I attempted to use Rex's spreadsheet for temp/muzzle velocity variation and came out with major discrepancies in my meaured results vs. the spreadsheet prediction. My equipment is:
-.308 semi-auto shooting molly coated 190 SMKs over IMR4064 with CCI 200 large rifle primers.
-.280 AI bolt gun shooting uncoated "mexican match" rounds consisting of the powder charge and bullet from Hornady 150gr straight .280 "precision hunter" rounds in a Hornady factory new .280 AI case with a federal magnum large rifle primer
-CED M2 Chrony
The procedure: I cooled rounds in for both guns in a cooler on ice during the two-hour drive to the range. After I set my gear up, I fired the rounds immediately after I unpacked them and took a temp reading. Ambient temp was somewhere in the 90-100F range.
The .308 ammo measured 71F when I took it out. Predictably, it chronoed exactly the same as last time I clocked it, when the ambient temp was 70
With ammo at ambient temp, the ammo measured at 100F
NOTE: Once I started shooting the group, I shot rapidly, to avoid anything heating by sitting in the chamber. However, I also test that during this same range session, (by letting rounds sit in the chamber after strings of rapid fire, heating the to 117F) and got zero difference in velocity results. My conclusion is that this might matter if it was cold outside, but at 100F ambient, it didn't. I let the guns cool somewhat between strings of fire.
For the .280 AI
Chilled ammo @ 62F
I inputed this stuff into the spreadsheet and got numbers that are not even close to my real world result (it said my .308 should be going 2600, and my .280 3,000). That data error will result in a miss for a 1000 yard engagement.
QUESTION: How can I fix this spreadsheet (or input additional data) to make it accurate inside my ammo SD, or is there recommended software I can get somewhere that will allow me to input multiple data points to get an accurate MV curve? I saw that Patagonia and Applied Ballistics both offer such software, but I thought I'd ping you guys first and see what the thoughts are.
Thanks a lot.