Extreme spread observations

[dauntedfuture]

I ran some numbers today on the JMB calculator in an attempt to confirm of deny some effects of velocity spread on drop and the corresponding effects on target. When I test ammunition for competition I do so in 10 shot batches and look very close at Extreme spread and to a much lesser extent SD along with the group size. There are some combinations that shoot small groups with high ES and these are not suitable for long range shooting and should I encounter such a load combination that where I cant improve the ES, I will check another primer to see if it helps with the given powder.

I ran numbers for my .308, 6.5x284 loads for 1000yds and .223 for 600 yds. 25 FPS ES @ 2975 (3000 FPS 1K drop - 2950 FPS 1K drop / 2) with a .308 Lapua 155g = 6" at 1000 yds, or slightly over 1/2 MOA. The same 25 FPS ES with a .264 142g SMK @2975 = 5.5" at 1000 yds. Or, if I'm shooting one of those loads at 1000 yds and they both have a 25 FPS ES, I can expect that ES to give me a total of 1/2 MOA elevation spread, or .25 MOA high on a fast shot and .25 MOA low with a slow shot.

Running a .223 80 JLK at 2675 (a conservative load) results in a ES induced 1.8" at 600 yds, or about 1/3 MOA.

There is less effect for faster, higher BC bullets.

Slight variations in length and weight of bullets has a smaller but similar effect as BC varies with length/ weight.

What does this mean? If you have a solid rifle and you have a load that produces a SD of 5-7 for 10 shots, AND an ES of 25 FPS, that one high or low shot will be about 1/2 MOA high or low at 1000 yds.

 

[LR338nut]

So what is your real ES and SD? Or are you just speculating what might happen?
You can also have a tight ES and SD but at the wrong point in your barrel harmonics (research barrel nodes) and it will still vertically string your groups at distance. That's why a lot of guys ladder test their loads before going crazy on the chronograph.

 

[dauntedfuture]

I ladder test too and crono at the same time. I have noted that the differences in velocity are closer together at the "node" then other points. If my ES is not 25 fps or less then the load will not work at 1000 unless I can get it there even if groups are good enough at 200yds, 10 shots 2", (max distance home range) I mostly see guys quoting SD only and not looking at ES. My real ES is about 20 FPS or so and i'm working on neck turning to uniform neck tension to reduce that, i'm also planning on replacing my well worn and old RCBS 5-0-5 scale when I return. This topic would be more appropriate in external ballistics but I put it here by mistake.