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?

 

[Pdvdh]

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]

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 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.

 

[MontanaRifleman]

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.

Good to know

 

[D.ID]

I have given up on chronographs all together. Between set up and alignment, unreliable or worse yet just inaccurate readings, I'm done. I thought about buying a more expensive one but came to the conclusion: gravity and repeatability do not lie, always give a reading and since I have to test for both anyway. I am done wasting time and ammo over a chrono. I have stepped back to the old fashion method of testing my loads at distance. It works in any weather, in any lighting and the results are confident. With todays ballistic computers and there simple ability to establish velocity based on proven known factors in known conditions and then modify it appropriately for other conditions. I have no love or use for the chronograph. My opinion based on my experience, good luck.

 

[Pdvdh]

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

I've not experienced this with my rifles. Place them in a -5F freezer overnight with ammo. Pull them out and shoot over the chronograph, which is at normal outdoor air and operating temperatures - say 60F. Velocities haven't increased to any significant extent. The more common result, and greater magnitude of change, is a decrease in velocity. But not by 100 fps.

Tighter bores usually result in decreased velocity. Lilja barrels have a reputation for being a little tighter than some others, with a corresponding reduction in MV at maximum loads, compared to looser barrels at their maximum charge loads. Increasing the pressure in the bore by increasing the friction factor (resistance) going down the bore isn't generally a beneficial use of the energy of the expanding powder gases.

I suppose others may, occasionally, have a different experiences. There are quite a few variables in-play. Those experiences could be due to cold temperatures affecting the chronographs, as extreme cold is known to affect the accuracy and reliability of chronographs.

 

[Lefty7mmstw]

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.

never been that lucky(higher velocities when cold), and I've treid a bunch of times to freeze my hands off shooting in sub-zero temps.. What I've seen is invariably some drop off in velocity.