From post #33 . This appears in the "conclusion" of the document.
243winxb,
We are actually in agreement, I don't know if you are aware of that? You are referring to the pressure created by the detonation of the primer only and I'm only concerned as a reloader to the overall chamber pressure and signs there of. So the fact that ARL measures "increased" primer pocket pressure by firing only primers and also when igniting fully charged loads, they were only concerned with the first several milliseconds of the time/pressure curve only and not when the pressure of the main charge kicked in. Their graphs clearly show this and its pointed on in the textual comments around the graphs.
Lets look at the complete firing sequence just in case it is not clear:
- pull trigger
- firing pin goes forward
- crushed the priming compound and the "first" bang occurs
With a small flash hole this is the increased pressure that this test you keep referring to, is being evaluated on. If this same ignition to this point was done with a flash hole doubled in size to the previous example... you now have less pressure inside the primer pocket because the larger flash hole lets the pressure bleed off with less resistance. I'm in agreement here to this point. But remember you are dealing with a much smaller PSI measurement of this primer compound ignition pressure as compared to the much higher PSI pressures to be created on the other side of the flash hole inside the actual case once the main charge is ignited.
- flash of the priming compound goes through the flash hole and ignites the main charge
- reach full pressure
Your PSI starts to rapidly increase inside the case on the OTHER side of the flash hole. This is the difference I've been trying to point out. If you are using a small flash hole at this point and you have 55K PSI pressure inside the case, the smaller flash hole does not allow as much pressure to bleed back through the smaller flash hole onto the spent primer cup... i.e. smaller flash holes show LESS pressure sign on the primer cup from the overall main charge PSI measurement. Because after all, aren't we as reloaders concerned about the overall time/pressure curve of our loads and NOT that little bump in the pressure curve in the first few mili-secs?
Now take this same 55K PSI load for example and we drill out the same exact case that was fired above that had a .060" flash hole for example and open it up to .125" and fire the same exact load using the same lot number of primers and powder, etc. That larger flash hole at .125" will allow more expanding gases from the main charge to bleed back through the larger flash hole and the primer cup will show increased pressure signs because of this. Disregarding the fact that a too large of a flash hole possibly allows kernels of powder to get back into the primer pocket. I'm only giving measurements for the sake of argument here.
Yes in both cases the primer detonation caused its own pressure BEFORE the main charge. That pressure is increased with a smaller flash hole, BUT for the very same reason primer pocket pressure increased allows let less pressure from the main charge bleeds BACK through the same small flash hole main the main charge is at full pressure. That is the point I've taken since my first post.
And the bottom line is this: I can recreate my old experiments and drill out flash holes and almost create pierced or ruptured primers on demand. Its a repeatable test. Not recommended due to safety reasons but it can be done. And the fact that if what you say is true in that small flash holes create more pressure, then the OP's reported results should have been just the opposite of what he reported. The Lapua cases with the the smaller flash holes should have shown the pressure signs and NOT the Hornady cases.
Steve
I should have thought to take a pic first. Looked in that can two days later but they emptied it already.
