Rifle Accuracy Facts by Harold R. Vaughn

Author:Harold R. Vaughn
Language: eng
Format: epub
Tags: General Fiction
Published: 1988-10-09T16:00:00+00:00

Muzzle Blast Pressure Reduction

The fact that a fast burning powder will result in a lower pressure at the muzzle than a slow burning powder was mentioned earlier. However, for the same muzzle velocity the faster burning powder will produce a higher chamber pressure. Nothing ever comes free in this business! I decided to test this contention by measuring the in-bore pressure at the muzzle using the strain gage method that we used to measure chamber pressure back in Chapter 2. Since we are only interested in the comparison of muzzle pressures resulting from the two different powders, we don’t have to go through the tedious calibration procedure used in measuring chamber pressure, and the theoretical calibration will suffice. We will use 49 grains of IMR 4064 and 57 grains of IMR 4831 for the test, which yield approximately the same peak chamber pressure. The 90 grain 270 hollow point bullet was used with both powders. The test results (Figures 7-22 and 7-23) showed that the muzzle pressure was 11,500 psi for IMR 4831 and 7,100 psi for IMR 4064, which means that the muzzle pressure was about 38% less for the faster burning IMR 4064 powder. The muzzle velocity for the IMR 4064 was about 150 fps less than that of the IMR 4831. The negative blip on the oscilloscope data is probably caused by a compression wave that runs a few inches ahead of the barrel expansion caused by the internal pressure behind the bullet. The base of the bullet passes under the strain gage at about 1.37 msec where the peak pressure occurs, and the base of the bullet exits at about 1.40 msec. Notice that the muzzle pressure of 11,500 psi agrees well with the pressure on the base of the bullet at the point of muzzle exit shown in Figure 2-21. So now we know that the choice of powder significantly effects the magnitude of the muzzle blast pressure.

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