Pressure is what makes bullets go. It can be a good friend, but becomes a dangerous foe when it exceeds your firearm’s safety margin. You can blow up the strongest modern firearm if your handloads are too hot, but pressure is what makes guns work. Everything from muzzle loaders to rimfires to centrefires rely upon a burst of burning gas to propel the projectile or shot charge out of the barrel. Without this pressure, firearms could not have been developed. But when guns are subjected to too much pressure, things are apt to come apart – often rather quickly!
The hunter who uses factory ammo only has to have enough sense to use the correct cartridge for his musket and keep the muzzle free from obstructions. But the handloader, the chap who concocts his own ammo, must necessarily be aware of the causes of excessive pressures and their results, and above all he must recognize the signs which indicate the point of pressure danger.
It is an inescapable fact that gun powder creates a volume of gas equal in weight to the charge of powder burned. However, the way that gas is generated has a lot to do with the way guns behave. Blackpowder shooters weren’t much bothered by pressure problems. They could use a case full of the stuff without any fireworks; but as the science of ballistics and the development of propellants moved into high-efficiency realms with smokeless powders developed for limited combinations of case volume, bullet size, and specific operating pressures; and as handloaders began to tinker with these complexities in their back sheds, so has the problem of pressure recognition become more critical.
With black powder, things were not so complicated, because black powder is a non-progressive propellant. It burns at a constant rate and develops a pressure level consistent with the amount of powder burned. When a black powder gun blows up, it is because too heavy a charge was used, the gun was too weak, or the bore of the gun was obstructed.
Modern smokeless powders burn entirely differently. They are progressive burning in that their burning rate (and the rate at which the powder is converted into gas) is largely determined by the heat and pressure in the chamber in which the powder burns.
You can compare the respective burning rates of black and smokeless powders by lighting a small amount of each in an unconfined space.
As soon as you ignite the black powder with a match it flares into a cloud of white smoke, whereas smokeless powder burns quite slowly when not confined. A normal charge of smokeless powder for, say, a .270 Winchester cartridge might require five to ten seconds
to be consumed – much slower than the few milliseconds in which it is consumed when confined in a cartridge case.
Obviously, we cannnot have today’s impressive ballistics with bullet speeds over three times the speed of sound, without pressure. The handloader, by a careful selection of powders that in a certain case volume and behind a given bullet will generate high but controllable operating pressure and maintain it for a longer time, (build up to a smooth-topped time-pressure curve) to obtain maximum power from the combination, and use all the energy in the powder.
With a properly loaded cartridge, the pressure in the barrel climbs to a predetermined peak and then diminishes as the bullet accelerates on its way down the barrel. The high point of the pressure load occurs during the first inch or so of the bullet’s travel. That is when the powder is most confined and the static inertia of the projectile is being overcome. As the bullet moves down the barrel, the combustion chamber (space in which the powder burns) is greatly enlarged and the pressure drops off.
Common to smokeless powders is a peculiarity that is best described as the higher the more. That is, as powder burns it creates heat and pressure. The higher those go the faster the powder burns. This in turn, creates more heat and pressure to make it burn even faster creating even more heat and pressure. Something has to give and thankfully it’s the bullet. But with an overload this escalating spiral can get put of hand in one helluva hurry, if you don’t read the signs correctly.
Other factors can cause pressures to spiral out of control in otherwise normal and safe guns and ammunition. One of the most common is an increase in ambient temperature. Early in my reloading career I loaded a batch of .25-06 ammo, and since I wanted to reach out across wide gullies and open mountainsides for goats, the cartridges were loaded to the maximum safe pressure limit. Here in the mountains where I live the loads grouped well and showed not the least sign of pressure, such as excessively flattened primers or heavy bolt handle lift. But when I took those reloads north into Queensland on a pig hunt some strange things happened. The climate was warmer than it had been where I used the reloads at home, and the increased temperature noticeably jacked up pressures. The bolt handle got hard to lift. Also, I was shooting fast at a mob of pigs and as the barrel warmed up, it heated the ammo even more. Soon I experienced leaking primers and my bullets were not kicking up dust to tell me by how far I’d missed.
I realized that the higher chamber pressure had increased the velocity beyond what the construction of that particular design of bullet could withstand. They were disintegrating in mid-air and a blue streak of smoke could be seen marking the bullet’s path as they came apart.
For many years the British loaded virtually all sporting ammo used in tropical countries like India and Africa. They took the heat factor into consideration and loaded cartridges for dangerous game on the mild side. A cartridge that produced too much pressure on a hot day might just jam the mechanism or stick in the chamber at a critical moment.
