Powder Variations...Why?

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nrobertb
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Powder Variations...Why?

Post by nrobertb »

For any given cartridge the loading tables show a heavier load for a lighter bullet and vice-versa. This seems counter intuitive. You would think that the heavier the bullet, the more powder it would take to push it out of the barrel, but not so.

Can anyone explain the scientific reason behind this apparent anomaly?

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Cattus Borealis
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Re: Powder Variations...Why?

Post by Cattus Borealis »

Max pressure.

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nrobertb
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Re: Powder Variations...Why?

Post by nrobertb »

Could you elaborate a bit?

DaleH
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Re: Powder Variations...Why?

Post by DaleH »

The force required to squeeze the heavier bullet down the barrel takes a lot more force. Consider bike riding on level ground in an easy gear ... but now leave it in that gear and go up a steep hill. How much extra force would you need to provide or feel in your legs to maintain a decent speed?

Said another way, the degree of effort (pressure) is proportionate to the load (weight) your trying to move.

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Re: Powder Variations...Why?

Post by Rapidrob »

The heavier the bullet, the longer it is and a tighter fit into the bore, hence the greater friction and resistance to being moved down the bore. Now add the extra weight and you must reduce the powder charge to prevent exceeding the maxim safe chamber pressure.
It all has to do with Newtons 3RD Law, and the laws of thermodynamics as well as a host of other laws.
So, a bullet weighs 50 grains, it has a short metplath, so less friction to move it down the bore, the rifling twist does play a large factor in the total amount of friction to move as well and spin the bullet. The calculated powder charge will build up to its maximum pressure in a known micro second amount. The bullet moves before this happens. The split second the bullet starts to move the volume increases and the pressure starts to drop.
Now the same powder charge is used for a 150 grain bullet. The bullets metplath is much longer,adding much more friction and a much better gas seal The bullet is also heavier. A body at rest....
Now the powder is still burning at the same rate. In the same time period the pressure increases to where the 50 grain bullet started to move. The 150 grain bullet does not, or at least cannot accelerates as fast, the powder builds more pressure, the more pressure the more heat, the more heat the better the combustion and the pressure will go from a normal operating pressure to an abnormally high pressure and will exceed the safe limits for the cartridge case,primer, and in the worse case,the breech of the firearm. Something has to give.
So the powder charge must be reduced to prevent the gas pressure from exceeding safe operating limits for the heavier bullets/increased time to get the bullet moving overcoming the bullet weight/friction and increase the breech volume to control the maximum pressure being built up.
Smokeless powder is a solid fuel. It is a propellant when loaded properly. Confine the smokeless powder for too long of a time, it is a low order explosive. The amount of energy per given weight/time/volume/temperature /friction of the bullet is a know factor. Alter any of those main factors and bad things can happen real fast. (there are at least 15 factors that I know of)
Case in point. Take a safe load fired in a 1:12" twist barrel and Fire it in a 1:7 twist barrel and you can blow primers, very hard to open the bolt,the fired case is badly dented by the ejector when ejected or even has marks from the face of the bolt. All because the 1:7" barrel increased three factors. Friction and the time to spin the bullet faster and move it down the barrel increasing volume.
I hope this makes some sense. When you pull the trigger there is a lot more going on then explained here. This is just a brief description on what is really going on.
More of the same powder,heavier bullet,=KaBOOM
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