Correct. If you read FALPhil's article carefully you will realize the table posted by Loader9 uses the old terminology from when PSI was still given to mean what we now call CUP, since they originally didn't know there was a significant difference. I have a number of older (80's and thereabouts) military tech manuals that make the same error. AFAIK, the whole issue wasn't even seriously sorted out in the laboratory until the DuPont sponsored U. of Michigan study of the '06 with round nose bullets in 1965, Absolute Chamber Pressure in Center-Fire Rifles, that determined absolute pressure by strain gauges. You can download a
PDF version here, but the salient plot, page 43, was apparently cut off in scanning the PDF version. I have a hard copy I will scan if I remember to? What is missing is the top portion showing that when absolute pressure was 100,000 PSI in their test gun, the copper crusher reading was only 67,000 PSI. This was the start, I believe, of copper crusher numbers ultimately losing the right to be called PSI, and coming to be called CUP instead. The copper crusher was good enough at black powder cartridge pressures and makes a good way to match pressures for different loads, but that's it. It only took 30 years for this correction to get into military documentation. They were still calling CUP PSI until then.
wogpotter said:
A note on "pressure differences".
The same bullet, driven at the same velocity from the same rifle will have the same pressure, it's simple math. You're compensating for the differences by reducing the powder charge. You WILL have pressure differences if you use the same powder charge for both.
Not correct the way most people think of pressure. What you said is true for the
average accelerating force provided by the
average pressure in the barrel during the time of the bullet travels in it (the barrel time), but it is not true for the
peak pressure in the chamber which can be varied somewhat independently of average pressure by changing the powder you are using to one with a different burning rate.
An example:
IMI case with 56.2 grains water capacity in gun with tight chamber and 22" barrel firing the Hornady 150 grain BTFMJ seated to 2.780" col:
IMR 4198, 38.5 grains, 58,600 psi peak chamber pressure, 2,750 fps MV
Muzzle pressure 6,880 psi
IMR 4895, 44.8 grains, 49,700 psi peak chamber pressure, 2,750 fps MV
Muzzle pressure, 8,360 psi
Average accelerating force produced by the two loads will be the same.
Case capacity also has an effect even if you are using the same powder to reach the same peak pressure, just because you can fit more powder in a larger case, which makes more gas, which sustains higher pressure longer past the peak. That increases the average pressure applied at the base of the bullet during its barrel time.
An example:
Loading the Hornady 150 BTFMJ to 52,000 PSI with IMR 4895 in a 22" barrel at 2.780" COL:
Winchester case, 59.7 gr water capacity
47.1 grains 4895, 52,000 PSI peak chamber pressure, 2,837 fps MV
muzzle pressure at bullet exit: 8,850 psi
IMI case, 56.2 grains water capacity
45.0 grains 4895, 52,000 PSI peak chamber pressure, 2,788 fps MV
muzzle pressure at bullet exit: 8,450 psi
Same peak chamber pressures, but about 50 fps MV difference because the larger quantity of powder in the bigger case sustained pressure longer.
Nick
