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What happens inside the cartridge until maximum pressure is reached?
New member Hermann Weideman of Peregrine Monolithics in the USA corrected my figure about the equivalent in the universe of the 60,000 psi inside a 30-06 chamber. I compared it to the pressure at the bottom of an ocean 132,000,000 ft deep. I must have done a decimal incorrectly somewhere: the actual comparison of 60,000 psi is at the bottom of an ocean 135,000 ft. deep.
Thank you Hermann.
We all know that rifle propellants do not meet the thermodynamic features to satisfy the common definition of an explosive. Having said that, go limit the expanding volume behind a departing bullet relative to the so called burn rate of the propellant, secure the rifle on a heavy vehicle tire, pull the trigger from behind cover with a string and see that rifle chamber explode in quite impressive fashion. So, despite all the rolling eyes out there - in this discussion I shall refer to the deflagration that takes place inside a rifle chamber as the explosion. :-)
The time of this burn is explosively quick and way outside our normal frame of reference, as is the pressure rise it causes. Misinterpreting this rapid development often is the cause of misunderstanding the dynamics of what happens inside the case and inside the ever-expanding combustion chamber behind the departing bullet. This post is all about that. Here are some first perspectives:
Pressure: The 60,000 psi in say a 30-06 chamber is a very HIGH and very dangerous pressure, being the same as at the bottom of an ocean 135,0000 feet deep.
Time: The application of this enormous pressure (the burn time of the propellant) - is explosively quick - like 0.0005 of a second in a rifle and 0.0001 in a handgun. For a rifle that is one half of one ten thousandth of a second. This means that after having fired one thousand rounds your rifle has experienced only 1.5 seconds of pressure through the average 24” barrel. It is a sober thought that some overbore rifle barrels have only a one second useful life.
In a rifle like the 30-06 this maximum pressure is already achieved after the bullet has only moved about 1.4 inches away from the case mouth, and in handguns it happens with the bullet still in contact with the case. With the relatively small case and large bore diameter of the .458 Win Mag the maximum pressure is attained when the bullet has moved less than an inch after having been expelled from the case mouth.
Despite the considerable amount of published affirmations about the benefits of wide shoulders, steep shoulder angles, short, and super-short, and ultra super short, wide bodied cases - real scientific research has not presented any empirical proof of either a thermodynamic or accuracy gain in such designs. In fact the .375 H&H, 30-06, .303 Brit and .300 H&H are right up there in the top 25 winners of the SA Hunting rifle group shooting competitions where the winner will have shot a .16" group and number 25 will typically have shot a smaller than .30" group. The case geometry has little if any influence on burn efficiency or accuracy during that 1/4 of one ten thousandth of a second of gas generation as discussed below:
1. Rate of Pressure Increase
This time taken by the explosion, or put more correctly: the rate at which the pressure is increased to peak value when the bullet is at the optimum distance away from the case is the most important performance requirement for the propellant. Rate of pressure increase is critical for every different combination of: 1) case volume, 2) the increasing bore volume behind the departing bullet into which the gas must expand, 3) bullet mass, and very important: 4) bullet frictional resistance, and 5) seating depth. Considering each one of these variables which influences rate of pressure increase in turn, and then the combined interaction of these:
1.1 Case Volume. The case volume determines the maximum volume of propellant it can hold and therefor the maximum gas volume it can develop and compress into the increasing chamber volume behind the departing bullet within a set time at the rate of its explosion. This determines the maximum amount of heat energy and therefor pressure that can be obtained, which determines the maximum velocity it can impart to the bullet.
1.2 The increasing bore volume behind the departing bullet. The case volume to barrel volume relationship has a direct bearing on maximum achievable velocity of a specific weight bullet.
1.3. Bullet Mass. Higher bullet mass means higher inertia to be overcome by the gas pressure to start the bullet going. Related to the increasing bore volume behind the departing bullet a heavier bullet in any calibre needs a slower burning propellant in order to prevent a too fast rate of pressure increase than what a lighter bullet would need.
1.4. Bullet Frictional Resistance. Bullets with relatively high surface friction like the original Barnes “X” have higher resistance to initial moving and this adds to the higher inertia or reluctance to start out, and typically needs a slower burning propellant than conventional bullets to prevent an overpressure condition. Conversely, bullets like the GS Custom series with narrow drive bands that have very low friction coefficients need propellants of typically 2-3 steps faster burn rate than standard bullets of the same weight to obtain the rated pressure needed for the required velocity.
1.5 Seating Depth. Together with the bullet’s inherent frictional resistance any other immediate mechanical hurdle which temporarily delays immediate movement will delay the rate at which the volume behind the bullet increases into which the gas can expand, will cause a quicker rate of pressure build-up and possible over pressure.
Bullets like the Barnes series and particularly the original “X” series with known high friction coefficients must have an easier start by not having them immediately against the mechanical resistance of the lands.
The very low friction of the GS Custom bullets with drive bands benefits from the extra delay offered by contact with the rifling. This is particularly so where the ratio of case capacity to bore volume is low like the 30-30 and the .458 Win Mag. Adding a tight crimp to the .458 bullet will also assist towards a steeper pressure slope.
2. The Complete Picture:
2.1 Gas Generation and Pressure (1). Pressure starts slowly increasing after the firing pin has ignited the primer, then a very short-duration, rapid rise in pressure at first ignition of the propellant follows, and then immediately a zero rise in pressure for a short while followed by a gradual rise to about 10% of maximum pressure, all in about ¼ of one ten thousandth of second.
2.2 Gas Generation and Pressure (2). At 10% of maximum pressure the bullet starts moving out of the case and the volume of the combustion chamber behind the bullet starts increasing exponentially. This is the critical and determining phase for under of over pressure. Too little bullet mass or a very slippery bullet with a lot of freebore ahead moving out too easily, combined with a propellant that has a slow rate of pressure increase due to low burn rate or low gas volume from a small case relative to bore volume (30-30 and .458 Win Mag) may not reach rated pressure and muzzle velocity.