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Old May 2, 2007, 10:13 PM   #1
xjz
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Random Reloading questions

I have a couple random questions I've been thinking about for a few months and have searched a couple time not fully answering any of them,

1. Is the internal volume of the cartridge what really matters with respect to pressure instead of COAL? For example if I have a 200 gr RNFP bullet and a 165 gr RNFP bullet which has the same profile but .050" shorter in lenght can I make the 200 gr bullet into a cartridge with a COAL at 1.250" then make the 165 gr bullet into a cartridge with the same powder charge but make the COAL 1.200" and maintain the same internal pressure? Would the pressure still go down some because the bullet weight is less even though the internal volume is the same?

2A. Is there a calculation you can use to calculate the internal pressure of a round from the measured velocity and weight of the projectile?

2B. I understand most people use a Chrony for just measuring velocity so they can try to match their reload velocity to a factory round velocity. Do you do that to be sure your reload doensn't have too high of a pressure as the factory actually measures their pressures but there is no real way to back calculate pressure from velocity and projectile weight?

3. Are signs of excess pressure seen in a pistol cartridge (flattened primer, back extruded primer into firing pin hole, unsupported case bulge) around 25000 psi seen when you wouldn't normally see those signs in a rifle cartridge until it reaches 60000 psi, or am I completely wrong?

Thanks for any clarification you can give me on these random thoughts.
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Old May 3, 2007, 06:14 AM   #2
Geeko
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First off, use the reloading books for reloading data. Don't exceed the mfg max loads. Things to note when using the reloading books, check what the mfg used for their test rifle. check what cartridge details they used for their test data. Different barrel lengths will get you different speeds. Different cartridge details (e.g. RP vs LC or primers, OALs, etc) will get you different velocities and pressures.

1. The initial internal pressure pulse will tend to be different between the two due to the different weight bullets not necessarily the seating depths. The heavier bullet will require more pressure to get it moving. This is why in the loading data, the heavier bullets are loaded will less powder verse the lighter ones. Think how much energy you would need to use to push a small car verse a large semi to get them moving from the stop. It is the inital energy (pressure) pulse to get the object moving.

- A specific powder amount will produce a specific amount of energy or gas. That fixed amount of gas in a fixed amount of volume will create a specific amount of pressure. The type of powder will determine the rate the pressure is created. Fast burning powder will create a fixed amount of gas quickly vs. the slower burning powder which will create a fixed amount of gas slower than the other powder. Thus the pressure curve for the faster burning powder will spike faster than the slower powder.

- As the gas pressure increases, that pressure is applied to the base of the bullet. The weight of the bullet will determine how much energy is need to start it moving. Also including the amount of holding force of the neck of the cartridge that also needs to be overcome.

- As the bullet begins to travel down the barrel, the internal volume will increase, thus the pressure will drop. Again, it will take more energy for the heavier bullet than the lighter one to force either down the barrel at the same speed. Since the the fixed of amount of gas and pressure will have two different weights to push (assuming same powder being used), the lighter bullet will travel faster than the heavier.


2a. There is an online pressure calculator: http://kwk.us/powley.html


2b. The chrony is used to measure speeds only. You can get different velocities from what the books says. The book will give you guidelines. But again, it is highly recommended never to exceed mfg max. With the measured bullet speed, you can calculate bullet drop over distance. This is important when either hunting or target shooting at distances. The bullet drop for a 150gr at 2650fps will be different than a 168gr at 2650fps.

3. Pressure signs for pistol can be different from rifle and different between pistol type (auto vs revolver). In revolvers it can be the amount of force to extract the cases from the chambers of the cylinder. In Auto, some of the things you have stated. But again, don't exceed mfg max. Using max loads maybe fun for a few rounds but it puts wear and tear of the pistols. Above max loads, well, new gun, new hand, new face...for the gun blewup.

Hopefully this helps some....
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Old May 3, 2007, 12:03 PM   #3
cdoc42
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Geeko,
I read your response very quickly over lunch and I have just one observation.

The heavier bullet "requires more pressure" but uses less powder, on its face, doesn't match. If the heavier bullet required more pressure, you'd need more powder, not less.

I think it is more likely the heavier bullet is harder to get moving (as you mentioned in your example) and the delay in getting it started causes the pressure curve to reach a higher point than it would with a lighter bullet. Thus, if you did use more powder, the pressure would be even greater, and that would cause problems.

The pressure in the barrel might (I'm surmising here) drop at a rather equal rate after the bullet is moving so that the heavier bullet ends up with a lower velocity even though the initial pressure was higher. It's hard to get a 180gr bullet going as fast as a 130gr bullet, as we all know.

Good points over all, though and I'm going to look into that link you offered. Good discussion.
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Old May 3, 2007, 12:13 PM   #4
cheygriz
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Pressure is effected by:

Powder burning speed and quantity
Bullet weight
Seating depth (Oal)
Bore Diameter
Bore friction (smoothness, or lack thereof)
Groove depth
Bullet bearing surface area
Bullet (or jacket) hardness
Core hardness
Primer brisance
Case thickness (capacity)
Expansion Ratio

Uhhhhh...... IOW, unless you have access to a ballistics lab, follow the manual.
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Old May 3, 2007, 12:44 PM   #5
Shoney
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Combinations of factors can cause great variances in pressure. Here are a few factors:

Primer: strength, brisance - is a measure of the rapidity with which an explosive develops its maximum pressure
Barrel: length; tightness of bore; height of the lands; distance of bullet to lands; temperature of barrel;
Bullet: bearing surface of bullet, alloy of bullet; shape of bullet;
Brass: new/used elasticity; manufacturer, volume;
Powder: new, aged, old, batch powder was from;
Weather: ambient air temp., barometric pressure, humidity
Other: I am sure I have not listed all

Now, mix and match them. Anyone care to calculate the number of possible combinations?
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Old May 3, 2007, 12:52 PM   #6
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Quote:
Now, mix and match them. Anyone care to calculate the number of possible combinations?
Not I, sir!
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Old May 3, 2007, 02:44 PM   #7
xjz
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Thanks for the info everyone ther was a few things I didn't think about that effect pressure.
I'm with you cdoc42 on the pressure with heavier bullet idea that pressure builds more as its harder to get it moving than the light round so more powder would just increase pressures past safe levels.

Just to keep is straight, I'm not someone who likes to experiment, I NEVER shoot max loads, I'm often on the bottom of the scale where I barely have enough powder to get the slide to cycle. I reload for money savings so I can shoot more for the same amount of money. I don't have a chrony yet but I always stick with my reloading manuel numbers.
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Old May 3, 2007, 03:17 PM   #8
cdoc42
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If you don't experiment you'll never maximize the potential of your particular firearm. Shooting is much more fun when most of your shots are under 1 inch in diameter and even better when 0.5". I have a 6mm PPC that I shoot for fun and I got a 5-shot group at 100 yards that measures 0.19" The more you reload the more you'll find it's not just about saving money.
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Old May 3, 2007, 06:15 PM   #9
Geeko
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cdoc42...I guess I didn't explain it well enough then. The amount of energy need to move a 10lbs weight one inch in one sec vs a 1lbs weight one inch in one sec, it will take more energy to move that 10lbs the one inch in that one second. Thus, to move a heavier bullet to the same velocity as a lighter bullet to the same muzzle velocity will require much pressure/powder which would be more than likely overpressure in the chamber. Therefore, one reduces the amount of powder to keep the chamber pressure down and in doing so, the heavier bullet will have a lower muzzle velocity.

I don't know if this helps? Nevertheless, thanks for pointing out the confusion.
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