For years the U.S ammunition industry has expressed pressure levels in pounds per square inch (p.s.i) indicating how much outward pressure was being exerted against each square inch of the interior of the chamber and bore. Conventionally published pressures are taken under hightly standardized conditions, with temperature controls and all such factors held to SAAMI (Sporting Arms and Manufacturer’s Institute) specifications which establishes safe maximum pressure standards for U.S made ammo. This is done in a pressure gun dimensioned to precise industry standards using special barrels with minimum specs in a universal receiver.
Chamber pressure is measured in a variety of ways. The British usually measure the rearward thrust of the cartridge against the breech block and express the pressures in tons per square inch. The Americans have traditionally used the p.s.i index which is rather arbitrary. It is more useful for comparing relative pressures than for determining actual pounds per square inch of chamber pressures. The p.s.i index therefore, is similar to an engine’s horsepower rating, which doesn’t actually tell us how powerful an engine is compared to a real, live horse.
The p.s.i system is gradually being phased out in favour of another index – C.U.P (copper units of pressure). The pressure figure itself is usually read from tarage tables based on how much a cylindrical chunk of copper about a 1/2 inch long and of known physical properties, is compacted by the gas force as it drives through a hole in the rifle chamber, a fitted piston against the anchored crusher. The higher the chamber pressure, the more the crusher is shortened. Squeezing it about .103 might mean a pressure of around 58,000, for example. The C.U.P figure is usually about 85 percent of the correct pressure and it is peak pressure only. However, it is also a relative pressure figure and not the true pressure.
For firearms that produce lower pressure levels, such as shotguns and handgun, a lead crusher is used. Shotshells usually produce pressures in the 6,000 to 12,000 p.s.i range.
Other methods of taking chamber pressures are by means of a transducer, an electronic system that not only indicates the peak pressure of the load, but also traces on an oscilloscope screen a pressure wave that shows the pressure at any given point along the barrel. Like the copper-crusher system the transducer mechanism is mounted in the chamber wall of the gun, but is quite different in the way it measures pressure.
There are two different types of transducer mechanisms. One is piezzo electric type, which utilizes a crystal that produces and electric charge when pressure is applied. The higher the pressure the higher its electrical output. By measuring the value of this electrical discharge, the amount of pressure on the crystal can be determined.
Another transducer type is the strain-gauge, which measures how much a delicate wire coil’s electrical resistance is altered when the coil is stretched by an expanding barrel.
All of these pressure measuring systems are beyond the means of most handloaders. Inexpensive systems of doping pressures, Homer Powley’s computation system and PMax gauge for example, will in qualified hands work with acceptable indications, especially on a comparative basis. Over the years there have been several other systems, but they are rather too complicated.
Wayne Blackwell’s Load From A Disk computer programme calculates starting and maximum loads with commendable accuracy, but only if it is correctly applied. Judging from some of the letters I’ve received this is not always the case.
Most reloaders don’t go so far, yet they do need to know to recognize the symptoms of pressure, though they don’t really have to know what the exact figure would come out at – 47,500 or 46,800 – so long as it is reasonable and safe for their equipment, brass and rifle, which are inevitably different in many details from the standardised pressure gun setup.
Few ammo and powder makers publish pressure data, but Norma and Hodgdon do. However, their figures should not be taken as gospel, particularly their maximum loads. If the Hodgdon table for example says that their .223 Remington load of 25.5gn of AR2208 powder and the 55gn Speer bullet gave a velocity of 3174fps at 41,300 C.U.P, that’s no reason to believe that your use of the same charge in your rifle would give a 41,300 CUP reading.
Might be lower, might be higher. It All depends on the internal dimensions of chamber, throat, and bore; on the diameter, the length of bearing surface, and hardness of bullets even of the same weight; on the size of the primer flash hole, and brisance of the primer pellet; on the thickness and hardness of the cartridge-case neck, the case’s general fit in the chamber, the thickness – hence capacity – the case body itself; even to some smaller extent on the temperature and the barometric pressure, not to mention the manufacturers specific lot of powder.
A good many older high-intensity rimless and belted magnum cartridges such as the .270 Winchester and 7mm Remington Magnum generate peak pressures in the vicinity of 50,000 to 55,000 p.s.i, but in recent years some cartridges (the 6mm Rem. for one) are being loaded to 60,000 p.s.i, and the new Winchester Short Magnums are being loaded to an average 63,000 p.s.i. Most modern bolt-action rifles, plus some autoloaders and the Browning BLR are designed to withstand a momentary pressure level of 55,000 p.s.i plus a safety margin. But it is not until pressures get up around 75,000 p.s.i that things begin to give.
Next month we’ll continue this discussion about pressure and how handloaders can recognize symptoms that warn they are loading too hot.
This article was first published in Sporting Shooter, June 2011.
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