Author Topic: Rate of Twist and Rifling Characteristics  (Read 18832 times)

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Offline Radom

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Rate of Twist and Rifling Characteristics
« on: April 08, 2003, 09:42:11 PM »
CZ Handguns: Rates of Twist and Other Rifling Characteristics
            As I hope that I will make clear, it is very difficult to discuss the rate of twist of rifling and its effect on accuracy without first speaking in terms of rifles.  It is extremely rare that one sees a handgun that allows for one complete rotation, due to their short barrel lengths.  The general principles used for accurizing rifles can be modified and applied to handguns.  
            The central idea behind rifling is that the bullet must be stabilized with a rotational spin, or it will not obtain uniform external ballistics.  In more crude terms, without the spin, a bullets path becomes too unpredictable for either precision (small groups) or accuracy (shooting to point of aim).  The bullet will tumble end over end, it will not reach its true potential range, and it will lose velocity quickly, due to the greater drag on its mass.  
            Rate of twist describes how fast the rifling grooves will spin a bullet from that barrel.  In the US, it is expressed in how many inches of barrel are required to make one complete (360-degree) rotation of the grooves.  Thus, rate of twist in expressed with numbers in this form: 1:9, 1:12, and 1:16.  In other words, one complete rotation occurs in nine, twelve, and sixteen inches, respectively.  The lower the number, then the twist is called faster.  That is, 1:9 is a faster twist than 1:16, and the barrel is imparting greater rotational force.  Obviously, a long barreled rifle with a fast twist might make three complete rotations within its barrel length.  It is exceptionally rare that a handgun barrel makes a complete rotation.  In order to receive an adequate spin, the bullet need not complete a full rotation before exiting the muzzle.  However, it is another factor explaining the relatively poor accuracy and precision of handguns, as compared to rifles.  
            The rifling of a firearm can be either right-handed (clockwise) or left-handed (counterclockwise).  Unlike barrel length and rate of twist, this has no effect on either accuracy or precision.  The vast majority of firearms made to date have a right-handed twist.  Traditionally, Colt has been a notable exception to the general rule.  
            Theoretically, each individual bullet has an ideal amount of spin that should be imparted for its exact velocity.  It is impossible for anyone to actually obtain this degree of a match.  Occasionally, someone might see a reference to an ideal rate of twist, and this is simply a fallacy.  Environmental factors affect accuracy, and none of us do much hunting or practice in laboratory conditions.  Bullets vary in weight and balance within the same production lot.  Fortunately, bullet manufacturers try to make bullets as uniform as possible, and most modern bullet styles are fairly compatible.  For example, one will notice that many different designs are available, but they all tend to have a similar external shape, relative length of bearing surface, etc.  The net result is that shooters typically speak of matching the rate of twist to a particular bullet weight.
            This is potentially misleading.  In of itself, the mass of a particular bullet does not play any great role in the equation.  In fact, the direct effect of mass is probably most significant in terms of the inertial resistance against spin, which is obviously a relatively tiny factor.  However, the bullet weight is a indirect indication of the length of the bullet and the length of its bearing surface, the portion that engages the rifling grooves.  That is, a heavier bullet will be longer, all factors being equal.  Furthermore, a longer bullet will have a longer bearing surface, all factors being equal.  Since bullets are sold by their weight in grains rather than their lengths, shooters usually speak of matching the bullet weight to a particular twist.  However, this approach requires that the overall length of bullet in question is representative of other bullets of its weight.  The overall length is the critical measurement, because even the portion that does not contact the rifling affects the center of gravity.  However, a relatively longer bearing surface, as compared to other bullets of the same length, does alter the bullets behavior; generally, this makes the bullet more accurate.  If the bullet design radically departs from the norm, it may require a different rate of twist.  
            Matching the rate of twist to bullet length will also require some compromises.  Obviously, if the rate is too slow, the bullet will not properly stabilize, which degrades accuracy, velocity, and maximum range.  Unfortunately, bullets fired from a barrel with too fast of a rate twist also have problems.  If the bullet is even slightly out of balance, too much spin will send it at a tangent that much faster, creating dramatic inaccuracy and imprecision.  Additionally, a bullet traveling at high velocity with too fast of a spin can literally tear itself apart in mid-air.  At slightly slower terminal velocities, this might still occur when the bullet impacts living tissue, which can prevent acceptable penetration.  These phenomena caused by too fast of a rate of twist are typically ascribed to over-stabilization, which is an unfortunate choice of words.  (A bullet is either stabilized, or it isnt.)  
            A firearm will normally have a slightly faster rate of twist than its so-called ideal rate, never a slower one.  In custom rifles, the practice in years past was to bore a rifle for one particular bullet weight, using its theoretical ideal rate of twist.  This is slowly changing for a number of reasons.  As bullet construction and manufacturing have improved, some of the negative results of so-called over-stabilization are becoming less critical.  The bullets of today are better balanced, more uniform, and have jackets less prone to separation.  During the smokeless powder era, testing procedures have improved to the point that it has become obvious that a faster rate of twist is desirable in adverse weather conditions.  As such, the industry now chooses to err on the side of a faster rate.            
