Range Report What are acceptable chrono results?

[AtownBcat]

I was doing a little load development and settled on a load that shot very well in my 300WM. The load is as follows:
RWS Brass
76.5g Retumbo
CCI Mag primers
208 A-Max

This is an accurate round. I have shot several 5 round groups between .259(almost able to call it a .25....)and .313.

I decided to shoot 2 ten round strings to get some velocity data.
My first string was with sunshine overhead and the second string was with the sun dropping down behind a wall so there was not direct sunlight.
The chrono is a CED M2. The unit was 15ft from the muzzle.

First 10 rounds
2894(cold bore)
2908
2925(this round the bolt was a little hard to close the bolt, wish I had replaced this round with another one)
2900
2892
2898
2909
2902
2888
2890

so the Average was 2900
ES was 37(that one round jacked this up)
and a SD of 10.5


The second string was a little crazy...
2913
2919
2868
2914
2900
2856
2886
2844
2848

average 2875
ES 110
SD 34.9

Im tempted to throw the second group out but it still messes with my head..
What are acceptable ES and SD numbers? Should I be using the light packs to get more reliable results? What are your thoughts about my results?

 

[The Mechanic]

Re: What are acceptable chrono results?

Direct sunlight always "F's" up a chrono.

 

[Greg Langelius *]

Re: What are acceptable chrono results?

Group size counts when using the chrono. The consistency of the group size can sometimes be related to the consistency of the velocities; and yet, sometimes the relationship is counterintuitive. In either, case the relationship tells us something useful; but my mony's on the group size.

Also bear in mind that true velocity displays can become distorted by the crono's own inherent error margin.

These margins are typically described in the chrono's user documentation, and tend to run in the +/-1% error margin range. This means that a velicty that's displayed as 3000fps can actually be a true velocity of anywhere between 2970fps and 3030fps.

This does not mean that such deviations of less than 60fps are meaningless. IMHO.

I believe that the velocity deviations can be considered more accurate than that, taken as relationships from shot to shot. I also believe that the <span style="font-style: italic">mean</span> displayed velocity is where the error margin comes into relatively more play.

Greg