Ignition, Combustion and Chamber-Pressure Curves

The hot gas jet from the primer enters the propellant charge and raises the temperature of the first layer of powder grains to their ignition temperature, typically 200-300 degrees Celsius for nitrocellulose-based propellants. This happens at the base of the bullet (in most cartridges) or at a central igniter tube in large-calibre charges. The initial ignition of a few surface grains releases heat that ignites adjacent grains, creating a thermal cascade that propagates through the powder charge.

The speed of ignition propagation through the charge is influenced by charge density (how tightly the powder is packed), grain geometry and size (more surface area per grain accelerates the cascade), charge weight relative to case volume (a case filled to 95% of its volumetric capacity ignites and burns very differently from one filled to 50%), and the temperature of the loaded cartridge. The NATO STANAG 1235 protocol for temperature sensitivity testing requires ammunition to be tested at -54°C, +21°C (ambient), and +71°C, because propellant burn rates shift significantly across this range, affecting peak pressure and muzzle velocity. At -54°C, many double-base powders burn measurably slower, reducing peak pressure and muzzle velocity; at +71°C (equivalent to a vehicle dashboard in direct sunlight in a hot climate), burn rates increase, pushing peak pressure toward or above MAP. Indian CFSL casework involving ammunition stored in hot, humid environments has had to account for temperature-induced pressure variation when evaluating whether a weapon failure was caused by the ammunition or the firearm.

The transition from a confined deflagration to a true detonation is a failure mode known as a detonation overload, and it is exceedingly rare in normal small-arms use but not impossible when a charge is dramatically compressed (as in a squib load where a projectile is lodged in the bore, building back-pressure). For practical internal ballistics, propellant combustion in a cartridge is a controlled deflagration: supersonic combustion (detonation) in the chamber would produce pressures orders of magnitude above MAP and would destroy any firearm.

The chamber pressure-time curve of a firing event has a characteristic shape that reflects the sequential physical events from primer ignition to bullet exit. Understanding each phase of this shape allows the forensic ballistician to extract quantitative information from test-fire data.

The curve begins at ambient pressure. In the first 0.1-0.2 milliseconds, primer deflagration raises pressure sharply from ambient to a low initial level. The propellant then begins burning; pressure rises steeply as gas evolves faster than the bullet can move (the bullet is constrained by its crimp and engraving force until it begins to move). Peak pressure is reached when gas evolution rate equals the rate of volume increase from bullet displacement. For 5.56x45mm NATO ammunition in an M4-length (14.5-inch) barrel, peak pressure is approximately 55,000 psi (SAAMI MAP) reached at approximately 0.4-0.6 milliseconds. For 9x19mm Parabellum in a 4-inch barrel, peak pressure of approximately 35,000 psi is reached at approximately 0.3-0.4 milliseconds.

After peak pressure, as the bullet accelerates down the bore and the combustion chamber volume increases faster than gas is being produced, pressure falls. The rate of fall depends on whether combustion is complete at peak pressure (which it typically is for well-matched loads) or continues past peak (producing a flatter, broader curve that pushes the bullet harder in the later part of its bore travel). The bullet exits the muzzle when pressure has fallen to the muzzle exit pressure, which for most rifle calibres in a full-length barrel is 5,000-15,000 psi, still substantially above ambient. For a short-barrelled firearm (a pistol or SBR), muzzle exit pressure is higher because the bullet exits earlier on the falling pressure curve.

The area under the pressure-time curve is proportional to the impulse delivered to the bullet (and the equal and opposite recoil impulse delivered to the firearm). This area is the metric that most directly determines both muzzle velocity and felt recoil.

Two principal methods are used to measure chamber pressure in small-arms ammunition testing: the copper crusher method (obsolete but still occasionally referenced) and the piezoelectric transducer (piezo gauge) method, which is now the standard in both SAAMI and CIP testing.

The copper crusher method used a copper cylinder of known dimensions placed in a port in the chamber wall. The pressure from a fired cartridge compressed the cylinder, and its deformation (measured by a micrometer and compared against a reference calibration curve) gave a peak pressure in "copper units of pressure" (CUP), also written as "psi LUP" (Lead Units of Pressure) for pistol calibres. Crusher pressures are generally somewhat lower than equivalent piezo measurements and are not directly comparable: a load producing 52,000 CUP is not the same as 52,000 psi (piezo). The transition away from crusher testing is largely complete, but legacy data in older loading manuals still uses CUP/LUP units, which creates confusion in forensic reconstruction when comparing published data across decades.

The piezoelectric transducer method uses a quartz or ceramic crystal mounted in a port in the chamber wall (barrel transducer) or in the case-head location (conformal transducer). When chamber pressure acts on the transducer face, the crystal generates a charge proportional to the applied force; this charge is converted to a voltage by a charge amplifier and recorded against time at high sampling rates (typically 100,000-1,000,000 samples per second). The result is the complete pressure-time curve, not just a peak value. SAAMI's standard reference barrel for 5.56x45mm uses a 24-inch test barrel with the transducer at the 1.000-inch position from the case head. CIP's reference barrel for the same cartridge uses a slightly different transducer position and test barrel length, which contributes to the different MAP figures published in SAAMI Z299.4-2015 (55,000 psi) versus the CIP TDCC specification (58,000 psi; approximately 400 MPa) for 5.56x45mm.

