Fill Ratio Formula

[6ShotsOr5?]

I trust the smart people that I’ve been reading who say fill ratio is important for accuracy. A few reloading manuals include fill ratio for the max load, but most I have looked at do not. I searched the forum looking for a fill ratio formula but I couldn’t find anything. It would be even better if it accounted for seating depth. Maybe QL calculates fill ratio - I don’t know, I don’t have QL yet. Have any of you seen a formula or tool that does a good job calculating fill ratio?

 

[Bart B.]

That may be another term for "load density." I don't know how it would be expressed as ratios normally are; x:y like 1:10" rifling twist ratios are.

There's no standard for load density but its generally the percentage of the cartridge case inside volume with the bullet seated. A minimum of 95% is common.

Full a primed case with powder to the point where the bullet base rests when seated. Weigh that powder charge. Its weight is that bulleted cartridge's 100% load density. If it weighs 50 grains and the loaded round charge weighs 45 grains, that charge's load density is 90%. 49.5 grains is 99%.

 

[dmoran]

It's the powder capacity under the bullet in relationship to the volume of the charge used:
Charge weight (gr.) / Powder Capacity (gr.) = Load Density (%)

Powder capacity under the bullet is computed with the powder type being used density.
You will need these specifications to compute "Load Density":

• Case Capacity - (water)
• Case Length
• C.O.L.
• Bullet Overall Length
• Bullet Diameter
• Boattail Length
• Powder Density
• Charge

 

[6ShotsOr5?]

It's the powder capacity under the bullet in relationship to the volume of the charge used:
Powder Capacity (gr.) / Charge weight (gr.) = percentage of Load Density (%)

Powder capacity under the bullet is computed by the powders density being used.

Donovan, very clear, thanks! Do you know where to get the powder bulk density values?

EDIT — Donovan, found a table on the www with a lot of powders, http://www.tacticoolproducts.com/powder.pdf. Thanks again.

 

[Bart B.]

There's another way that might be easier, not needing to measure bullet & case dimensions nor acquire case capacity. Possibly more precise. And it's adjustable for different bullets and seating depths.

It does sacrifice one case that won't be reloadable afterwards.

 

[Jeff A]

QuickLoad shows fill ratio as one of the outputs. One of the many reasons to use QuickLoad.

 

[Webster]

I trust the smart people that I’ve been reading who say fill ratio is important for accuracy. A few reloading manuals include fill ratio for the max load, but most I have looked at do not. I searched the forum looking for a fill ratio formula but I couldn’t find anything. It would be even better if it accounted for seating depth. Maybe QL calculates fill ratio - I don’t know, I don’t have QL yet. Have any of you seen a formula or tool that does a good job calculating fill ratio?

What happened to shooting at targets to find the best load?

 

[Bart B.]

What happened to shooting at targets to find the best load?

Is it the fascination of minimizing a variable whose effect is smaller than that of the variables that aims the rifle?

 

[dmoran]

What happened to shooting at targets to find the best load?

In the planning stages of load development, knowing the Load Density can be useful in choosing powders and to what charge levels one may proceed with. Example:

​

It is my experience, best accuracy to bolt-action rifles normally come from higher load density levels of +88%, and personally like to be 94 to 98%. While I find even higher density to still achieve great accuracy, I personally don't like to have much compression to the powder column, which can attribute to hard seating and/or seating-depth variation.

 

[Webster]

In the planning stages of load development, knowing the Load Density can be useful in choosing powders and to what charge levels one may proceed with. Example:

View attachment 1045071​

It is my experience, best accuracy to bolt-action rifles normally come from higher load density levels of +88%, and personally like to be 94 to 98%. While I find even higher density to still achieve great accuracy, I personally don't like to have much compression to the powder column, which can attribute to hard seating and/or seating-depth variation.

Isn't that what you end up with shooting at targets and looking at groups. I do ladder test or powder and jump. Loads based on pressure and 1 - 1.5 gr under max must have good fill. Amounts to the same thing. Anything 2-3 grains under max most people consider near starting loads. No-one loads to 70-80% fill.

 

[dmoran]

Isn't that what you end up with shooting at targets and looking at groups. I do ladder test or powder and jump. Loads based on pressure and 1 - 1.5 gr under max must have good fill. Amounts to the same thing. Anything 2-3 grains under max most people consider near starting loads. No-one loads to 70-80% fill.

Why do you use a published max... what happened to shooting at targets to find the best load?
Oh wait.... you use it as a tool, to base your load development from.
Same as I use Load Density for: a tool to base load development from.

In fact, myself don't use books or online load data at all, and choose to calculate minimums, maximums, pressure, and velocity from the actual dimensions and capacities of the cases that I will be using, in conjunction with the actual chamber dimensions, bullet dimensions, and powder properties.

Good luck with your way and your choices of doing things

 

[6ShotsOr5?]

Thanks for the information Donovan.

As to the value of using equations and spreadsheets vs. just shooting to see what you get, there’s plenty of room in the shooting sports for different approaches to things, and room to learn something from all of them. As long as we are enjoying ourselves, it’s all good, except when it’s not, right?