243winxb
Member
No, not odd at all. In post #13 i said >
In other words, the brass has not yet expanded to the TRUE SIZE of your chamber.
.223 Backed out primers
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You've already noted that the generated pressure is not enough to stretch the overly-resized case head back to the bolt face to reseat the primer. It does have enough pressure, however to swell the case to the chamber walls. This swell-but-not-stretch situation causes the case to actually shorten lengthwise by a bit to maintain constant volume. Perfectly natural (and you're seeing the evidence youself).
As far as pressure goes, Quickload calulates a 75gr Hndy HPBT over 22gr of Varget in a "standard" 28.8cc case volume at 2.25" OAL will generate 46,500psi -- more that enough to strech the case -- at 70 degreeF. Even though Varget is supposed to be a temperature-stable powder, I suspect Zero-degree F weather is driving what you're seeing. (QuickLoad can also calculate a gross temperature effect that could be as much as 5,000psi less for that zero-degree temp). BTW: This is one reason not to develop loads in the Winter to shoot in the summer
If you are not already doing it, try a magnum primer under 23.0gr VARGETbefore you start up'ing the powder charge significantly over that, and see if you get a different result. Either that, or try again with an equal weight of IMR4895.
Finally, to translate CUP to PSI, see here: http://www.shootingsoftware.com/ftp/psicuparticle2.pdf
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post script: I'd not poo-poo Hornady's Max Load info (of 23.5gr VARGET). Quickload tells me that can generate a bit over 57,000 psi is a 55k-rated cartridge -- one not noted as being too kind to primer pockets to begin with. Run that load on a hot 105-degree day and you're looking at ~61,000 psi. Naturally, chamber size can lower that a bit, but only a chronograph will tell you where you sit on that curve.
As far as pressure goes, Quickload calulates a 75gr Hndy HPBT over 22gr of Varget in a "standard" 28.8cc case volume at 2.25" OAL will generate 46,500psi -- more that enough to strech the case -- at 70 degreeF. Even though Varget is supposed to be a temperature-stable powder, I suspect Zero-degree F weather is driving what you're seeing. (QuickLoad can also calculate a gross temperature effect that could be as much as 5,000psi less for that zero-degree temp). BTW: This is one reason not to develop loads in the Winter to shoot in the summer
If you are not already doing it, try a magnum primer under 23.0gr VARGETbefore you start up'ing the powder charge significantly over that, and see if you get a different result. Either that, or try again with an equal weight of IMR4895.
Finally, to translate CUP to PSI, see here: http://www.shootingsoftware.com/ftp/psicuparticle2.pdf
.
post script: I'd not poo-poo Hornady's Max Load info (of 23.5gr VARGET). Quickload tells me that can generate a bit over 57,000 psi is a 55k-rated cartridge -- one not noted as being too kind to primer pockets to begin with. Run that load on a hot 105-degree day and you're looking at ~61,000 psi. Naturally, chamber size can lower that a bit, but only a chronograph will tell you where you sit on that curve.
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