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
-Infrared thermometer
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.
Results:
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
5 shots
AVG 2487
SD 13.1
With ammo at ambient temp, the ammo measured at 100F
10 shots
AVG 2514
SD 6.9
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
AVG 2956
SD 14.6
Ambient 90F
AVG 2926
SD 19.3
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.
I believe that the 101 series will tell us to take the opportunity to shoot in as many ambient air temps as we can then make corrections to our muzzle volicty on the XRLS sheets and also to the muzzle volicty sheet as well in order to track multiple rounds over time and barrel life.
That is exactly what the 101 series says, and very sensible advice it is. However, it looks like the spreadsheet is setup for a single input at 70 degrees. I attempted to put a second input in for my 100 degree rounds and instead of adjusting the curve off both data points, it just adjust the curve for the single data point I entered, giving me a crazy curve.
A few thoughts in no particular order. I wouldn't worry too much about having a difference in what the spreadsheet predicts and what your MVs are. As long as you measure the MVs the same way every time, then go with that data. If the bullets don't pass over your chrono in pretty much the same way every time, MVs won't be as accurate as they could be. Don't forget that a good representation of your MV curve needs many data points in order to be as accurate as possible. Lots of variables can play a role, including how big an SD you have, in addition to the extreme spread. If you can keep your SDs and ESs lowm you'll be better off. How new are the barrels? Are they going to speed up as they foul? As the other commenter said, shoot in as many different temps as you can, log what you see, and verify the results on target if you can. Finally, as I recall, a huge point of making the chart is to know if you need to remove the copper fouling or rebarrel your rifles when the MVs get to be wildly different than they were in the past under the same environnmental conditions.