how to reduce velocity extreme spread?

[joe0121]

I'm a little late chiming in here but I reloaded extensivley for a 300 Win Mag
in a SAKO factory model 75 rifle.

Best load was using:
BERGER 210 seated 4 thou from rifling
LAPUA brass, Federal 210 match Gold Medal primers,
80.0 grains H1000
avg 2853 fps ES 35.67 / SD 13.11
3/8" group @ 100 yds.

Hope this might help your situation in some way !

Regards
AL

I'll throw mine in was at the range today. Rem 700 28" Krieger #10, 28 degrees out 75% humidity.

Mixed brass mixed amount of firings, but all fired at least once in my chamber.
208GR Amax not sorted in any way picked at random out to the box.
79GR H1000 Loaded .010 off the lands
CCI BR2 Primers.

I bet if I went all bench rest on case prep, weight sorted brass etc I could get better, but this is darn good.

Avg 2932
ES 27
SD 11

.640 groups for three shots.

No pressure signs might bump up to 80GR and see if I cant get some compressed loads.

 

[reefraker]

boss hoss, I need a written basic method to reference to practice reducing my exstreme spread. ive been reloading for my hunt rifles for 3 yrs. and need to advance my skills. I'm not a competition shooter just a hunter. thanks. my email is [email protected].

 

[reefraker]

can some reply with a basic brief step by step method on extreme spread reduction. I have been reloading for my hunt rifles for 3 yrs. and recently obtained a chronograph. I want to start advancing my skills but cannot fing any step by step method in any instructional book or reloading manual on the subject of reducing extreme spread. thanks. my email is [email protected]

 

[dsculley]

Just to clear up a couple of things:

As said, ES (Extreme Spread) is the difference between the highest and lowest velocity in a particular string of fire whether that is a 3,5,10 or 100 shots.

Unfortunately this data is reliable for this string only and can not be interpolated into the entire population of shot (the rest of the shots you take for the life of the barrel).

SD (Standard Deviation) is much more useful. Assuming a random sample, then roughly 68% of the total population will fall within +/- 1 SD. 95% will fall within +/- 2 SD's. 99.7% will fall within +/_ 3 SD's.

With an ES of 50, on that day there was a difference of 50 fps in the string you shot.

Assume Joe 0121's SD of 11 fps.

11 fps * 3 = 33 fps 99.7 % of the shots from this load and rifle can be expected to fall within 33 fps of the average muzzle velocity of 2932 fps. This would give a max mv of 2965 fps and a min mv of 2899 fps, or an actual extreme spread of 66 fps for the total population.

As you can see, using this real world data as an example the actual extreme spread of the population is very different from the ES of the sample. ( 66 vs 27).

Standard Deviation is much more useful than Extreme Spread.

When you look at the statistics of shooting, don't forget to look at your target. It is not uncommon to have very nice group started, say 3, 4 or 5 shots that group within 1/2 moa. You are shooting over a chronograph and the mv is consistent with very little dispersion, say extreme spread for these is 15 fps. Then you have a flyer! Did you pull it? Check the chrono and the mv is 60 fps more or less than the others. For a string like this, ES would be high at ~60 fps, but SD would be low because there was a group with low dispersion followed by a single flyer. (If the mv of the flyer had been consistent with the others I would have assumed that I pulled it.)

One other point to consider. Say you shoot a group that is less than 1/2 moa, but your chronograph shows a high ES or SD. Do you trust what you see on the target or do you trust the chrono?

FWIW, just a few things to consider when looking at the statistics from your chrono.

 

[dsculley]

And the best way I know how to explain how to get SD down is consistency throughout the reloading operation. Prep brass the same, anneal necks for consistent neck tension. Whether you weigh charges or charge by volume be as consistent as possible. When seating, occasionally check the length to ogive for consistency. Use good components, perhaps test different primers if SD is too high.

 

[Okanogan]

If you want to up your reloading capabilities to lower SD, I would suggest trying to get a copy of Precision Reloading Shooting Guide by Dave Brennan. It is out of print so I borrowed it through the local library. It has a lot of good information that is considered by many to be classic guidelines.

Brian Litz also had a very good section in his recent book Modern Advancement in Long range Shooting Volume II which provided useful objective information on the impact of several reloading variable on SD. There are a lot of variables in reloading which affect SD and precision so Brian's scientific approach to isolate a single variable and verify associated impact on SD was something I found very useful. You can also check out the Precision Rifle Blog for information on what the PRS elite are doing.

Because of the variety of different reloading factors that effect SD, I think you may find it challenging to get a simple set of steps that works in every case. In my experience improving reloading results has been a journey, which has been helped greatly by getting a good chronograph (Lab radar/ Magnetospeed) and other tools (Gempro scale, concentricity micrometer, head space measurement tools, etc) to check with precision many of the various steps in reloading. Working with top quality components has also helped in my case (Federal primers, Lupua/Norma brass, Redding/Foster dies, etc)