Explanation for why heavier bullets get lighter charges

H

horseman308

New member
I have yet to reload a single cartridge, but in reading through my Lyman 49th in preparation for loading some .45-70 (it'll be used in a Sharps), I noticed that the powder charge decreases as the bullet weight increases. My initial expectation was that heavier bullets would have needed more powder to move, not less. Can someone provide a little info on why?

I'm thinking that it has to do with the inertia of a heavier bullet makes it slower to move, staying in the barrel longer and allowing more pressure to build. Thus, you'd have to use less powder to keep pressures from spiking to unsafe levels. Yes or no?

Is this also the case with black powder or just with smokeless?
 
Heavier bullets take up more space in the case. Which means it takes less powder to achieve same pressure as lighter bullets with more powder.
 
Yes the heavier the bullet the greater the pressure generated by a given charge. Most any cartridge, for this reason, is more "efficient" using heavier bullets....requires less propellant to achieve the same energy level as would be attained with a lighter bullet and a greater charge.
 
jwrowland77 said:
Heavier bullets take up more space in the case. Which means it takes less powder to achieve same pressure as lighter bullets with more powder.
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Nope. If it was that simple, I could seat a heavier bullet further out so there is equal space in the case and use the same charge as a lighter bullet.

How much science do you want?

Simple explanation:

Smokeless powder burns faster as pressure increases. Heavy bullets accelerate slower than lighter bullets. Slower bullets mean less space behind them which means higher pressure which means faster powder burn which means even more pressure. If there is enough powder to continue burning the pressure will exceed the strength of the gun barrel and it will blow up.

More complete explanation:

You need to understand 3 basic facts to figure out why heavier bullets use lighter loads.

Fact #1: Every gun is designed for a specific chamber pressure limit.

Those limits can be found here if you're interested:

http://saami.org/specifications_and_information/index.cfm

Fact #2:

About 400 years ago Isaac Newton figured out that FORCE = MASS x ACCELERATION (Newton's Second Law).

You can verify that here:

http://teachertech.rice.edu/Participants/louviere/Newton/law2.html

Fact #3:

The higher the pressure, the faster smokeless powder burns, therefore creating even more pressure!

SAAMI explains smokeless powder properties here:

http://www.saami.org/specifications.../download/SAAMI_ITEM_200-Smokeless_Powder.pdf

When it burns under pressure, as in a cartridge fired in a gun, smokeless powder produces very little smoke, a small glow and leaves very little or no residue. The burning rate of smokeless powder increases with increased pressure.
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The highlighted part is the key point.

Now that you know these 3 basic facts, it's pretty easy to figure out.

Burning powder creates pressure in the chamber and barrel of the gun. From FACT #1 we know that each gun is designed for a specific max pressure. We do not want to go above this pressure because we would prefer that our gun remain a gun and NOT turn into a bomb.

The pressure created by the burning powder exerts a FORCE on the base of the bullet. This FORCE is what causes the bullet to move. For example, a cartridge with a pressure limit of 20,000 PSI (pounds per square inch) would exert a force of 10,000 pounds on a bullet with a base area of 0.50 square inches at this pressure.

Since we know from Fact #2 that FORCE = MASS x ACCELERATION, we know that given equal forces (10,000 pounds in the example) the heavier bullet (more mass) must accelerate slower. This means that given equal max chamber pressures (equal forces), there is LESS space behind the heavier bullet than the lighter one at any given time because the heavier bullet is accelerating slower.

We also know from Fact #3 that more pressure means that the smokeless burns faster. This less space (therefore more pressure) behind the heavier bullet means that the powder will be burning FASTER thereby creating even MORE pressure behind the heavier, slower bullet.

If you had equal amounts of a given powder behind a heavy and a light bullet, the powder behind the heavier bullet would keep burning and creating pressure until the pressure could easily exceed the strength of the steel used in the gun, at which time the gun turns into a bomb and goes BOOM.
 
45 auto gave you your best answer so far. I'm glad you asked the question. Way too many new reloaders today want instant gratification and don't take the time to read the book. It's important that you start off knowing the basic concepts even if you don't understand them. Even the ones who are less technical on their answers understand the concept and need to follow the book in load development. Keep reading, keep learning, and stay safe. Have fun while doing it.
 
The muzzle energy of a bullet exiting the barrel is one half the mass of the bullet times the square of its velocity.
Decreasing the mass of the bullet decreases its muzzle energy and efficiency,
to maintain muzzle energy then,velocity must be increased and the only way to
do this is to increase the charge.
 
