help reading chronograph results please

[Shortdraw]

I went to the range this morning to get some numbers from my new chronograph. I have never used one before to any real extent. Please help interprate my results. I was unable to record any decent groups, the wind was swirling and variable 15-25mph with some gusts over 30. The same load produced some very nice groups at 100yds last week between .12"-.32" in zero wind.
Federal 2x fired brass neck sized and trimmed
175 SMK (308)
42.5gr RE-15
Federal Large rifle primers
Shot 5 strings of 5 rounds through 20" built Rem 700
Temps were 16 degrees, 64% humidity at sea level.
1st group AV 2543fps ES 16 SD 7
2nd group AV 2545fps ES 15 SD 7
3rd group AV 2540fps ES 5 SD 2
4th group AV 2520fps ES 15 SD 7
5th group AV 2517fps ES 10 SD 4
I was suprised at the consistency, I figured the spread would have been higher but I don't know what a good number is for sure. I also am not grasping exactly what standard deviation is so not sure there. I was also wondering why my average velocity was getting lower the longer I shot. All I can think is the barrel started off close to car temperature and the steel shrunk as it cooled off to the 16 degree outside temps or maybe as the ammo got colder, the velocity decreased. Any and all thoughts, questions, insight, or comments are greatly appreciated. I am fairly new to this.

 

[Lindy]

Re: help reading chronograph results please

Well, you know what an average is.

Take each individual shot, and calculate the amount that the velocity of it is different from the average.

The SD is basically the average of each of those numbers.

Read this for more details, and how to interpret your results:

http://riflemansjournal.blogspot.com/2010/07/statistics-for-rifle-shooters.html

 

[doc76251]

Re: help reading chronograph results please

I for one would be very pleased with those numbers. If nothing else it shows that your reloading technique is very good. the drop in speed could be definitely attributed to the ammo getting colder. You "could" hypothesize that your velocity at ~20*F is around 2520 and your velocity at ~50*F is around 2540 ish. Did you go check targets or take a break between the 3rd and 4th string?

Cheers,

Doc

 

[ChadTRG42]

Re: help reading chronograph results please

Those are very good low ES numbers. For a 175, it's running slow, though. If you plan to shoot longer range, I'd try to get more speed.

 

[Shortdraw]

Re: help reading chronograph results please

Thanks for the replies so far guys. I did take about a 15 minute break between my 3rd and 4th string, my hands were going numb from the cold. That seems to explain the velocity gap and hearing the same theory from someone else validates it for me. I am running a little slower than I would like so I am going to bump it up further and see if accuracy continues. I am not as concerned as most people here, I realistically do most of my shooting at either the 100 or 500 yard lines. I rarely get past 1000 but would like to occasionally. Does low ES and SD equate to accuracy, or can a load have good numbers and still shoot poorly?

 

[doc76251]

Re: help reading chronograph results please

If you have really low ES's like that and crappy groups there is something wrong w/ the weapon system or the nut behind the trigger. FWIW you should find another node up around 43+ish, watch testing in cold conditions, if it ever warms up where your at things could get "interesting". I'd start paying attention to temps above 44.0gr.

Cheers,

Doc

 

[vman]

Re: help reading chronograph results please

I agree with doc...

Low SD and ES would indicate that your bullet drop should be very consistent even at long ranges. Provided there was no wind you get very nice groups provided you do your part.

So if your groups are crappy I would say either the wind is playing havoc, the shooter is having a bad day or theres something wrong with the weapon system itself.