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Old May 7, 2012, 02:42 PM   #18
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 9,991
SL1,

.45-70 is a funny cartridge in that it's copper crusher and Piezo transducer pressure magnitudes are the same. The old Dr. Lloyd Brownell study shows 30,000 being a number below which the two are about the same in .30-06 loads of at least one component combination, and the at a magnitude of 28,000 for both CUP and psi, the .45-70 is close to that range. The crossover point isn't the same for all cartridges, though, because it's affected by the pressure sample port location. .30 Carbine, for example, is 40,000 CUP/40.000 psi in SAAMI's information. I don't find any other rifle cartridge where they're exactly the same like that, though for the 35 Remington and .444 Marlin the psi numbers are lower than the CUP number, same as with .357 Mag and .44 Mag in pistol (mainly) cartridges.


Quote:
Originally Posted by frumerious
I am a little concerned about throat erosion and "burning up the barrel" in general (is that just throat erosion?) but I figure the ultra-high-end loads won't be as accurate as the loads that are more like book max or a little below. So I won't be firing enough cartridges at those high levels to do any damage...I will work down past them quickly enough. I plan on firing 25 cartridges per load level as I am not a good enough shot for 1 or 2 groups at a given load level to mean anything.
How hot the loads are depend on as-fireformed water capacity of your cartridges. It also depends on the length of the particular 168 grain match bullet. There are differences between match bullet brands and, in .308 in particular, between capacities of different case brands.

If you read through my pressure signs, you'll find sudden deterioration of accuracy as you increment the charge is one of them. If you have a chronograph, you'll find that at the lower loads (which you can increment about 2% usually in a load work up) will usually give you very close to the same number of feet per second added velocity with each equal increase in charge weight. But if an increase fails to produce the expected velocity increase, it is a sign the metal is stretching and it's time to back down. That should happen in the 1895, too, based on McPherson's test.

Primer punctures and the like are tough indicators to trust as the cup thickness and hardness (plated vs. non-plated, for example) can vary and the shape of your firing pin nose can affect it.

40,000 psi is a commonly given as a Marlin pressure, but McPherson and several others have reported going over 50,000. Garrett ammunition keeps it down to 35,000 psi, and I've always thought that was a reasonable compromise between battering the shooter and performance.

In your .308, Geoffry Kolbe says throats burn out noticeably faster as pressure gets above 58,000 psi, so I don't like to max that chambering out. QuickLOAD suggests you could get the same velocities as you have been with 2520 using Reloader 17, but get there at about 5,000 psi lower peak pressure. It might be a good move to make.

Also, don't forget to put the hot load warning in when you discuss over-published pressure loads.
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