Bart,
Some time ago Denton Bramwell concluded the strain gauge had the most reliable repeatability. From his article
PRE, CHE, RIP:
Quote:
σe is a figure of merit for a measurement system. It is the standard deviation of the random error in the system. Smaller is better. In terms of PSI, I have measured or calculated from published data the following σe numbers for a single cartridge test. This provides the simplest way of comparing several systems side by side.
{in decending order of measurment error-prone behavior}
CHE method, 7,500
PRE method, 6,800
Copper crusher method, 1827
Commercial piezoelectric, 1366
PressureTrace™ strain gauge, 667
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I read somewhere that Hornady is now using strain gauge gear in their routine testing. If you have access to reference loads, it is probably the most accurate way to work and is certainly more economical than crushers and transducers and, as the table shows, more than 10 times better than the pressure signs on cases that manual authors used back in the day.
Doyle,
I wrote that on the assumption the user wanted to stay within load data pressures. When you don't, you incur shorter barrel life and more fatigue of the gun steel. That can be worthwhile if the gun genuinely shoots more accurately that way or if you have a real need for a faster bullet, but it should be an informed decision.
If I want to stay inside SAAMI pressure numbers, the reasoning behind backing off the load from getting a higher velocity is this: If you load starting with data developed in a pressure test gun and are using the same barrel length, you will most often get lower velocity in a production gun, but occasionally you find one that gets higher velocity. Obviously, If your gun gets lower velocity, your gun is experiencing less pressure from the load than the test gun did. If your gun gets higher velocity, your gun is experiencing more pressure than the test gun did.
In the case of getting lower pressure, increasing the powder charge to match the published velocity (and this assumes you know your chronograph is accurate) will still leave you with lower peak pressure because, with higher powder charge, you are making more gas and therefore a greater portion of your total bullet acceleration is in the barrel
after the pressure peak. Thus, your peak pressure is still a little below the published one and that leaves room for going still higher without exceeding the standard limits. But in the reverse case, the opposite is true. Where you get more velocity than the load source publishes, even if you back the charge down to get the same velocity as the load source got, your peak pressure is still higher than the test gun saw because you now have less gas and therefore less post-peak bullet acceleration contributing to the final bullet velocity. Therefore, more of the velocity is being provided by pressure occurring up to the peak value, which therefore must be higher than the test gun saw. In this case you are incurring more throat wear and shortening barrel life as compared to the standard load so your peak velocity should be a little lower than book to avoid that. Probably about as much lower as your initial velocity was too high.
Incidentally, it can simply be your powder lot or different components that make the velocity difference. My main point is that if you match velocity with less powder in the same barrel length, your peak pressure value is higher than the book load produced.