Velocity spread... Is this acceptable?

Evil_Dark

Evil_Dark

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I did my load developpement before having a chrony, so I found that my .308 savage 10TR loves the 168gr Hornady match with 40.4gr of IMR-4895.
I was able to do 3/4 groups with it, surely outshoot me as a relatively beginner shooter.
I ran that load through the chrony and it gave me these readings, I want to know your opinion if these resulta are acceptables or if I should continue the developpment...
2710
2684
2689
2698
2712
2650
2675
2659
2666
2695 (all in fps obviously...)
Meteo was 44.1F, pressure 30.43 at 38.4% of humidity.

What do you think?

Dark
 
Here's some more details about my reloading procedures:
The COAL variations are +- .003", the cases lenght are all trimmed with the Lee gauge and trimming tool, winchester brass are neck sized by a RCBS die, winchester large standard rifle primers.
I try to be consistent as I can be, the powder is mesured by a RCBS 10-10 scale, which I like, I'm capable of +- 1 granules precision.

Dark
 
That gives you a Standard deviation of 20FPS

Mine is 12FPS for a reference point. If you try really hard you can get it down into single digits but I don't have the time for that.

Another reference, looking at your extreme spread.

(Numbers based off my 175gr projectile and load out, this is just for illustrative reference)

At 800 yards @ 2712 drop would be 23.74MOA at 198.86 inches
At 800 yards @ 2650 drop would be 25.06MOA at 209.92 inches

So an 11.06" difference at 800 yards. BUT, remember that is extreme spread. Your average would put you in the middle of that saying some shots might 5.5" high, some might go 5.5" low.
 
Last edited:
SD: 20
ES: 60
average: 2683

Seems pretty high SD. Majority of my loads are around 8-12.

I'd look into getting lapua brass(10-20 reloads = 9.5-4.75 cents per), and FGM match primers. Those two things should make your rounds that much more consistent, as that is the name of the game.
 
Usually for me when jumpy results like that show up(testing in mid day sun...), I consider its not a real good measure of velocity variation, I only use that as an average speed idea.
Only time I've gotten the chrony to give numbers that don't jump all over or error is in the evening when there's still just enough light for it to work well, but not too much, and not direct over it. Then a load that was +/-30 goes to +/- single digits.
Though I hardly care about it anyhow, good scale to .02gr, decent reloading procedure, how horizontal they fall at distance tells the rest usually.
 
As SND mentioned, the discrepancy could have to do with the Chronograph itself. There are many factors that affect the accuracy of a chronograph, distance from muzzle, llighting etc. The other possibility could be crimp, if the crimping is a bit inconsistent, perhaps a tighter crimp on some rounds and less on others. Some of the cartridges could develop slightly more pressure and some develop less pressure. In any case, whether or not it is sufficient is entirely dependant on what you are using that load for. Obviously, any shooting activity that is distance or accuracy based is going to need a lower SD in order to have reliable results. However, plinking or short to medium range activities are less dramatically affected by a slight velocity inconsistency.
 
On my TRG 20" barrel with magnetSpeed. 5 shots groups. ~1/2" grouping 1/2MOA excluding the called flyers (blame the shooter)
168 SMK, Varget, 100m, Lapua Brass. Fed LR primer
Grs, SD, ES, Avg FPS
43.5, 11, 29, 2548
43.8, 10, 28, 2571
44.1, 5 , 14, 2587
44.4, 7, 18, 2597
44.7, 7, 19, 2613

My rifle likes 44.1
Will try 43.9 to 44.4 on Wed to see if ES will go down below 10 (the magic number).

Precision is consistency.
 
What is the age of the rifle, how many shots through it?

I find that new rifles/barrels are very inconsistent at the start.

Also , how the sun moves over your chronograph van and may effect your readings.
 
skokie said:
Obviously, any shooting activity that is distance or accuracy based is going to need a lower SD in order to have reliable results. However, plinking or short to medium range activities are less dramatically affected by a slight velocity inconsistency.
Click to expand...

^^
That was kinda my point with my ramblings.
Even with a 20 SD if your only shooting 400 yards your speed difference will drift you about an 1inch from center. Some are happy with that.
(Some obviously are not)
 
Thanks guys for your inputs.

I think I know how to calculate Extreme Spread, as it is 62fps (lowest 2650, highest 2712). But how do you determine/calculate the SD?

Thanks

Dark
 
Kelly Timoffee said:
What is the age of the rifle, how many shots through it?

I find that new rifles/barrels are very inconsistent at the start.

Also , how the sun moves over your chronograph van and may effect your readings.
Click to expand...
My rifle is almost new, got exactly 156 shots through it. (savage 10TR)

Dark
 
Evil_Dark said:
Thanks guys for your inputs.

I think I know how to calculate Extreme Spread, as it is 62fps (lowest 2650, highest 2712). But how do you determine/calculate the SD?

Thanks

Dark
Click to expand...

Easiest way is to just key your numbers into excel and on a different cell type =STDEV(A1:A7) <Adjust the A1:A7 to fit your range proper>
 
Basically it is how much your will probably drift from your average number.

IE, it is how consistent your load is. Doing any really distance shooting you want to remove as much "Change" as possible.
 
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