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Rifles, Reloading, Optics, Equipment
Equipment Discussions
Oehler Model 43 chrono and pressure reading unit
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<blockquote data-quote="Brent" data-source="post: 45081" data-attributes="member: 99"><p>All SAAMI test barrel bores are on the tight side, as well as their chambers. This simulates a "highest" chamber pressure situation a manufacturer might chamber a given cartridge for, and ammo manufacturers might make ammo for, or provide us loading data for, just to keep us on the max end, or less, of what SAAMI rates the specific cartridge for. </p><p></p><p> </p><p></p><p>The various ammo manufacturers send their ammo around in a circle to every other member of SAAMI to test the ammo for maximum pressure in each test barrel. The maximum pressure arrived at in the one test barrel that shows hottest will dictate how hot this load can be loaded, a ceiling if you will. Testing of loads in one barrel at one facility at the place of manufacturer is the starting point, and then they make the rounds to be tested at the other facilities. This is what the ballistician at Federal Cartridge Company has told me. </p><p></p><p> </p><p></p><p>What we end up with, is our rifle will produce at, or less pressure with SAAMI approved commercial ammo when manufacturers use barrels and chambers within SAAMI specs. Most often factory ammo produces less pressure, and thus MV than it would in the rifles barrels and chambers were held to the tightest of specs. </p><p></p><p> </p><p></p><p>When we chamber for a factory or wildcat cartridge and want to know the chamber pressure that OUR barrel/chamber combination is producing, not what SAAMI's tight spec barrels would be, we can glue on a strain gage over our rifle's chamber. This will measure the stretch of our barrel in order to calculate the peak pressure as well as the entire pressure curve from ignition to bullet exit. </p><p></p><p> </p><p></p><p>A strain gage which is super glued over our chamber at the mid-point between the face of the receiver and the shoulder of the cartridge as it sits in the chamber will change (increase) electrical resistance as the thin wire circuit inside is stretched when the chamber expands upon firing. </p><p></p><p> </p><p></p><p>We have two wires that attach to the strain gage, one on each end of the circuit and run to the a device which measures the voltage change when a trigger level has been reached upon firing. The unit captures multiple data points during the approximate two millisecond firing cycle and a pressure/time graph shows us this plotted pressure curve. </p><p></p><p> </p><p></p><p>Converting voltage change to chamber pressure is not difficult; it is really two rather simple formulas. A Thick Wall Pressure Vessel and a Thin Wall Pressure Vessel formula are used to calculate the pressure contained in the two vessels, the barrel chamber and the brass chamber. </p><p></p><p> </p><p></p><p>Barrel steel of known thickness and diameter will expand a specific amount with a specific pressure. Brass of a known thickness and diameter will contain a specific amount of pressure before it yields to cause further expansion upon the chamber wall. Together these two formulas determine the total expansion we will measure at a specific chamber pressure. We measure this expansion with a calibrated gage that will produce a specific voltage change with a specific change in length. The barrel expansion causes the gage bonded to it to expand with it and increase in length, thus expansion is measured. </p><p></p><p> </p><p></p><p>With PressureTrace's super fast processor, it collects and plots 100 data points (micro strain) per millisecond on a graph, which is sensitive enough to capture even the smallest changes in pressure along the pressure curve. </p><p></p><p> </p><p></p><p>When comparing commercially available factory ammo for a reference, hand loads can be held well in the safe zone if careful measurements are taken and you stay below the reference ammo's measured pressure. Reference ammo is not "needed" but it does help in determining if you've made a mistake with barrel measurements or bonding the gage over the chamber. </p><p></p><p> </p><p></p><p>Of every chamber I've done testing on, I have never found a factory load that has exceeded SAAMI maximum, ever. Usually they are just below, some loads are a few thousand PSI below. I get about what I'd expect in my hand loads for MV considering barrel length at the indicated pressure, and also pressure signs on the cases when I exceed them by very far. </p><p></p><p> </p><p></p><p>Testing so many different ones this far, I see no problem with the measurements given without reference ammo; I rarely even buy reference ammo any more FWIW. If I wasn't confident as a result of testing so many loads the last few years I still be using the factory stuff for a reference, it has always seemed a total waste of money and time it's so predictable though. When I jumped into the 30-338 Lapua Imp wildcat, I had no reservations what so ever about not having factory ammo for a reference… </p><p></p><p> </p><p></p><p>Hope that helps some. 70,000 psi is a good point most classic pressure signs begin to be observed, generally not any lower than about 68,000 psi. I'll post a little more when I get back.</p><p></p><p>[ 05-23-2004: Message edited by: Brent Moffitt ]</p></blockquote><p></p>
