Testing Firearms: Measuring Chamber Pressures In this post, we will look into the technique of measuring pressures inside a barrel, when a firearm is discharged. It is important for people to determine pressures of a firearm with various types and brands of ammunition, because some ammunition may produce too much pressure and therefore cause the firearm to explode. A system that enables one to determine the pressure is therefore very useful to determine which ammunition types may be safely used by a firearm.
The maximum pressure is exerted at the breech end in the firing chamber and decreases further down the barrel tube. That's why published numbers usually only list the chamber pressure. It must be noted that there are a couple of different standards in exactly how the pressure is measured. In our first post about proof testing, there were two organizations mentioned, CIP and SAAMI, that publish firearms data. Unfortunately, they have slightly different ideas about how chamber pressures should be measured. We will study these differences a little while later down in this post. Suffice it to say that CIP doesn't care about the shape of the cartridge when deciding which point to take the pressure reading from, whereas SAAMI has different locations to measure the pressure from, based on the shape and diameter of the cartridge. Therefore CIP and SAAMI numbers don't match up for this reason. The classic method of measuring pressures goes back to the 1800s and early 1900s. It was the method most in use until about the 1960s. It uses crusher gauges to determine pressures.
It consists of a device that allows mounting of a gun barrel. The gun barrel is drilled at various points where pressure measurements are desired. To each of these holes is pushed a tight fitting stopper and the other end of the stopper is held in place by a precisely machined cylindrical piece, which in turn is supported by a steel screw. The precisely machined cylindrical piece is of uniform density and made of copper or lead, depending on the firearm type. For lower pressure weapons such as shotguns or smaller pistols, a lead cylinder is used, whereas copper cylinders are used for higher pressure applications such as rifle or most handgun cartridges. When the cartridge is fired, some of the gas pushes upwards and drives the stoppers out of their holes. This has the effect of squeezing the copper (or lead) cylindrical pieces against the steel screws holding them in position.
The amount of deformation of the copper or lead cylinders is measured very precisely and compared with a chart of similar cylinders which were deformed previously under known pressures and the corresponding value is called the Copper Unit of Pressure (CUP) or Lead Unit of Pressure (LUP) value. While CUP or LUP values are meant to be compared with the crushing power of a known pressure, this is not always the case. For example, the same amount of deformation can occur from a short duration high-pressure pulse as from a longer duration, but lower pressure pulse. Also, these numbers tend to be a bit lower than peak pressures measured using transducers. Therefore, when
measurements are made using crusher gauges, the pressure is listed in mega-pascals (MPa - the SI standard unit of pressure) or pounds per square inch (psi - the imperial standard unit of pressure) followed by the letters CUP or LUP to indicate that the measurements were done using crusher gauges. The crusher gauge method was the only reliable method of measuring chamber pressures until the 1960s, when cheap piezoelectric devices became available. This method is called the Conformal Transducer method or Piezo method. A piezoelectric transducer (a.k.a. sensor) has the property that it generates electricity as it is crushed. The setup is similar to the crusher gauge method, except that instead of a copper cylinder, a quartz crystal transducer is used. Quartz is a material that exhibits piezoelectric properties. Thus, when the cartridge is fired, the transducer transmits electricity, which can be measured and then compared against values generated by similar transducers, when subject to known amounts of pressure. This method has the advantage of measuring pressures at different instants of time, as the bullet leaves the barrel.
Measurements by this method generally read about 15-20% higher than those shown by the crusher gauge method. As piezoelectric sensors have become much cheaper now, this is the method of choice for measuring chamber pressures, though crusher gauge measurements are still around as well. Here's where the differences between the CIP and SAAMI standards show up. CIP uses a transducer made by the Swiss firm Kistler and requires a hole be drilled into the cartridge case and fired by a specially prepared barrel. SAAMI uses a different transducer, called a conformal sensor, mostly made by a US Company PCB Piezotronics. These sensors don't need holes drilled in the case, but they are more expensive as each one needs to be designed based on the diameter of the cartridge. CIP standards state that the transducer should be positioned 25 mm (1”) from the breech face, when the cartridge is long enough. If the cartridge is too small, then CIP decides to position the transducer at a shorter distance from the breech face, based on the cartridge model. CIP
standards do not care what the shape of the cartridge is.
SAAMI, on the other hand, does care what the cartridge is shaped like. For bottle-necked
cartridges, the center of the transducer is placed 0.175 inches (4.4 mm) behind the shoulder of the cartridge for large diameter (0.250 inches, 6.4 mm) transducers and 0.150 inches (3.8 mm) behind the shoulder of the cartridge for small diameter transducers. For cylindrical cartridges, the transducer is located behind the base of the seated bullet at a distance of (0.5 * transducer diameter + 0.005 inches). Since the two standards have different methods and different points from where to take measurements, therefore the CIP and SAAMI pressure numbers are different from each other for the same cartridge type. Using a transducer is less expensive than using crusher gauges because the same transducer can be reused over and over again, as long as the cartridges used are the same type. So if a tester wants to repeat the test multiple times to get an average pressure reading, he or she only needs one transducer, if using the piezo method. Compare this to the crusher gauge method where each test needs a new copper cylinder and the costs begin to add up.
