Accuracy vs standard deviation in velocity

[jeep45238]

Has anybody noticed a correlation between their most accurate hand loads, and having low standard deviations for velocity? It clicked for me when reading my Lyman manual, and their reasoning for highlighting loads with accuracy potential based on consistency in velocity.

I haven't compiled to my targets yet, but some of my hand loads are around 10fps deviation in my g19, and I have three in the 7fps deviation for my 226 a both of which occur at opposite ends of the spectrum for both guns.

[Mark Housel]

It would seem that at least in theory, variations in velocity, absent other mechanical effects would be the factor in variations in elevation POI. I may be missing some other elements but this seems like fundamental physics.

Just as it would seem that, absent any other mechanical effects, in a zero wind environment windage variation should be pretty small.
These sorts of thing always remind me to re-read the The Secrets of the Houston Warehouse (http://precisionrifleblog.com/2013/10/18/secrets-of-the-houston-warehouse-lessons-in-extreme-rifle-accuracy/), which struck me as a study in shooting in as controlled environment as one could imagine.

[Odin Bravo One]

This is why we chrono our sniper guns and loads every time we get a new lot number. Everyone makes crap once in awhile, and we got an early batch of .220 SMK from BHA that had deviations of upwards of 75-110 fps. At extended ranges, one could not even guess where a round might impact, making the math and holds irrelevant. If you can't accuratly predict what velocity the bullet will leave the gun, you can't predict what it will do as it travels along its trajectory.

[MSparks909]

On my precision rifles I like a SD of less than 10. I don't think much of it for handguns.

[Jim Watson]

Low standard deviation is a mark of good QC in pistol and shortrange rifle ammo. A PhD friend analyzed the statistics of .38 Special and found the Good Stuff had a coefficient of variation (standard deviation as a percent of the average) of 1%. Say 7fps for 750 fps wadcutters. But it did not show up on the target, 2%-3% is not enough for a reject.

At long range, it isn't the standard deviation that matters, it is the total spread. Unless you are getting vibration cycle compensation, the fast bullet is going to shoot "flatter" and strike higher, the slow bullet is going to shoot lower. This can run into several inches for mediocre ammo at Long Range.

[wrmettler]

If you run some calculations on the JMB Ballistics website, you’ll find that a Winchester 230 fmj going between 750 and 950 f/sec has a difference of drop at 20 yds of .1”, 30 yds .2”, 50 yds 1.5” and at 100 yds a difference of 9”.

At 25 yds, with a drop difference between 750 and 950 ft/sec of about .15”, I don’t see how that will affect a group of shots, given the ability of even the best hand gun shooters.

If you do the calculations for a normal pistol powder designed for a .45 acp, you should see a deviation of about 150 – 200 ft/sec per grain of powder. That is a very big difference for almost any progressive reloading press, it seems to me.

At 1000 yds, it is another matter.