NOTE: I was composing when you answered the shot count number was 10. You can find it on the table below. I'll leave that table up as the ratios for shot counts hold for anyone.
Once you get the arguments correct, the ballistics program repeats the calculation the same way each time, getting the same result each time, so it's standard deviation is alway zero, too, no matter how many times you run that calculation. It uses a mathematical model to attempt to find your average or mean value.
Velocity standard deviation is a measure of how irregular your velocities are that lets you predict what percentage of all future shots you take with that load will have what velocities. Since that irregularity depends your primer seating technique, your powder throw consistency, your crimp consistency, and upon how settled fouling in your barrel is, upon whether or not your cases are the same brand and have the same reloading history (number and pressure of reloads) so the hardness of the brass is the same, upon your cylinder's chambers being exactly the same size and aligned equally well with the bore, etc., we have no way to know that from a computer calculation for an idealized gun with perfect load consistency, which is what the computer calculation represents.
On a bell curve, average deviation is about 1.25 times bigger than standard deviation, so if you have 8 fps average deviation you'd expect standard deviation to be about 6.4 fps if the sample size is extremely large. If it's not very large, then, based on the extreme spread of 28 fps and different numbers of shots involved in getting that ES, estimates for more practical numbers of shots would be:
Code:
shots fired ES SD Estimate
2 28 24.8
3 28 16.5
4 28 13.6
5 28 12.0
6 28 11.1
7 28 10.4
8 28 9.8
9 28 9.4
10 28 9.1
11 28 8.8
12 28 8.6
13 28 8.4
14 28 8.2
15 28 8.1
16 28 7.9
17 28 7.8
18 28 7.7
19 28 7.6
20 28 7.5
21 28 7.4
22 28 7.3
23 28 7.3
24 28 7.2
25 28 7.1
∞ 28 6.4
The reason the number gets smaller as you fire more shots is your extreme spread pair has more chance of being a larger and less likely deviation on the bell curve. For 10 shots the 9.1 ES estimate is probably about as good as you'll get.
Estimating pressure is tough. Alliant doesn't tell you about the 7.5" barrel length they claim for their two recipe loads with 250 grain Keith style LSWC's. It is not a standard pressure barrel length for .44 Mag, according to the SAAMI documents. If it is a single-shot barrel, then it's measured from the breech of the gun, so it includes the chamber. If it's a revolver it's measured from the start of the forcing cone, after the chamber. Not knowing which they used affects the pressure estimate by about 30%.
For your SRH, Lyman's old and new data for 2400 with your bullet is quite different. Comparing the two allows that you may be anywhere from about 26,000 psi to about 37,000 psi. That would agree with Alliant's recipe for the 250 grain Keith, which is 20 grains under CCI 300 primer. My guess from your velocities is that you are nearer the low side, but for safety you should assume the high side.