Horseman 308, I pulled down thousands of military surplus 8mm57 rounds. Most of the ammo had 49 grains of powder with a 150 grain bullet. The rest of the ammo had a 180 grain bullet with 45 grains of the same powder. The 150 grain ammo was traveling close to 2.900 fps. That is fast. Had I switched bullets as in had I seated a 180 bullet in one of the cases that was loaded for 150 grains I could have rendered the rifle scrap.

I pulled the ammo down because of the corrosive primers and the cases were suspect meaning too many of them did not survive the first firing.

F. Guffey
 
Its a matter of chamber pressure. Most listed max loads are max because they reach near max chamber pressure. If you increase bullet weight, or you increase the powder charge, you increase pressure. If you increase one, you have to decrease the other to maintain the same chamber pressure. Yeah, it's a little more complicated than that, but not much.
 
Thanks y'all. I love the scientific answers. Newton's law and the max safe pressure on any given gun were the two facts I knew. I did not know that smokeless burns at a different rate as pressure increases, but it makes sense.

Now, does black powder do the same thing, or does it burn more consistently/equally?
 
Now, does black powder do the same thing, or does it burn more consistently/equally?
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As luck would have it, someone answered that very question in another thread this very afternoon.

Apparently, BP burns at a consistent rate be it compressed in a chamber or in a pile on the floor.
 
I would suggest you read the Lyman 49th manual cover to cover before doing ANYTHING, the question your asking is a very fundamental question that could have been answered by reading first before asking. Often times we want to shorten the learning curve but it is best to start at the first step before progressing to something more complicated. Reading the book will help you to develop the vocabulary and understanding of what we do plus this is a hobby that could damage your firearm or injure you if you screw up, the more you know the safer you will be. Think of it as the infant stage that you will eventually make it thru to adulthood without to many scars.. You might try finding someone in your area to serve as a mentor to assist you and shorten the learning curve, the more you know the safer you will be which is a huge plus.. William
 
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I wouldn't limit the major factor to bullet weight.

Heavier bullets are longer bullets, which means more bearing surface, which increases pressure. Sometimes this can be seen in the loading manuals where a 190 gr. boat tail bullet will accept a greater charge weight than a lighter 180gr. flat base bullet.

Bullet construction/hardness is also a contributor. This can be seen where charge weights for a solid copper bullet are less than those of a lead core bullet of the same weight. Again, the solid copper bullet generally has greater bearing surface.
 
I would suggest you read the Lyman 49th manual cover to cover before doing ANYTHING, the question your asking is a very fundamental question that could have been answered by reading first before asking.
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The OP did read Lyman's 49th. It was in doing so the OP noticed the trend that led to the question we're discussing.

Whilst I too recommend reading that or another reputable manual, I see nothing wrong in asking.

For example, I found info on crimp less than satisfactory in manuals and sought clarity on here.
 
In fairness to William, I probably didn't read it as close as I could (should) have, and I'll take the good advice to go back to the book. I will say that I often find myself questioning whether I've understood things correctly, and getting clarification from places like this is immensely helpful.
 
Horseman308 - Thanks for posting this question. I just had a conversation with the people at my local gun supply store about this topic.
 
In fairness to William, I probably didn't read it as close as I could (should) have, and I'll take the good advice to go back to the book. I will say that I often find myself questioning whether I've understood things correctly, and getting clarification from places like this is immensely helpful.
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I think you'll find out that the answer you get to a lot of questions asked here is "get a manual and read it" no matter how many manuals you own and have read. This is what makes me reluctant to ask questions in reloading forums. That statement makes me feel like folks think I'm ignorant in reloading even though I own 4 manuals and have read them several times but still have questions.
 
If you use Newton's math you can see that F=M*A, if F=35,000 psi and M=115 grains and A=4.6 grains then 35,000 divided by 115 gives you 304.3479 or 37,339 psi if you use the same amount of powder for a 124 grain bullet.

You need to use 93.74 % of that 4.6 grains of powder to obtain the safe 35,000 psi or 4.3 grain of powder for a 124 grain bullet.

You are always trying to stay within a safe pressure range to keep you or your gun from having to go to the hospital to be fixed.

Stay safe.
Jim
 
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If you use Newton's math you can see that F=M*A, if F=35,000 psi and M=115 grains and A=4.6 grains then 35,000 divided by 115 gives you 304.3479 or 37,339 psi if you use the same amount of powder for a 124 grain bullet.
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This is absolute bunk.
First: Force F is not measured in psi,that is the unit for pressure P, force is measured in Newtons that is one Kg at one meter/sec squared.
Second:A stands for acceleration,that is m/sec squared not grains which is a unit of mass.
Please check your facts before you post such nonsense.
 
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