In forensic reconstruction, the critical issue is which standard applies to the weapon being examined. A firearm chambered to SAAMI specifications should be evaluated against SAAMI data; a weapon to CIP proof standards against CIP data. Indian ordnance specifications are aligned with CIP standards for most NATO-type calibres, given India's historical and current procurement of European-designed weapons and the Proof of Arms Acts 1934 (as updated) administered by the Proof Houses at Delhi and Kolkata.

Muzzle velocity is a consequence of the work done on the bullet by the propellant gas as it travels down the bore. The work done equals the integral of force (pressure times bore area) over the distance of bullet travel. Since bore area is constant, muzzle velocity is proportional to the integral of pressure over bore length, which is the area under the pressure-distance curve (not the pressure-time curve). These two integrals are related but not identical: a broader, lower pressure curve can deliver the same area under the pressure-distance curve as a sharper, higher peak.

This relationship explains the diminishing returns of adding powder charge to a load. When charge weight is increased, peak pressure rises steeply while the post-peak pressure tail rises less proportionally: the bullet is past the peak by the time the extra gas is generated. Beyond the SAAMI or CIP MAP limit, additional charge weight produces little additional muzzle velocity (the bullet is already moving fast enough that dwell time in the barrel is short) while driving peak pressure well above the proof standard of the firearm, risking case rupture or action failure.

Barrel life is primarily determined by two mechanisms: erosion of the bore surface by hot propellant gas (thermal and chemical erosion, concentrated at the throat where pressures and temperatures are highest), and mechanical wear from projectile engraving. Double-base powders, with their higher flame temperatures, erode bores faster than single-base powders at equivalent pressure. The STANAG 4172 specification for 5.56x45mm NATO service rifles specifies a minimum barrel life of 10,000 rounds before the projectile's average muzzle velocity has dropped by more than 30 m/s from new. In forensic reconstruction of a weapon's service history, bore condition (measured by a bore gauge or assessed visually by the examiner) is one indicator of the number of rounds fired through the barrel, though the relationship is not linear and depends heavily on ammunition type.

In casework, barrel life matters when an expert is asked to opine on whether a weapon was capable of producing a specific muzzle velocity on the date of an offence. A heavily eroded bore will produce lower muzzle velocity at the same chamber pressure because the bullet's engraving is incomplete (gas bypasses the projectile through the worn throat), the bore is slightly oversized, and in extreme cases the rifling is nearly absent, producing an essentially unrifled barrel with corresponding loss of stability and accuracy. CFSL New Delhi and CFSL Hyderabad both use test-fire protocols to document the muzzle velocity of recovered weapons; in the UK, the National Ballistics Intelligence Service (NABIS) and FSR-accredited laboratories maintain similar test-fire capability.

Three principal failure modes arise from disruptions to the normal ignition and combustion sequence. Each leaves characteristic physical evidence on the cartridge case, the firearm, and potentially the target.

A squib load (also called a squib fire) occurs when the propellant charge fails to ignite or burns incompletely, generating insufficient gas to propel the bullet fully out of the barrel. The bullet is driven partially down the bore by the residual pressure of the primer deflagration alone, and it lodges in the barrel, usually between 2 and 8 inches from the chamber end. The fired case shows a normal primer indent and primer cup deformation. If the shooter cycles the action and fires again without detecting the obstruction, the second round impacts the lodged bullet, producing a bulged or burst barrel, a sudden sharp report, and potentially a dangerous case-head separation. Squib loads result from: missing propellant charge (double-pull at the loading machine depositing no powder), severely degraded or contaminated powder that fails to ignite, or primer-only ignition on reloaded ammunition where powder was accidentally omitted. In India, CFSL forensic reports have documented squib-induced barrel failures in recovered country-made weapons that used improvised, partially-burned powder substitutes. In the US, squib fires are one of the more common catastrophic firearm failures investigated by ATF technical examiners.

A hangfire is a delayed ignition: the firing pin strikes the primer, but ignition does not occur immediately. There is a perceptible delay, ranging from a fraction of a second to several seconds, before the round fires. Hangfires result from: degraded primer compound (moisture-contaminated, aged, or improperly stored ammunition), primer compound partially displaced in the cup, or a weak firing-pin strike that partially crushed the compound without fully initiating it. The standard response to a presumed misfire is to keep the weapon pointed downrange for a minimum of 30 seconds (US military protocol, derived from ATF and SAAMI guidance) before opening the action, because a hangfire may fire up to several seconds after the strike. An examiner may detect a hangfire from the primer cup: a delayed ignition often shows a less-sharp primer crater (because the cup was re-pressurised after the partial strike during the delay) or a slightly off-centre firing impression. In the context of the Bombay 1993 firearms forensic examinations, CFSL analysts had to distinguish between hangfires and misfires in assessing recovered cartridges from seized weapons.

A slamfire is the opposite problem: the firearm fires when the bolt closes, without a separate trigger pull, due to a free-floating or broken firing pin contacting the primer with sufficient force during chambering. Open-bolt submachine guns (the Sterling L2A3, the Uzi, the MAT-49) are inherently slamfire-capable if the primer is excessively sensitive or the firing pin is damaged. Semi-automatic weapons with free-floating firing pins (including the AK pattern, the SKS, and several early M16 production variants) can slamfire if the firing pin inertia drives it forward hard enough against a highly-sensitive primer during chambering of a round. Slamfires produce full automatic or burst fire from a semi-automatic weapon, which is a serious legal and safety issue; the examiner documenting a slamfire must distinguish it from an intentional modification (the addition of an auto-sear or disconnector bypass) versus a mechanical failure.