The Percussion System, Part 4: The "Fuel"—Engineering the Percussion Cap
In our series on the percussion system, we've established the "engine" (the hammer lock) and the "nozzle" (the nipple). Now we analyze the component that makes it all work: the percussion cap.
The cap is the true genius of the system. It's a self-contained, weather-resistant, all-in-one detonator and gas seal. It's the "fuel" for the ignition, and its chemical and mechanical properties are a significant leap in engineering.
1. The Chemistry: Corrosive vs. Non-Corrosive
A cap does not spark. It explodes. It contains a shock-sensitive primary explosive. The exact composition of this "primer" is the most critical chemical variable.
Historic "Corrosive" Caps: The original 19th-century caps were charged with fulminate of mercury or, later, a potassium chlorate mixture.
How it Works: When crushed, these compounds explode, producing a hot, violent jet of flame.
The Flaw: The chemical byproduct of this explosion is potassium chloride (Source: Calibre Magazine). This is a salt. This salt residue, when combined with the slightest humidity, is hyper-aggressive. As we noted in Part 2, this is the corrosive "blowback" that fouls the lock and rusts the breech.
Modern "Non-Corrosive" Caps (NCNM): Today's caps (like those from CCI or RWS) are "non-corrosive" and "non-mercuric." They are based on the lead styphnate primer formulas developed for centerfire cartridges around the 1930s.
How it Works: The composition is a balanced mix of an initiator (lead styphnate), an oxidizer (barium nitrate), and a fuel (antimony sulfide). (Source: Office of Justice Programs, "Primer Composition and Gunshot Residue").
The Benefit: This compound produces a hot, reliable ignition without the corrosive potassium chloride salts. This is the single biggest reliability improvement in modern muzzleloading. A modern "non-corrosive" cap is far less destructive to the nipple, bolster, and lock internals.
2. The Mechanics: The Cup and The Foil
The cap is a brilliant piece of mechanical design, serving two purposes.
The Cup (The Gasket): The cap's body is made of a soft, malleable metal (typically copper or brass). As we discussed in Part 3, this is by design. When the hammer strikes, it swages the cup's skirt against the nipple, creating a gas seal at the moment of ignition. This seal is what contains the explosion and directs it down the flash channel.
The Foil (The Weather Seal): Inside the cap, the primer compound is protected by a thin layer of foil or shellac. This is the component that makes the percussion system an all-weather design. It is the true barrier that solved the flintlock's "open pan" flaw, sealing the sensitive primer from humidity, sweat, and rain.
3. The Variable: Fit is Everything (#10 vs. #11)
The percussion cap is not a "one size fits all" component. The system's reliability is 100% dependent on the cap-to-nipple fit. A poor fit re-introduces all the problems the system was designed to solve.
Unfortunately, there is no universal "mil-spec" for caps and nipples. Dimensions vary by manufacturer.
#10 Caps: Generally have a smaller diameter. They are most often used for revolvers or nipples with a smaller cone.
#11 Caps: The "standard" for most rifles. They are slightly larger in diameter and height.
Musket Caps: A much larger, "top hat" or 4-wing cap designed for military nipples. They contain a significantly larger (and hotter) charge, a fact noted in American Longrifles forum archives.
Failure Modes:
Fit is Too Loose (e.g., #11 on a #10 nipple): The cap is not a "seal." It's just sitting on top. This is the #1 cause of cap splitting, as the ignition gas blows out the side instead of down the channel. This causes hangfires, vents pressure, and throws shrapnel.
Fit is Too Tight (e.g., #10 on a #11 nipple): The cap will not fully seat. The hammer's energy is wasted seating the cap before it can crush the primer compound. This is the #1 cause of "dented cap" misfires.
The Solution: The cap must be pushed onto the nipple and fit snugly, with no "wobble," but without needing force. The only way to find the right combination is to test your specific nipple against different brands (CCI, RWS, etc.) of caps.
Conclusion
The percussion cap is the self-contained "cartridge" of the muzzleloader. It's a chemical detonator and a mechanical gasket in one. Its "non-corrosive" modern chemistry is a major advantage, but its performance is totally dependent on a precise mechanical fit with the nipple.
Tags: Percussion, Engineering, Primer, Corrosive, Nipple