Using case fill to determine Charge Best accuracy and lowest ES?

[Kocher]

While reloading 6.5 Creedmoor with popular extruded powders, I also noted that 556/223 was also the same case fill where the accuracy is typically seen when using varget or a copy powder.

when you look at the two cases in the picture the fill of the powder is almost identical, just below the neck/ taper case or just above.

40 grains for the IMR 4350 in the 6.5 Creedmoor was the lowest SDS and also the most accurate for me in my rifle.

when you randomly filled the case up to the neck taper …=24.5 grains of varget. In a Starline 556 brass.

This also seems to be what most people get for accuracy when using the 69 gr smk’s and using24.5-25 grains of varget. (a quick search will come up with like 50 replies of the same recipe)

Shooters world long rifle had a charge of 40 grains as max this shot lowest SD spread in my gun and appeared to be most accurate and that was almost the exact same case fill as IMR except a little bit lower in the neck taper…fill line.

I tried this experiment with Vittori 555 and I got 41.6 grains of powder.

load 140 grain bullet with this powder charge and see what type of accuracy I get an extreme spread as an experiment

I’ve never loaded this powder in my gun, Charge and bullet wt. are within the extremes of VV max Charge and I do have some lower charges. I will try first just to be safe.


Obviously, ball powders may differ, but I think they’re in the same category as 90% fill seems to be where most accuracy of known powders ? Or at least up to the case taper…
take from this what you will,
For me, it was an eye-opener something clicked so we’ll see how accurate my guess Charge of the 555 Vivit powder shoots.

 

[Doom]

Well, you have it partially correct. Case fill is very important in that like most things reloading, consistency is paramount. The first thing that happens in firing a round is the ignition of the powder and consistency in the burning. If that doesn't happen then then SD will suffer. Most experts will tell you that case fill should be ~90% or greater. In conjunction with that it is desirable that the powder burn out completely in the barrel. This is where burn rate can come into play as it's a balancing act. If the powder is <90% then its orientation in the case won't be as consistent. Also, some powders are more sensitive to this than others.

So the partially correct? Selecting a powder based solely on case fill may lead to using a slower powder which may not burn out completely in the barrel. On muzzle exit the unburnt propellent bombards the base of the bullet with high velocity particulates that may/can lead to an increase in the dispersion of the shots.

 

[RegionRat]

When I was still very new in this topic, I asked the seniors what they thought about case fill percentage in terms of the quality of the velocity stats and accuracy for direct fire.

They showed me as many examples that were exceptions to the rules as there were ones that followed the rules.

Moral of the story.... nobody had all the answers as to why things worked the way they did, but life would be hard if you argued with real world results. Do what is proven to work, even if you can't explain why. YMMV