Implications of case fill percentage

[Hobbit]

Hi

I just wanted peoples thoughts on the impact of case fill percentage - particularly low 90's in percentages

I am experimenting with two new calibers (for me) 300 win mag and 7x64

I tend to stick to the RS and Vhit powders given they have a range to do a job and can be got hold off

I am using high BC/heavy for caliber bullets (225gr ELD-M and 162.175gr ELD-X respectively and have found some good, sub Pmax loads that give me good ES/SD, velocity flat spots and accuracy with typical 20 thou jump. 

However - the urge to tinker is always there and a youtube video has made me aware of the 123 button on quick load so now I am aware of other possibilities including other RS and Vhit powders

Questions

1, Does a low fill percentage (90-97%) give you uneven burn and higher SD/ES's? or any other issues?

2, If you go the other way - how far over 100% can you typically go with a sensible crush - I have heard up to 105% typically depending on powder and die (redding competition micrometer dies not being guaranteed with any crushed loads)

Thanks

[Hobbit]

Using RS62 for 7x64 given I had it for the creedmoor and it works well and N165 for the 300 WM - got a 30 inch barrel so can get mid level velocities

[Andrew]

Not that I've noticed. The 300WM loads I shoot have so much bullet seated into the case (210 RFD Noslers) that case fill is offset to some degree. In the end, what works, works and small numbers at the chrono are no assurance of accuracy.~Andrew

[Hobbit]

Agree on accuracy point but I have seen long rage reloaders suggest that you manipulate load charge to find a low ES/SD node that is also stable with regard to small variations in charge (velocity/charge weight flat spot) and then vary distance to lands for accuracy

Given that ES/SD is a direct contributor to accuracy (vertical spread) at long range by question would be does fill percentage have any effect?

[Andrew]

My Creedmoor with a full charge of H4350 still has a lot of room and yet gives single digit SDs. ~Andrew

[Popsbengo]

20 hours ago, Hobbit said:

Agree on accuracy point but I have seen long rage reloaders suggest that you manipulate load charge to find a low ES/SD node that is also stable with regard to small variations in charge (velocity/charge weight flat spot) and then vary distance to lands for accuracy

Given that ES/SD is a direct contributor to accuracy (vertical spread) at long range by question would be does fill percentage have any effect?

Isn't that the root of the problem, does comparatively low fill volume upset good SD/ES? Received wisdom seems to be lower fill may promote the possibility of variable burn therefore variable velocity. Has anyone ever scientifically established that? There are so many variables not accounted for in most tests I've seen to lead me to conclude it's not much better than guess-work to claim any single factor is having a clear effect. Finding an 'accuracy node' seems to be more important to good SD/ES irrespective of fill % .

Try it and see what works for your combination of variables.

I wonder how many barrels have been well on the way to being shot out while experimenting for the optimum accuracy!

[Hobbit]

Fair enough - So it’s not proven or even generally accepted? 

I personally just do a load ladder and look for the low SD and velocity flat spot with an easily available powder that suits the caliber but then I found that 123 button in quick load and wondered if it was a better way to choose a powder and if so should I care about fill%age as well as P and V??

[Hobbit]

I think Laurie voiced his skepticism of the suggestion that the short fat cartridges were more accurate/consistent than the longer ones - 284 vs 280 on another blog

thought i’d Get an empirical draw vote on the fill percentage

 

 

[DaveT]

Good info on this in Litz second book on modern advancements in long range shooting.

 

[Hobbit]

On 12/17/2018 at 6:14 PM, DaveT said:

Good info on this in Litz second book on modern advancements in long range shooting.

 

Thanks, just ordered it from brownells

[Laurie]

On 17 December 2018 at 6:14 PM, DaveT said:

Good info on this in Litz second book on modern advancements in long range shooting.

 

................... and which couldn't find any correlation between fill ratios and ES/SD results. In general though, a faster burning powder is usually required for low fill-ratio when allied to lower pressure / performance loads to work consistently. So go a step faster burning for 'downloaded' cartridges as well as choosing grades known to burn consistently at low pressures.

Over the years, I've found many combinations using smaller charges of faster burners that gave surprisingly good results. There is nothing new in this. Read Ken Waters' 'Pet Loads' articles of a generation or two ago and relatively fast burners were widely used in the USA in 30-06 short-distance match rounds with the 168gn Sierra MK, this practice carried on in the 308 after it replaced the old Oh-Six. IIRC Waters has one of his best 308 Win accuracy loads from the long discontinued 190gn Winchester HPBT Match above Hodgdon H322, a powder we'd regard as far too quick for this combination these days and which will come nowhere near filling the relatively large case. In fact mentioning the Oh-Six, its standard M2 military ball loading of a 152gn flat base FMJ over bulk-lot IMR-4895 must have produced very low fill-ratios, especially as this '4895' varied enormously between lots in its actual burning rate, some lots equivalent to IMR-3031, and had to have charges reduced accordingly in the ammunition factories.

[Hobbit]

Laurie

Thanks - does this then, by extension,  apply to powder column? The short fat cases are touted as 'efficient"

I recently got a 7x64 barrel for my R8 as the 7mm option given it had the twistrxte for heavy bullets and you can get lap cases without any necking up.

is there any reason a .284 win (in a standard action which it would need to be to seat the long, high BC bullets) would be more accurate than a 7x64 given case shape - I take it it might use less powder for the same velocity