Top 3 variables for small extreme spread?

[Shadow9mm]

Does it matter the powder used? I.e. is a fast double-base powder worse than a slow dingle-base powder?

That's a tricky question. There a lot of variables that go into powders. Some are more consistent than others. Some are more temperature stable than others. you can get variations in velocity by leaving a casing in the sun vs shade, or letting it sit in hot chamber and warm up, compared to a load that was chambered and fired quickly. From what I understand you will see match shooters storing their ammo in a lunch cooler until ready to use to keep temps stable.

With that said, narrowing down to top 3 is hard, I still say powder are the most important 3 as they are the easiest to check, control, and maintain in general.
charge weight, it does not matter what or how good the powder is if you don't load it consistently
primer, some are more consistent
case capacity(consistent) case volume effects pressure and how the powder burns, different volume different burn, different velocity.

 

[Nathan]

Does it matter the powder used? I.e. is a fast double-base powder worse than a slow dingle-base powder?

Powder matters. It seems like sometimes powder, primer and bullet don’t quite deliver good accuracy or sd. Changing primers helps sometimes, but changing powders can be worthwhile.

IMO, ball powders have their place in plinking, but I generally avoid for max accuracy or hunting loads where temp stability matters.

 

[Unclenick]

Anyone ever sighted in their rifle very precise and never put a bullet hole dead center on the target shooting offhand without a sling?

No fair, Bart. I have to raise my sights a full 2 moa for offhand as compared to sitting or prone. Apparently, my brain discounts the extra area of movement below the 6:00 hold that exists in offhand. I don't know why, but at least I've learned how to compensate: dial the sight up by 2.

Standard deviation is the measure to use for samples of 8 or more. Extreme spread depends on just the two sample values furthest from the mean, which makes them the least probably events in the sample. One of which might be a highly improbable outlier. For that reason, the extreme spread is not an adequately consistent representation of the average extreme spread you get over multiple samples of the same load.

Standard deviation mitigates the influence of outliers, and the bigger the sample, the better it does that. The problem with standard deviation is that the extreme spread is what determines the actual size of groups. So you need a workaround. The solution is to take your standard deviation and multiply it by ξ(n), a statistic that predicts the average value of extreme spread for a particular sample size as a multiple of its standard deviation. A table of values for sample sizes from 1 to 50 is here. Just multiply the standard deviation of shot velocity by the value of ξ(n) corresponding to the number of shots, n, in your velocity sample to get an estimate of the average value of extreme spread you would get if you fired the same size sample over and over again. It will only be off by as much as the standard deviation is off, and that is generally less than you can count on getting from extreme spread. At least, it is for samples of more than 7. At 7 and below this becomes less certain and some feel the extreme spread then becomes just as likely to work out.


The variable I see missing is primer seating. You want to compress the anvil into the primer by about 0.003" to get the best ignition. This is accomplished by fairly hard seating.

"There is some debate about how deeply primers should be seated. I don’t pretend to have all the answers about this, but I have experimented with seating primers to different depths and seeing what happens on the chronograph and target paper, and so far I’ve obtained my best results seating them hard, pushing them in past the point where the anvil can be felt hitting the bottom of the pocket. Doing this, I can almost always get velocity standard deviations of less than 10 feet per second, even with magnum cartridges and long-bodied standards on the ’06 case, and I haven’t been able to accomplish that seating primers to lesser depths."

Dan Hackett
Precision Shooting Reloading Guide, Precision Shooting Inc., Pub. (R.I.P.), Manchester, CT, 1995, p. 271.​