just wondering, in most of my reloading books, the max loads listed for the same bullt can have different max pressures. what i mean is with one type of powder, the max load is listed at 62000, while some other powders are listed with max loads at 50000. both in psi. same bullet, same book. shouldn't the max pressures be the same?
diff max pressure-same book
- Thread starter bruno
- Start date
Different powders expand at different rates. Hence different pressures. - dan
Slower burning powders will normally build up pressure more gradually as opposed to faster ones.
Think of it this way - when powder ignites it expands and thus starts moving bullet forward but it doesn't all burn instantly, so the pressure grows, volume does too as bullet keeps moving. So, not so much of a spike in pressure compared to faster burning powder.
Also, with slower burning powders you can achive higher bullet velosity as it keeps pushing it longer.
Think of it this way - when powder ignites it expands and thus starts moving bullet forward but it doesn't all burn instantly, so the pressure grows, volume does too as bullet keeps moving. So, not so much of a spike in pressure compared to faster burning powder.
Also, with slower burning powders you can achive higher bullet velosity as it keeps pushing it longer.
rnbra-shooter
CGN Ultra frequent flyer
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For modern cartridges, max pressures are normally in the low 50,000's CUP range, or if PSI is used, in the low 60,000's PSI. Sometimes load manuals show both, a max load of one particular powder might give 52,400 CUP, and on the next line another powder's max load might give 61,300 PSI. (these pressures are pretty similar, BTW, even though there is no direct translation of CUP to PSI)
As to one max load being 62,000psi and another max load being 50,000psi, it could be that the 50,000psi load is maxed out for a reason other than "it's not safe to put in more powder than this". The most common reason could be that the case is full of powder and you simply can't put in any more.
For example, look up .308 Winchester at http://data.hodgdon.com/cartridge_load.asp
In the "150 Nosler BT" area, you will see:
IMR 4007 SSC max 46.0C 2648fps 48,000PSI
IMR 4895 max 47.3C 2920fps 57,700PSI
Both loads are probably at or near their volume limit; the "C" after the charge weight indicates a "C"ompressed load. They might be lightly compressed, or they might be at the upper practical limit.
In the case of the IMR 4895, it is quick enough burning that it is able to reach 57,700PSI, which is near the max specified pressure for .308 Win (which is in the low 60,000s PSI).
The slower burning IMR 4007 SSC, even with a full case of powder, reaches a pressure substantially less (more than 10,000PSI less) than can be safely handled.
No they shouldn't be the same. The data you're looking at is the highest pressure recorded for that load, not the SAAMI maximum pressure for that cartridge.