The third method involves using a strain gauge. This is a thin, flat piece of wire whose electrical resistance changes as it is stretched or strained. The strain gauge is attached on the outside of the barrel near the front of the chamber. When the firearm is discharged, the barrel expands slightly, which stretches the strain gauge and the change in the electrical resistance can be measured and the pressure calculated. This method is fairly accurate, but not as reliable as the other two methods. However, it has the advantage of being the cheapest method of the three and not requiring as much special equipment as the other two methods.
It must be noted that no test can give a truly accurate pressure reading because there is no way to know what the actual pressure should be. Even if the tester uses 100 identical cartridges with equal amounts of propellant and using the same firearm and test setup, the tester can expect up to a 5% variance in pressure values from cartridge to cartridge, when using the crusher gauge method to measure pressure. With the conformal transducer method, the variance is up to 3% from cartridge to cartridge. This is why the conformal transducer method is proclaimed as more accurate than the crusher gauge or the strain gauge method. Posted by The Editor at 12:51 AM Labels: Chamber Pressure, Testing Firearms
Sunday, January 9, 2011
Testing Firearms: Measuring Trigger Pull Force The term "trigger pull force" is defined as the amount of force that is needed to cause the trigger to release in a firearm. If a firearm has a very light trigger pull a.k.a. a hair trigger, then it can be shot very rapidly, as it takes very little force to activate the trigger. However, it also has a greater chance of accidentally discharging, because of the same reason. On the other hand, if a firearm has a very heavy trigger pull, the user will not be able to shoot rapidly and will also not shoot accurately, because the force of pulling the trigger will usually cause the user to shake the firearm a bit more.
So how does one measure trigger pull force. Well, if one is a gadget junkie and has money to spend, then one could acquire a trigger pull gauge like the two examples below:
The first is a mechanical spring gauge, much like an old fashioned spring balance and the second is a digital gauge. To measure the trigger pull force, the tester cocks the weapon (making sure it is unloaded first), simply fastens the hook end to the trigger and pulls the gauge backwards until the trigger releases the firing mechanism. The reading then shows how much force was needed to release the trigger. The illustration below shows how this is done.
Then there is another simple technique as the image below shows. This one is from a Life Magazine issue from 1937, but the same technique is used to this day in many official shooting competitions.
The tester simply adds weights until the trigger releases. Simple and easy to perform. Before the reader assumes that this is an outdated method, this technique is still used in official NRA shooting competitions to make sure that no competitor is shooting with too light a trigger.
The above image is an NRA official trigger weight system and is available from some sporting goods stores. Of course, for the casual user who doesn't want to spend $50-$100+ for gizmos like the ones above, there is a much more lower tech way of measuring the trigger pull force, which gives fairly accurate results as well. The homebrew tester simply acquires some heavy wire, such as a wire clothes hanger, cuts it to length and bends it into a S-shaped hook, using a pair of pliers.
The tester first unloads the gun and makes sure it is empty and then cocks the trigger. Then the tester simply hooks the trigger to the large end of the S-hook and then hangs a plastic shopping bag or a tin can from the small end of the S-hook. The tester then adds weights to the plastic shopping bag until the trigger releases. Then the tester employs an ordinary weighing scale (such as the one used in kitchens) and weighs the hook, the plastic bag and its contents. Multiplying this mass
with the acceleration due to gravity gives the trigger pull force. For testers who don't have access to weighing scales, they simply load the plastic shopping bag with known weights, such as grocery items. For instance, 500 gm bags of beans, 100 gm boxes of cocoa powder etc. can be loaded into the plastic bag until the trigger pulls. The weight of the S-hook and the plastic shopping bag and the bags that various grocery items come in are considered to be of somewhat negligible compared to the weight of the groceries themselves, so this method can give pretty good approximate results. For example, using 500 gm bags of beans, it may be possible to determine that the release point of a certain firearm is between 2 kg. and 2.5 kg. (i.e. between 4.4 and 5.5 pounds) because the trigger didn't release when four 500 gm bags of beans were in the plastic bag, but adding a fifth one did. Then the tester resets the experiment, adds four 500 gm bags of beans back into the plastic shopping bag and then starts adding 100 gm cocoa powder boxes until the trigger releases, at let us say 2.3 kg. Now the tester knows that the trigger release point is between 2.2 and 2.3 kg. Then the tester can repeat the experiment using four bags of beans, 2
boxes of cocoa powder and something else that weighs say 20 gms etc. to get more accurate results and so on.
Posted by The Editor at 10:11 PM Labels: Testing Firearms, Trigger Pull
