By Duane Thomas
When we say a primer is “sensitive,” we’re referring to the amount of force – firing pin impact energy – required to cause the priming compound to ignite. There are multiple factors determining a primer’s sensitivity, like the thickness of the cup itself, and the shape of the anvil (a piece of metal inside the primer – the firing pin compresses the priming compound between the cup and the anvil, causing it to ignite), but in this series of articles we’re going to focus specifically on the actual composition of the priming compound itself, which has five components: the initiator, sensitizer, (sometimes) frictionator, oxidizer, and fuel. We’ll begin with the initiator.
Priming compound is a pressure sensitive material – commonly referred to as “shock sensitive” – in other words, we hit it, and it burns. The initiator is the material that actually detonates under impact. Early priming compounds used mercury fulminate or potassium chlorate which were unfortunately corrosive and would attack and damage steel unless the gun was thoroughly cleaned, in short order after being fired, but today it’s lead styphnate (by far the most widely used) or diazodinitrophenol (DDNP) found in lead-free primers.
Lead styphnate is moderately sensitive. It does a fine job balancing safety, ignition-reliability, and non-corrosive properties, which is why it’s been the standard initiator in non-corrosive priming compound since the 1920s.
DDNP is the initiator used in most lead-free primers. It’s less sensitive than lead styphnate; but, not containing lead, it gets us around worries of inhaling airborne lead particles, and its lack of sensitivity can be, to a large extent though not completely, compensated for with sensitizers.
The initiator is basically stored energy. When it’s subjected to shock (firing pin impact), that breaks the initiator’s chemical bonds, at which point it undergoes a rapid chemical reaction (i.e. detonation). This detonation causes the initiator’s molecules to decompose almost instantly, releasing a burst of heat (often exceeding 2,000°C), hot gases, and a shockwave.
The initiator’s role is to provide the initial spark, with minimal energy input compared to what is released, making its sensitivity critical. The entire process occurs in milliseconds and is baseline necessary for reliable primer ignition.
