You heard wrong. That would defy the laws of physics. The volume/pressure relationship is very well understood in a system like a cartridge case and the only way this would hold true is if the bullet had initially been seated hard against the lands. In that instance giving it a little room to initially accelerate by seating it deeper might lower pressures as compared to before, but it would raise pressures otherwise because the volume inside the case was reduced.
Well stated.
I soon discovered that a "flaw" in QuickLOAD is that it takes only the internal ballistic of the cartridge into account UNTIL the loaded on the lands. Thinking about it is would be nigh impossible for QL to calculate this due to the varying angle of the lands vs. the ogive shape so no criticism of QL here.
QL does not take the effect of approaching the lands into consideration. So QL tells you to add some 7 200psi to the Start Initialisation pressure if seated on the lands but does not offer any advice as you get closer to the lands and we know the effect of inertia will increase as we approach the lands.
So I loaded up a bunch of 7.62 rounds at varying COL's and headed to the range get some data to plot. I reworked the average speed back to a pressure and plotted the pressure in psi vs. the distance from the lands in mm.
Here is what I discovered, forgive the metric units.
As you set the bullet further out the pressure drops, well no surprises here. But as one approaches the lands that the pressure then started to increase in an exponential fashion, despite their being more volume in the case as you are now introducing an external resistive component. It is for this reason that playing close to the lands must be done with respect.
I have two plots, the "cartridge pressure" is QL and the "inertia pressure" is the plot from actual results. Clearly one can see that from 1mm (0.040") the pressure begins to rise. So in this example the pressure difference for the same load but seated 0.040" off vs. on the lands is 6 000psi, close to the QL allowance).
I must state here that I DID NOT shoot the last result off the lands, I assumed QL to be correct and I merely plotted the last result fictitiously. It may be that the actual pressure increased more than QL. I have never shot off the lands and am reluctant to do so, so forgive the last data point that I manipulated.
Whats the ES stand for?