[QUOTE="Brent, post: 45081, member: 99"] All SAAMI test barrel bores are on the tight side, as well as their chambers. This simulates a “highest” chamber pressure situation a manufacturer might chamber a given cartridge for, and ammo manufacturers might make ammo for, or provide us loading data for, just to keep us on the max end, or less, of what SAAMI rates the specific cartridge for. The various ammo manufacturers send their ammo around in a circle to every other member of SAAMI to test the ammo for maximum pressure in each test barrel. The maximum pressure arrived at in the one test barrel that shows hottest will dictate how hot this load can be loaded, a ceiling if you will. Testing of loads in one barrel at one facility at the place of manufacturer is the starting point, and then they make the rounds to be tested at the other facilities. This is what the ballistician at Federal Cartridge Company has told me. What we end up with, is our rifle will produce at, or less pressure with SAAMI approved commercial ammo when manufacturers use barrels and chambers within SAAMI specs. Most often factory ammo produces less pressure, and thus MV than it would in the rifles barrels and chambers were held to the tightest of specs. When we chamber for a factory or wildcat cartridge and want to know the chamber pressure that OUR barrel/chamber combination is producing, not what SAAMI’s tight spec barrels would be, we can glue on a strain gage over our rifle’s chamber. This will measure the stretch of our barrel in order to calculate the peak pressure as well as the entire pressure curve from ignition to bullet exit. A strain gage which is super glued over our chamber at the mid-point between the face of the receiver and the shoulder of the cartridge as it sits in the chamber will change (increase) electrical resistance as the thin wire circuit inside is stretched when the chamber expands upon firing. We have two wires that attach to the strain gage, one on each end of the circuit and run to the a device which measures the voltage change when a trigger level has been reached upon firing. The unit captures multiple data points during the approximate two millisecond firing cycle and a pressure/time graph shows us this plotted pressure curve. Converting voltage change to chamber pressure is not difficult; it is really two rather simple formulas. A Thick Wall Pressure Vessel and a Thin Wall Pressure Vessel formula are used to calculate the pressure contained in the two vessels, the barrel chamber and the brass chamber. Barrel steel of known thickness and diameter will expand a specific amount with a specific pressure. Brass of a known thickness and diameter will contain a specific amount of pressure before it yields to cause further expansion upon the chamber wall. Together these two formulas determine the total expansion we will measure at a specific chamber pressure. We measure this expansion with a calibrated gage that will produce a specific voltage change with a specific change in length. The barrel expansion causes the gage bonded to it to expand with it and increase in length, thus expansion is measured. With PressureTrace’s super fast processor, it collects and plots 100 data points (micro strain) per millisecond on a graph, which is sensitive enough to capture even the smallest changes in pressure along the pressure curve. When comparing commercially available factory ammo for a reference, hand loads can be held well in the safe zone if careful measurements are taken and you stay below the reference ammo’s measured pressure. Reference ammo is not “needed” but it does help in determining if you’ve made a mistake with barrel measurements or bonding the gage over the chamber. Of every chamber I’ve done testing on, I have never found a factory load that has exceeded SAAMI maximum, ever. Usually they are just below, some loads are a few thousand PSI below. I get about what I’d expect in my hand loads for MV considering barrel length at the indicated pressure, and also pressure signs on the cases when I exceed them by very far. Testing so many different ones this far, I see no problem with the measurements given without reference ammo; I rarely even buy reference ammo any more FWIW. If I wasn’t confident as a result of testing so many loads the last few years I still be using the factory stuff for a reference, it has always seemed a total waste of money and time it’s so predictable though. When I jumped into the 30-338 Lapua Imp wildcat, I had no reservations what so ever about not having factory ammo for a reference… Hope that helps some. 70,000 psi is a good point most classic pressure signs begin to be observed, generally not any lower than about 68,000 psi. I'll post a little more when I get back. [ 05-23-2004: Message edited by: Brent Moffitt ] [/QUOTE]
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Oehler Model 43 chrono and pressure reading unit
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