            In a given caliber, longer bullets require a faster rate of twist than a shorter bullet in that same caliber for best accuracy.  This general principle always holds true, regardless of the bullet weight or velocity.  As was stated above, this is because there is some relationship between the bullet weight and the overall length.  There is also an extremely direct relationship between the velocity of the bullet and the velocity of its spin.  The 1:12 rate of twist provides the best example.  In this rifling, the bullet makes one complete rotation in 12 inches (a foot).  This means that the bullet is rotating 1,000 times per second at a muzzle velocity of 1,000 fps, 2,000 rotations per second at 2,000 fps, etc.  In actual industry practice, the fact that a given cartridge is expected to perform within a certain range of velocities does factor into the decision of which rates of twist will be manufactured.  
            In rifles, a manufacturer may offer several different models with different rates of twists, or they may standardize on a particular rate.  Since their are a great number of cartridges that use bullet diameters in the range of .220-.224, these cartridges provide some excellent examples.  In the .22LR cartridge, it is virtually a universal practice throughout the arms industry to manufacture rifles (and even many handguns) with a 1:16 rate of twist.  This is because the ammunition manufacturers have deliberately made the 30gr JHPs have the same overall length as the more traditional 40gr target loads.  In .223 Remington (5.56x45), the actual bullet diameter is .224, and the available bullet weights range from under 40gr to over 80gr, although bullets within the 40gr-80gr range are most often used.  This has a profound impact on the overall length of the bullets and the lengths of the bearing surfaces.  Furthermore, not all bullets currently used in this cartridge are of the FMJ spitzer type found in military loads.  As a result, some manufacturers produce rifles in a dizzying variety of rates of twist.  Olympic Arms alone has made rifles with the following rates of twist (in inches): 1:7, 1:8.5, 1:9, 1:10, 1:12. 1:14, and 1:16.  In contrast, new production Ruger Mini-14s have a relatively fast compromise rate of 1:9 twist, which partially explains their reputation for mediocre accuracy.
            In the late 19th century, a British mathematics professor, Sir Alfred George Greenhill developed a mathematical model for determining the optimal rate of twist for heavy ordnance.  He revised this into a form for small arms now known as the Greenhill Formula.  1) Determine the bullet length in calibers (hundredths of an inch).  2) Divide 150 by the bullet length.  3) Divide this result by the diameter of the bullet in calibers.  The Greenhill Formula works exceptionally well up until 2,200 fps.  In Dr. Greenhills lifetime, firearms rarely achieved 2,000 fps.  As velocities approach 3,000 fps, most modern experts advocate a Greenhill Factor of 180, rather than 150.  
            All of the principles discussed thus far apply to handguns, but they do so in varying degrees.  Handguns made for target use and hunting are made more or less like low-end rifles.  In some extreme cases, barrels with different rates of twist are actually made available as options by the manufacturers.  In the case of service pistols, there are typically one or two standard rates of twist in a given caliber, and the majority of manufacturers use these same one or two rates.  Unlike arms intended for game, a wide variety of handgun loads were not considered necessary or desirable until the last few decades.  With the exception of some of the most popular service pistol calibers, such as 9mm, there was typically one standard military loading.  As such, military and police pistols were simply designed around this load, and the commercially available loads duplicated the military offerings.  
            The rates of twist found in service pistols are often faster than they would be in a rifle firing the same bullet design.  For many decades, designers believed that handguns required a faster rate of twist in order to offset their shorter barrels.  This has become a minority view.  There are some advantages to the faster rate of twist.  Faster compromise rates minimize the accuracy problems caused by some of the more exotic bullet designs found in modern defensive loads.  Compared to rifles, handguns are extremely low in velocity, and problems of over-stabilization are encountered far less often.  In particular, the short effective ranges of most handguns prevent unbalanced bullets from creating significant deviations.  Most importantly, handguns have some inherent features that degrade accuracy and precision: short sight radius, short barrels, light weight, etc.  Conventional wisdom has been that any accuracy loss caused by a slightly faster than ideal rate of twist pales in significance to these other factors.  
            In terms of barrel design, there are two important reasons why CZ pistols have a reputation for outstanding accuracy.  First, CZ pistols have six, rather than four, rifling grooves.  Although six grooves have now become more or less standard, four grooves were the norm for many decades.  Smokeless powder had been around for nearly fifty years before designs with six rifling grooves were adopted in significant numbers.  Ceska Zbrojovka appears to have been ahead of the technology curve by at least a decade or so.  (For example, the Mk. I Hi-Powers have four rifling grooves.)  All CZ pistol designs, including the vz/22 and vz/24, have had six rifling grooves.  Second, CZ 75/85 series pistols have a faster rate of twist than many other 9mm designs at @1:10 (as opposed to 1:12-1:14).  This makes them better suited to 124gr and heavier bullets than most other 9mm pistols.  This has been an important factor in their success, since most nations began switching over to 124gr bullets about twenty years ago.  Commercial loads heavier than 124gr began to appear on the market at about the same time that CZ pistols became readily available in the West.  Although 147gr loads are not as accurate in a 9mm CZ as bullets in the 115-135gr range, the CZs handle them much better than some of their competitors.            
The artist formerly known as FEG...