Quote:
Originally Posted by glowe
I think we are talking about two different issues. The number that SAAMI documents is a single pressure and not a time lapse pressure gradient throughout the barrel. Peak pressure rise with the addition of more powder is what I consider linear. Your graph and article refers to a pressure curve over time. As a bullet moves down the barrel, the area behind continues to increase, relieving the pressure and resulting in a lower pressure at the muzzle. I refer only to the maximum instantaneous pressure that will increase at a constant rate as powder charge increases.
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We are talking about the same thing:
Peak pressure rise with the addition of more powder. It is approximately exponential, not linear. I debated about including the first reference, since the article includes a graph of pressure change as the bullet moves down the barrel , but that is NOT what I am talking about.
Several factors change as the powder charge inceases, such as efficiency of combustion, expansion ratio, % filled, and
even the rate the powder burns. So while common sense says if I add X amount of powder the pressure increases by Y regardless of what pressure I start with, it just isn't true. Common sense does not work in internal ballistics, where the rate of change of one factor changes the amount of another factor.
Adding more powder also makes more of the powder burn (efficiency), and makes the powder burn faster,
effectively increasing the peak pressure as if you had changed to a faster powder. Thus at low pressure, 6% increase of powder might give 6% increase in peak pressure, while near max recommended pressure, the same increase in powder gives 20% increase in pressure. At even higher pressures , the same increase in powder can cause 50% increase in pressure, which can cause failure.
The change of burn rate of powder with containment is the joker in the deck of powder burn rate charts, and why powders appear in different order depending on the specifics of the test, and why they cannot be applied to specific cartridges.
Internal ballistics is not only more complicated than most understand, it is perhaps even more complicated than most can understand. That's why we all use specific load charts generated by actual pressure lab equipment. I have done years of calculus classes, teach college math and physics, and understand the equations, but I still use the load charts, and am perhaps more aware than most of how dangerous it is to wander off tested data and guess that a load is safe.
We haven't even addressed such issues as pressure waves and nodes during combustion, which are factors in such rare observed anomalies as loading a bullet LONGER or reducing a powder charge and having the pressure go UP, contrary to "common sense." Normally, we expect it to go down, and usually it does. But not always.