Mechanical Calculation of Internal Ballistics
There are some "manual" methods of computing a fairly close approximation of the actual internal performance (i.e.: chamber pressure and muzzle velocity) of a given combination of cartridge, firearm, bullet, and powder but their details are beyond the scope of this page. The "Powley Computer," a slide rule type device has been available for many years and gives rather good results when used with the "IMR" type of powders and cases of similar capacity to the .308 and .30-06. It is based on some mathematically derived "constants" which provide a close approximation to the results derived by some rather complex mathematical gyrations. Several computer-based adaptations of this program are available from different sources.
At one time there was available a "do-it-yourself" pressure measuring device using a "crusher" type tool. Rather than utilizing a drilled chamber this unit was mounted on a firearm in place of the telescopic sight and when the firearm was fired the recoil impulse compressed the crusher which could be measured and compared to a calibrated table. Unfortunately, the unit was tricky to set up, required very precise measurements of the firearm's weight, and if memory serves me right worked best when the firearm was unrestrained in recoil (as in hanging from support wires). It was also only really suitable for rifles.
There are also electronic strain gauge units available that do a better job and allow one to see the pressure curve on an oscilloscope or a computer screen and which don't require a special free swinging support. The new Southwest Products/RSI $190 PressureTrace and the The $800 Oehler Model 43 "Ballistic System" with its $170 add-on a strain gauge pressure sensor are examples of this technology.
Strain gauge units with real-time microprocessor compensation such as the new PresureTrace have been show to actually be more accurate than either piezo or crusher measurements. By utilizing certain easily measurable parameters of the chamber wall thickness and cartridge case wall thickness in the firmware and software, the measurements from strain gauge units give results that are accurately given in PSI. (If no compensation is utilized with a strain gauges, the measurements are in arbitrary units, such as "coconuts" as my friend says, which still can be used for comparison readings.
Strain gages have been used in industrial instrumentation for several decades and is fully developed, proven technology. Denton Bramwell, (physicist and statistician with a great deal of test instrumentation experience) recently compared the results of various pressure measurement methods . He used a standard measure of how likely a system will give correct answers called "Sigma E" for comparison. (Smaller Sigma E values are best.)
System Sigma E
Fabrique Scientific Peak Strain Meter
Microprocessor compensated strain gauge 1827 psi
The PressureTrace unit gives you the actual readings in your firearm, while the Oehler unit convert its measurements to what would probably be obtained by the load in a SAAMI specification test barrel. For the details on the PressureTrace go to http://www.shootingsoftware.com/pressure.htm
and for the Oehler go to http://www.oehler-research.com
I have worked with RSI on the PressureTrace unit and seen it in action and it is VERY neat. If you do order one I'd appreciate it if you would let RSI know that I sent you either by giving Jim my name or my affiliate number - AF10291954.
NOTE: While you can reliably compare the pressure generated by your handloads with the pressures generated by factory ammunition fired in your gun, or compare the pressures from different handloads in the same gun, strain gauges or recoil based pressure devices do not generate " SAAMI" specification pressure measurements (either PSI or CUP) even thought the results are probably more accurate, because SAAMI specifications currently requires piezo or crusher pressure technology and certain statistical analysis of the data.
We know that as pressure goes up, so does velocity. Pressure goes down, so does velocity. If you are worried about pressures, and don't have access to pressure equipment, develop your loads using a soft primer like CCI and a chronograph and find the top node or stop when you get flattening or cratering. Then switch to a harder primer and adjust your load to give the node velocity.
While these devices can be very informative for someone with a interest in interior ballistics and the effect of things like different powders and seating depths, most folks who buy these sort of devices don't understand barrel harmonics and velocity nodes. (See the discussion of "load development" on my home page). They hope to be able to load all the way up and wring the last foot-second of velocity out of their guns. They forget that the top velocity/accuracy node reachable without signs of excessive pressure (primer flattening/cratering) is the top practical limit for a firearm.
So, what good is being able to "read" peak pressure? It is mainly academic. Using one of these devices can let you see the effect of burning rates on pressure curves and and the effect of the change of the shape of the pressure curve all of which have an effect on velocity. For many folks it is simply fun to see what is going on. With a highly sensitive unit with a high sample rate like PressureTrace you can also see the effect of many other things like neck tension, seating depth and powder/cartridge match.
The best way to develop loadings data for an cartridge for which data isn't available still remains careful load testing using a fully instrumented pressure test barrel. However, if some "factory" ammunition is available in the "new" caliber one can use a strain gauge and a chronograph, choose some powders listed for a similar cartridge with known data that will achieve a similar loading density and then verify the results with the equipment.