Summary: This blog post explores the concept of fill rate in ammunition reloading, its importance, and the reasons why maintaining a fill rate between 80% and 100% is desirable. We discuss the potential dangers of press loads, the risks associated with low fill rates such as in subsonic loads, and how these risks can be mitigated using filler materials. Additionally, we examine how the positioning of powder within the cartridge can impact ballistic performance, referencing research by Reload Swiss.
What is Fill Rate in Ammunition Reloading?
Fill rate refers to the volume of the cartridge case that is occupied by the powder charge. A high fill rate means the powder nearly fills the case, while a low fill rate leaves significant empty space. Achieving the correct fill rate is essential for both safety and performance, making it a fundamental consideration in the reloading process. Also, see more information about the fill rate definition here.
Why Fill Rate Matters
A consistent fill rate is crucial for reliable ballistic performance. Powder burns more uniformly when it is compacted within the cartridge, leading to predictable pressure curves, velocities, and accuracy. Poor fill rates can result in inconsistent ignition, velocity deviations, and potential safety hazards.
The Ideal Fill Rate: 80% to 100%
A fill rate between 80% and 100% is often recommended because:
It promotes consistent ignition and burn rates.
Reduces the risk of powder shifting within the case during handling or firing.
Minimizes pressure fluctuations that can occur with partial loads.
This range ensures that there is enough powder to generate the required pressure without leaving excessive empty space that could cause erratic performance.
What Are Pressloads and Why Are They Bad?
Pressloads occur when powder is excessively compressed within the case, leading to:
Irregular ignition due to excessive compaction.
Inconsistent velocities and poor shot-to-shot accuracy.
Potentially dangerous pressure spikes that can damage the firearm or injure the shooter.
Maintaining a fill rate within the recommended range helps avoid press loads and ensures safe, reliable performance. It is essential to carefully measure powder charges and avoid over-compression during the reloading process.
However: There are often exceptions from the general rule. Sometimes a slight pressload can achieve the best accuracy and velocity. It depends on the caliber, powder, and gun. This is why testing various loads for a specific gun is so important. An example of a very well-working press load is i.e.: 6.5 Creedmore, Hornady brass, .264 | 143gr | Hornady ELD-X, 42.5gr | 2.75g ReloadSwiss RS 62, COL: 2.80" | 71.1mm.
Risks of Low Fill Rates
Low fill rates, such as those used in subsonic loads, can lead to several issues:
Inconsistent powder burn due to uneven distribution within the case.
Dangerous pressure variations can affect accuracy and safety.
Increased risk of detonation (rapid, unintended combustion) which can cause severe damage.
Subsonic loads often require lower powder charges, making fill rates challenging to manage. Understanding these risks is essential for reloaders working with specialize. Filler materials, like cotton or foam wads, can be used to:
Keep powder positioned near the primer for consistent ignition.
Ensure uniform burn rates by preventing powder migration within the case.
Reduce pressure variations and improve shot-to-shot consistency.
Proper selection and placement of fillers can make low-fill cartridges safer and more reliable, especially when working with subsonic ammunition.
Powder Positioning and Its Impact on Ballistics
The position of powder within a cartridge can also significantly affect ballistic performance. Needless to say, this is more relevant for low-fill rate situations. Research by Reload Swiss has shown that pressure differences can occur depending on how the powder is distributed inside the case. Powder resting against the primer (see above image 1) ignites differently than powder settled at the front of the case (see above image 2) , leading to variations in pressure and velocity.
Here is an example of how the positioning of the powder influences muzzle velocity and pressure (.308 Winchester, RS14 10.5 gr, Fill rate 74%, Lapua Brass, Lapua Subsonic B416 200 gr, S&B LR primer, C.O.L.: 66.6 mm):
| Position of the Powder | |||
|
| 1 -on Primer | 2 - Horizontal | 3 - at the bullet |
Velocity | m/s / fps | 315 / 1033 | 320 / 1050 | 325 / 1066 |
Pressure | bar / psi | 1651 / 23946 | 1924 / 27905 | 1996 / 28950 |
To ensure consistent ballistic performance, reloaders should:
Use filler materials when necessary.
Handle cartridges carefully to prevent powder shifting.
Experiment with different powders and charges to find the most stable combinations.
Conclusion: Maintaining an optimal fill rate is essential for safe and accurate ammunition reloading. By understanding the importance of fill rate, avoiding press loads, mitigating risks associated with low fill rates, and considering powder positioning, reloaders can achieve more reliable and precise results. Reloading is both an art and a science, and careful attention to fill rate ensures that every shot is consistent, accurate, and safe.
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