## Monday, November 12, 2012

### How to Calculate the Twist Rate - III

In the last couple of posts, we saw the Greenhill formula and the Miller twist rate formula as two methods to calculate the twist rate of barrels. There are also a few computer programs that were written to help calculate the barrel twist rate.

A well known program called McGyro was written by the late Robert L. McCoy in the 1980s. Bob McCoy was a distinguished scientist employed by the Ballistic Research Lab (BRL), now called the Army Research Lab (ARL) at Aberdeen Proving Grounds in Maryland. For nearly 30 years, it was his job to conduct both theoretical and experimental ballistic research on everything from .22 caliber to large cannon shells. During this time, he received 3 of the highest civilian awards from the US government for his work and even wrote a book on the subject called Modern Exterior Ballistics. So this is certainly a person who knew what he was talking about. In the 1980s, he wrote a series of small programs in the BASIC programming language to estimate the twist rate. The most famous of these are McGyro and IntLift. As an item of historical interest, his original programs are available here.

Bob McCoy's programs were later improved by William Davis Jr., who was also another well known ballistic engineer. Bill Davis qualified as an expert shooter in rifle, carbine and pistol when he was enlisted in the US Army during WW-II. After the war, he worked at the above mentioned Ballistic Research Lab (BRL) for a few years and later at Frankford Arsenal and Rock Island Arsenal. He was involved in the development of the 5.56x45 mm. cartridge and the M16 rifle as well (He held the ad-hoc title of 'AR-15 Project Directory' during its development). It was Davis' team that identified many of the early ammunition problems with the M16, including primer sensitivity and cyclic rate issues due to high port pressures; they also diagnosed and offered solutions to the ill-fated change to ball-type propellant. He later went on to found Tioga Engineering in 1980 and was ballistics editor for American Rifleman magazine for several years until his death in 2010. He wrote the NRA's book on handloading ammunition and contributed to the Encyclopedia Britannica for the Ammunition section.

The original programs that these gentlemen developed were programmed in the BASIC programming language, mainly because it was available on practically every home computer model sold in the 1980s and therefore could be run by a wide audience. Unfortunately a BASIC interpreter doesn't come distributed with computers these days, but many free versions can easily be found by using google and searching for "BASIC interpreter". The programs have also been translated into more modern programming languages such as JavaScript and may be found online at several places on the web.

There are also other translations, such as WinGyro, which is McGyro modified to run under a Microsoft Windows environment.