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This is a question I have wondered why the heaver the bullet the lighter the powder load, Like this 168gr sierra MK I load at 42gr R-15 and 175 MK would load 40gr of the same R-15. You would think the heaver the bullet the more powder it would take?? Some of you pros's put some light on this for me..
the bigger bullet sits deeper in the hull for one, and it takes more pressure to get it started because it is heavier.
It's all about pressure. As the bullet weight goes up the powder charge must go down to maintain the same pressure.
Charlie
Let me explain it differently.
As the bullet moved down the barrel, the volume of the space behind the bullet increases. We want the pressure to increase quickly to the SAMMI maximum pressure and to maintain the pressure as the bullet moves down the tube. Therefore the powder needs to be the slower burning type which will supply the hot gasses to keep the pressure up as the volume expands. A light bullet accelerates faster than a heavy bullet, therefore you need to be burning more powder (and a faster burning powder) to keep the pressure up.
The heaver bullet with the same powder charge as a lighter bullet will not accelerate as quickly, but the additional powder will supply more hot gasses and the resulting high pressure can be a major problem. When I just started reloading when I decided to change the weight of the bullet from 110gr to 125gr in the .357. I didn't change the charge and the result was flat primers and difficulty ejecting the brass. I was way over pressure, but the revolver was forgiving.
It does seem strange that a lighter bullet uses more powder. Also as the heaver bullet stays in the barrel longer, the pressure is acting on it longer, therefore it had more energy for less powder!
Good explaination Warthog.
Quote:
Originally Posted by Warthog
You got it!
Diesel Tech... sent that info to you in a PM. If you have any questions, let me know.
K. Schilling