, 2020 at 02:51 PM.
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Old May 20, 2020, 03:09 PM   #6
Gawp
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Quote:
Formula is one of the basic hydraulic ones: pressure per square inch = force in pounds/area in square inches.

If an inflated car bare tire supports 1000 pounds of weight with 31.623 psi air pressure, its contact patch is 31.623 square inches.
Oh thank you, makes sense!

Do you have any idea about my question above?

Quote:
Could you explain why QuickLOAD sets a limit for the Shot Start Pressure number? For example, the default Shot Start Pressure for a Standard Jacketed Rifle Bullet is 3625 psi. However, if I set a low charge (e.g. 5.09 gr of Vectan A1 for a 170 gr bullet), QuickLOAD will show a popup: "Shot start pressure for this data too high" and I will need to lower the Shot Start Pressure from 3625 psi to about 2769 psi.

What does this translate to in real life?
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Old May 20, 2020, 03:35 PM   #7
Bart B.
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Call Quickload support at 218-722-3113 but be patient for an answer.
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Old May 20, 2020, 07:15 PM   #8
Unclenick
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QuickLOAD assumes the start pressure listed in order for the powder model to produce a realistic burn and pressure curve. If you lower the start pressure too much, you'll find the rate of rise in pressure is much slower to develop and it no longer matches the shape of the curve you get making an actual pressure measurement. I can tell you from having used the Pressure Trace instrument to measure a Savage ML10, a muzzleloader that used smokeless propellant and employed a very low friction polyolefin sabot, that I needed to drop that start pressure to zero to get curves in QuickLOAD that matched the shape of the curves I got from the actual gun. So what the starting presure number actually is, is a compromise that describes the net effect of the force needed to break the bullet loose, plus the force needed to overcome its inertia and get it moving in the very short period of time it actually happens in, plus some portion of the force needed to assist it in its entry into the lands.

If you measure all that stuff statically (approximates everything moving slowly), you would, indeed, get Bart's 260 psi number for breaking the bullet loose and you would get about 15,750 psi to produce the 1200 pounds for forcing the bullet into the rifling at a slow rate afterward. In reality, in tests done by H.P. White Laboratories in the '70s, the bullet in a high power rifle cartridge could not be detected to have started moving until pressures are in the 10 to 15 kpsi range.
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