rck,
The problem stems from your input into your ballistic program. The chrono number seems reasonable for a 16" 308. There is no way you are anywhere close to 2,700ft/s let alone 2,750ft/s.
inputting the proper density altitude is the way to do it. Density altitude is pressure altitude corrected for non-standard temperature.
Here is how you should estimate your density altitude;
1. Standard atmospheric altitude is 29.92" Hg (mercury).
2. Pressure drops by 1" Hg per 1,000ft (1"Hg/1,000ft) increase in altitude.
3. To obtain your pressure altitude, divide your sea level elevation (5,100ft) by the above.
4. Therefore, At 5,100ft, your pressure altitude is (5,100ft/1,000ft) x 1"Hg = 24.82"Hg.
5. Last step is your temperature correction factor; sea level is 59F, and temp varies by 3.5degF per 1,000ft of altitude.
6. your standard temp should be 59F-(3.5F*5.1) = 41.15F.
IFF the conditions at the day you chronoed your load was not 24.82"Hg and 41.15F, you have non-standard conditions and need to correct for them.
If your temperature was hotter, your bullet would encounter less air resistance and act as though it was shooting with a higher muzzle velocity. If the temperature was colder, then your bullet would encounter more air resistance however, the air resistance while higher than on a hotter day would still be significantly better than at sea level.
I don't know if you are able to input your conditions in the program, what i would do is enter the pressure i calculated above (if you did not record your pressure reading on the day of your shooting) and the temperature on the day you took the readings (or the temperature above if you did not record that info also).
Those inputs would "true" your ballistics to more closely match what you will experience in real life.
As I noted in one of the posts above, I inputed the current conditions into Shooter as measured by my Kestrel. I also use station pressure. The measured pressure was 24.6. That is a tinge lower than the calculated number you give, but I was just south of a thunder storm too! So I think we are on the same page there.
Re temperature, it is my understanding (could be wrong) that Shooter does do the temperature correction, but that is an interesting point to look at. I ran the numbers again to compare the results at 41F as well as 77F. At 41F Shooter called for 0.1 mil more elevation than at 77F. So a smidge and I have noticed as big a difference in just how the iPhone rounds the numbers vs the iPad! I did the calculation for both 2500 and 2750 fps with the same result.
2500 FPS, 425yds, 41F, 2.7 mil required
2500 FPS, 425yds, 77F, 2.6 mil required
2750 FPS, 425yds, 41F, 2.2 mil required
2750 FPS, 425yds, 77F, 2.1 mil required
I fiddled with humidity but that is a big so what.
Now at this stage, I have to wonder about the ammo too, but I have shot this load for over 30 years and it has been stable as a rock. I double checked my log and the only difference was a new scale that reads 2-3 tenths lower. I throw with a Harrel though and the number of Harrel clicks is a constant since I choose to control volume rather than mass. All that said, the numbers reported by the chrono past the smell test.
One last bit to toss in. This is a fast barrel and it chronos 50fps faster than my 20 Remington.
