Unexplained chrono reading...

[20PPC]

Last weekend I shot the 600 yard match at Palmetto. I had my Garmin set up to check my numbers and see that everything was looking right.
On Sat starting out we shot heavy gun first, on those first 4 targets my ES was between 40-50 fps and SD was 25 plus,the vertical was bad as you would expect.
Needless to say I ended up only 1 or 2 spots up from last place.
All my ammo was loaded on a V4 and was weighed to the kernel, I don't have a pressure gage on my seater but they all felt very consistant.
In the afternoon my numbers straightened out with a ES of 12-14 and a SD around 5-6 and I finished midway in the top 10.
Sunday the numbers remained as they finished out in the afternoon the day before and I had a good finish on light and heavy.
The only difference between the good and the poor numbers was a different lot number on the bullets.
I'm shooting a 6 BRA, Varget and 105 hybrids so I guess my question is has anyone ever seen such a swing in numbers like that just by changing lot numbers?
I would understand a little deviation but nowhere near what I was getting.

 

[Krogen]

What was the change in ambient temperature from morning to afternoon?

 

[Straightshooter1]

Last weekend I shot the 600 yard match at Palmetto. I had my Garmin set up to check my numbers and see that everything was looking right.
On Sat starting out we shot heavy gun first, on those first 4 targets my ES was between 40-50 fps and SD was 25 plus,the vertical was bad as you would expect.
Needless to say I ended up only 1 or 2 spots up from last place.
All my ammo was loaded on a V4 and was weighed to the kernel, I don't have a pressure gage on my seater but they all felt very consistant.
In the afternoon my numbers straightened out with a ES of 12-14 and a SD around 5-6 and I finished midway in the top 10.
Sunday the numbers remained as they finished out in the afternoon the day before and I had a good finish on light and heavy.
The only difference between the good and the poor numbers was a different lot number on the bullets.
I'm shooting a 6 BRA, Varget and 105 hybrids so I guess my question is has anyone ever seen such a swing in numbers like that just by changing lot numbers?
I would understand a little deviation but nowhere near what I was getting.

I don't shoot that caliber of rifle, but over the years of measuring and sorting my 30 and 6.5 caliber bullets I've seen substantial difference from one lot to another with regards to length of bearing surface's and BTO's, even with the likes of Berger's. A combination of differences can add up to different seating depths and amount of blowby along with other things can make that much difference. . .particularly if the cartridges are loaded for a jump. Given that the results on target pretty much confirmed your chrono readings, apparently it's not a chronograph issue.

 

[20PPC]

What was the change in ambient temperature from morning to afternoon?

In the morning we started out in thrower 50's and ended up at the end of the day close to 70.

 

[Krogen]

In the morning we started out in thrower 50's and ended up at the end of the day close to 70.

Pretty mild temp swing . . . . That's not it.

 

[Bill Norris]

I don't shoot that caliber of rifle, but over the years of measuring and sorting my 30 and 6.5 caliber bullets I've seen substantial difference from one lot to another with regards to length of bearing surface's and BTO's, even with the likes of Berger's. A combination of differences can add up to different seating depths and amount of blowby along with other things can make that much difference. . .particularly if the cartridges are loaded for a jump. Given that the results on target pretty much confirmed your chrono readings, apparently it's not a chronograph issue.

Along with those aspects mentioned above different lots of bullets sometimes will have a difference in pressure ring measurements which will change the actual neck tension if bushing adjustments are not made accordingly.