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308Loader said:
Generally speaking, pressure is related to velocity, I know there are variables, but... At approximately what velocity with this setup do I cross the line into +p?
There's no fixed answer to that. When the velocity is equal, a slow powder produces less peak pressure than a fast powder does. All the equal velocity prooves is the average pressure on the bullet base during its travel down your barrel was the same. Here's a sort of ridiculously extreme example. It would not work well in real life because 4227 will not be modeled correctly by the program at such low pressures, but it demonstrates the principle.
We have kind permission from Ed Dillon at NECO, which controls the QuickLOAD copyright in the U.S. market, to put up the occasional output table from QuickLOAD.
Since the +P designation is determined only by the peak value of pressure, and not the velocity the powder produces from it, there is no fixed velocity associated with it.
In your test of 4.8 grains, if you have the average (mean) velocity of the test, divide it by 4.8 to get the average velocity increase per grain of your lot of 231 powder. Multiply that by the difference in the velocity between your 4.8-grain load and the velocity listed for a maximum charge fired from a 4" revolver barrel in the Lyman or Hornady data. As long as their velocity is higher for the same amount of the same powder, your average pressures are lower than theirs. But if it looks like you will get more velocity from that amount of powder, your pressures will be higher.
Example:
Suppose your average velocity for 4.8 grains was 825 fps. 825 fps/4.8 grains is 172 fps/grain. Hornady says 5.4 grains should produce 900 fps. The difference between 5.4 grains and 4.8 grains is 0.6 grains. If I multiply 172*0.6, I get 103 fps added velocity. If I add that to 825 fps, I get 928 fps. This is faster than the 900 fps Hornady says you should get, so your lot of powder would produce higher pressure than theirs did.
So, apparently, your lot is faster burning than Hornady's was. This means that even if you were to match their velocity, your pressure would still be higher than theirs was. At 5.4 grains, it appears you would have average pressure that is about 6% higher than theirs. QuickLOAD, with burn rate, tweaked to a velocity match, suggests your faster powder lot would match Hornady's pressure at 5.17 grains, with a velocity of 896 fps. Using the simple 172 fps/grain number suggests that velocity will be reached at 5.21 grains. 0.05 grains is pretty good agreement and well-within shot-to-shot powder dispensing capability, so I would judge 5.2 grains to be a good number to work with as your non-+P charge weight
IF 825 fps IS YOUR ACTUAL AVERAGE VELOCITY FOR THAT TEST. If not, we have to do that over again.
Note that there is no safety issue with getting to +P as SAAMI now uses the same proof pressure range for both standard pressure and +P in the 38 Special. So if you slip ever so slightly past standard and toward +P no physical danger will arise.
