The Percussion System, Part 6: Engineering the Main Charge

In this series, we have engineered the percussion rifle from a concept (Part 1) to a complete machine. We have analyzed the engine (the lock, Part 2), the nozzle (the nipple, Part 3), the detonator (the cap, Part 4), and the plumbing (the breech, Part 5).

The final component in this system is the main powder charge.

The percussion system's primary advantage is not just its speed (~25ms), but the quality of its ignition.

• A flintlock ignites the charge with a low-pressure, "cloudy" flash of burning FFFFg.

• A percussion cap ignites the charge with a high-pressure, high-velocity jet of incandescent gas and particles.

This "jet" of flame completely changes how we should engineer the main charge, especially in regard to granulation, breech design, and substitutes.

1. The Granulation Debate: FFg vs. FFFg

In our Ignition Chain series, we established that a flintlock must use a FFFFg prime. In a percussion rifle, the cap is the prime, so we only need to select our main charge.

The "rule of thumb" from the NMLRA and other sources is:

• FFg (2F): For .50 caliber and larger rifles.

• FFFg (3F): For .45 caliber and smaller rifles, and pistols.

The Engineering Reason: This is about burn rate and pressure curves.

• FFFg is a finer grain. It has more surface area, so it ignites and burns faster, creating a faster pressure spike. This is ideal for a short pistol barrel or a small .40 caliber bore, where you need to build pressure quickly.

• FFg is a coarser grain. It burns slower (relative to 3F), creating a longer, more gradual pressure curve. This "push" is ideal for accelerating a heavy .54 caliber ball down a long rifle barrel.

Using FFFg in a large-bore rifle can, as noted in American Longrifles archives, be harder on patches, as the sharp pressure spike can "burn out" or "tear" a patch, destroying accuracy. Conversely, using FFg in a pistol may result in a "bloop" and unburnt powder, as the ball leaves the barrel before the powder has time to fully burn.

2. The Breech Design / Ignition Column

This is where the percussion "jet" shines.

• A flintlock's "cloudy" flash is poor at igniting a deep, full column of powder. It's like trying to light a cigar from the side with a match.

• A percussion cap's "jet," when paired with a patent breech (as discussed in Part 5), acts like a blowtorch.

As detailed in Canadian Gun Nutz archives, the Nock Patent Breech (the predecessor to most patent breeches) was designed for this exact purpose. It directs the ignition flame in a linear path, sending a "spurt of flame throughout the main charge, more or less igniting it all in one go."

This is why a well-designed percussion rifle with a patent breech is more efficient and consistent. It ignites the entirepowder column more uniformly, wasting less powder and producing more repeatable velocities.

3. The Substitute Problem: Heat vs. Flash

This is the most critical concept for the modern shooter. Why are black powder substitutes (like Pyrodex) notoriously unreliable in flintlocks, but (mostly) functional in percussion rifles?

The Physics: Ignition Temperature.

• True Black Powder: Auto-ignition temperature is relatively low, around 600°F (315°C).

• Pyrodex & Substitutes: Have a much higher auto-ignition temperature, around 750°F (400°C). (Source: American Longrifles / Reddit r/blackpowder archives).

A flintlock's pan-flash is a low-pressure, low-temperature "flash" of burning FFFFg. It is often not hot enough to reliably cross that 750°F threshold, resulting in misfires and hangfires.

A percussion cap, however, is a detonation. Its "jet" is not just hot; it's a high-pressure, high-velocity stream of incandescent particles. This "torch" is far more effective at delivering the intense, focused heat needed to ignite stubborn substitutes.

This is why Pyrodex Pellets, for example, have a small "black ignitor" on one end—it's a real black powder booster charge (Source: Hodgdon Basic Muzzleloading Manual). It's an admission that the substitute itself is too hard to ignite without help.

Conclusion

The percussion system is a complete engineering solution. Its fast lock and hot, "jet-like" ignition from the cap are perfectly matched to the linear ignition path of a patent breech. This combination creates a system that is not only faster and more weatherproof than a flintlock, but also fundamentally more efficient at igniting the main powder charge. It is a masterpiece of 19th-century systems engineering.

Tags: Percussion, Engineering, Black Powder, Breech, NMLRA