Re: Question about pressures
Probably something to do with burn rates and pressure curves. I jinned up these in QuickLoad using the first whole grain below SAAMI max:
I hadda shrink them to make them presentable so they've lost some detail so here's the numbers:
IMR 4320, #72 on powder burn rate chart
PMax: 59,598 psi
Distance down barrel: 1.2"
Velocity @ PMax: 934 fps
Time to PMax: 0.346 ms
Pressure @ Z1: 58,896 psi
Distance to Z1: 1.6"
Velocity @ Z1: 1084 fps
Time to Z1: 0.376 ms
Hodgdon Varget #105 on powder burn rate chart
PMax: 57,912 psi
Distance to PMax: 1.2"
Velocity @ PMax: 916 fps
Time to PMax: 0.352 ms
Pressure @Z1: 57,771 psi
Distance to Z1: 1.1"
Velocity @Z1: 834 fps
Time to Z1: 0.339 ms
QL, for whatever reason, sees no need to limit Varget to 50K psi.
I'm just guessing but a couple of possibilities come to mind.
One thing that sticks out about this comparison is that Varget's Z1 occurs considerably sooner than 4320's. Z1 is the point at which the production of gas ceases to accelerate (transitions from progressive burning to digressive burning). In fact, Varget experiences Z1 <span style="text-decoration: underline">before</span> it reaches PMax. 4032 experiences Z1 considerably <span style="text-decoration: underline">after</span> PMax.
I backed the Varget charge down to one that only had a PMax of 50K psi and the Z1 <span style="text-decoration: underline">still</span> occurred before PMax but the time difference narrowed from the 0.013 ms above to just 0.008 ms. The 50K Z1 also occurs after 1.2" of bullet movement (vs the 1.1" above), which makes 9% more bore volume available for combustion. That makes me wonder if Hodgdon knows something evil (for lack of a better term) happens when there's so little volume available and Z1 is so far in front of PMax. Like maybe the powder would detonate.
Another possibility is that is has something to do with bullet length and obturation. I didn't bother to measure one but according to QL, the 168-gr SMK is 1.215" long. Maybe Hodgdon was manipulating the Varget pressure curve to delay Z1 until the bullet has moved nearer its own length as a means of accounting for differences in seating depths and to ensure that full obturation has occurred before digressive burning begins.