The Powley Computer John Tartell
atomic device. However, at The VARMINT HUNTER Maga- zine®, there are other interests. But for a formula, or set of formulas, that are really useful to us since the vast majority of the readers are handloaders, a guy by the name of Homer Powley, starting in the 1940s, developed a set of formulas that for handloaders is of great value. Homer graduated from college in the latter part of
Einstein gave the world its most famous formula: E=MC2 , and that’s great if you want to build an
the depression with a degree in physics, and a passion for internal ballistics. Unfortunately, upon graduation ammu- nition companies weren’t hiring. But thanks to World War II the military was, and Homer spent the war years in D.C. as a Naval Intelligence offi cer. During those years, he became friendly with key
people at the H.P. White Ballistic Laboratories in Maryland. Prior to his leaving the Navy, as Homer told me, it was Mr. White himself who gave him 25,000 sets of ballistic data: caliber, bullet weight, powder type, powder charge, bullet weight, muzzle velocity, barrel length, and chamber pressure in copper units (CUP). From here, and with years of patience, Homer derived an amazing set of empirical formulas. The end result: By plugging in a few simple and easily obtained weights and dimensions, one now had, for any modern cartridge, a starting load with the optimum powder charge for DuPont powders, the muzzle velocity this charge generates, and the chamber pressure in CUP (copper units of pressure) this charge generates (usually in the range of 46,000 CUP, which is well within the starting loading data for high-intensity cartridges in modern rifl es). Regarding the input data, it’s simple. Barrel length
from the base of the seated bullet to the muzzle, caliber, bul- let weight, and the weight of water in the case to the base of the bullet. Caliber and bullet weight — we obviously know these from the start. Barrel length (for the Powley Formulas it’s from the base of the bullet to the muzzle). Measure the barrel length from the bolt face, add the seating depth of the bullet into the case, and subtract the case length (all measurements in inches). Weight of water to the base of the bullet: weigh an
empty case holding a spent primer, and then fi ll it with water to the case mouth and weigh it. Subtract the weight of the case and you have the weight of water in the case. But we need to subtract the weight of water that the bullet will displace when seated into the case to its normal seating depth — and here is how we do this. Above, we’ve already determined the length of the
bullet below the case mouth, so multiply this number by the weight of water per inch per caliber from the chart below and subtract this small weight for the weight of water in the case, and this is the water weight in grains to the base of our bullet. Now we’re ready to plug our numbers into the computer (water weighs 252 grains per cubic inch).
Table of Water Displacement
In Grains — Per Inch of Bullet Per Caliber Caliber Grains Caliber Grains Caliber Grains 0.223 9.877 0.284 16.02 0.375 27.931 0.243 11.728 0.308 18.842 0.458 41.663 0.257 13.118 0.323 20.722
Long before the age of electronic computers, the Powley Computer accurately selected the proper charge. Page 148 October — December 2011
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