Bullet/charge combos

[smokjunkee]

Hello all,
I'm a first timer just getting started and in looking at some starting loads it seems that the heavier the bullet, the lighter the charge. Now this just doesn't make sense to me. Can anyone shed some light on the subject.
SJ

 

[tbtrout]

The heavier the bullet the more space it takes up inside of the case. If you used the same amount of powder as with a smaller bullet there is less space for combustion to happen and the result will be higher pressure,which could damage your gun or you.

 

[Walkalong]

You only have so much pressure to work with and cannot push a heavier bullet as fast within allowable parameters. (case capacity as tbtrout said) To push that heavier bullet substantially faster we need a bigger case.

We can, of course, push a heavy bullet a bit faster with "slow" powders vs "fast" powders in the same caliber.

 

[smokjunkee]

Thanks for the replys, more to think about. I've read the Lymans & Lees manuals & I guess I missed the chapters on case space and how it works. Here I thought that you were getting your bullet propulsion from powder charge alone but, it's pressure build-up also. It's starting to make a little sense. I'm going solo on the adventure with only manuals for guidance, so the forum is a great help. Thanks again!
SJ

 

[Shoney]

This is an over simplification, because the volume is costantly changing, but:

The state of an amount of gas is determined by its pressure, volume, and temperature according to the equation:

pV=nRT

where

p is the absolute pressure [Pa],
V is the volume [m3] of the vessel containing moles of gas, here it is the volune you allow in the shell case
n is the amount of substance of gas [mol], here it is in solid form as powder
R is the gas constant [8.314 472 m3·Pa·K−1·mol−1],
T is the temperature in kelvin [K].

Think of the amount of gas n as being in solid form as powder until the "big bang".

Now do some math. Presume you are shooting your pet load

1. Increase the amount of gas n (powder that produces the gas), while R and T remain constant,then remembering that the volume V remains constant (at least initially). That means the pressure must increase to match the increase in gas.

2. Back to the pet load, Now decrease the volume V (by seating the bullet deeper) and keep the gas n the same, in order for the equation to ballance, the pressure p must go up.

Now, isn't that as simple as pi R round.

 

[Walkalong]

It's all clear now.

 

[Shoney]

Sorry about that Walkalong. I could have been more clear had I used 6751 words to say what I said in less than the 200 above. But then I feared that a lot more reader would have to DucTape their head and treat the ailment with large amount of Medicinal Icein order to understand.

The only thing I shoud have added to that post was "what you do to the numbers on one side of the equation affects the numbers on the other side".

Caveat Emptor - my dear departed grampa used to liberally translate that phrase to read, "Never trust a naked used chariot salesman."

 

[Walkalong]

"what you do to the numbers on one side of the equation affects the numbers on the other side".

I do remember that much from math class.

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What did you learn in school son? "Pi R square" the young man told his father. Gosh, I bought him books and sent him to school and he didn't learn a thing. Everybody knows Pie are round.