Temperature or chrono. to blame?

[Tangodown911]

Went out testing some .260 loads today. But ran into some issues.

24" Krieger 1/8.5" twist barrel
0900 hours...temp. 45 degrees.
Norma brass
4831sc 46.1 grains 2692 was avg. velocity over 5 shots. As it warmed up to 64-68 degrees I began seeing speeds of 2750-2770fps. My issues is that the chrono was having problems with picking up shots due to what I think was a sun issue. My question is: Is it likely that the temperature change of 20-23 degrees caused a 60-80fps increase? Or does the chrono need to be addressed. I also shot my 20" .308 and it went from 2464 to 2546 as the temp went from 45-65 degrees. What gives?

I have little experience with a chrono.

 

[david777]

Re: Temperature or chrono. to blame?

Not really sure about the temp sensitivity of 4831sc, but I had the same issue with RL17 with my 260. Chronoed at 2965 fps at 10 AM / 50º F and had an elevation of 7.0 Mil @ 1000yds confirmed. I came back to at 2:30 PM / 83º F and shot the same load and it was .3 Mil higher (adj should have been around 6.7 Mil) . The chrono was fine, it was the powder / temp combination. I would bet that is what you are experiencing.

 

[wnroscoe]

Re: Temperature or chrono. to blame?

Velocity is a product of pressure. As out side ambient temps rise, so will pressure, velocity will follow. That’s why loads that work great in the winter aren't so great in the summer.

 

[AQC440]

Re: Temperature or chrono. to blame?

Unless your loads were laying out long enough to become heated by the sun (wnroscoe's pressure), then I would say you had lighting issues. The temperature change itself should have only been about a .3 fps change at chrono distances.
On the same note, David777's statement also holds true. The .3 fps difference at chrono distances would account for 50 fps. at the thousand yard mark.
At least that's what sticks out in my chrono notes

 

[Tangodown911]

Re: Temperature or chrono. to blame?

Well I guess it is a chrono issue. It always works better on shaded days.

 

[pupdawg]

Re: Temperature or chrono. to blame?

Or you could use a sky screen if it came with one. Most chrono's do, that's what the sky screens are for: direct sunlit conditions.

Somethin about the optical sensors and the sky, blah blah blah. I don't know the technical reason. haha