Velocity dropping each shot?

[stvnbrg]

So, i was out shooting today and it was pretty bitterly cold. (-20 or -25 Celsius ) Shooting over a chrony, my first shot in each string was quite a but faster than successive shots getting slower each firing. Results are (in order):

Group 1:
2831
2789
2777

Group 2:
2829
2802
2744
2729

As you can see in group 2 the difference is 100 FPS!!!!! I am in need of some guidance once more! What is the cause? Is it shooting cold bore in extreme cold yields higher velocities? i need some help cause im stumped!

[MontanaRifleman]

Have you ever shot this load under other conditions?

[phorwath]

Could be the chronograph is getting too cold. Check your owner's manual and see what temps it's rated for.

The first group is within reason. But the continuing decline to 100 fps difference in the second string seems excessive.

You didn't tell us if the ammo and gun started warm and then cooled off. Or if everything was cooled down to -20 before the first shot was fired. Same with the chrono. Did it start warm in your back yard or was it completely cooled down to outdoor air temperatures with the first shot fired.

[jimbires]

could the battery in your chrony be getting weak ?

[RT2506]

Could also be barrel temp. When that barrel is COLD it is smaller in diameter in the bore and thus will create more pressure and more velocity. After firing it has warmed up and expanded and thus less pressure is built up and velocity is less.

I had trigger issues once going from warm to cold. In the summer I set my trigger and it work fine. When I went hunting that winter and it got down into the 20* the rifle would not stay cocked. Metal had contracted just enough to not allow the sear to engage. Had to reset the trigger.

[MontanaRifleman]

I was thinking the same thing. Another shooter, in another forum reported similar results shooting in zero degree weather.

[Greyfox]

I just went through the same thing the other day with my Pro Chrono in freezing temps, 25-30. I brought it instead of my Oehler because it get usually gives very similar data with less set up time. I got 100 FPS spreads, also starting high then dropping. This occured even on loads that were previously tested. I usually bring loads that have confirmed velocities just to have a standard of comparison. Went and got the Oehler 35, and all was well. The chronograph was definitely effected in my case.