How to Estimate the Accuracy of any Given Rifle and Cartridge
By Lt. Colonel Townsend Whelen
Outdoor Recreation, vol 62, No. 4, Apr 1922, page 296
THE accuracy of commercial rifles and commercial ammunition is dependent upon certain factors. If we know these factors we can determine in advance just what average accuracy we can expect, and we can thus go over the whole list of commercially made rifles and ammunition and estimate very closely the accuracy value of each without firing a shot. This will assist materially in the selection of a rifle and a cartridge. The factors which determine accuracy, or really the peculiar design which limits accuracy in advance, are not well known, hence this article. '
It must be understood that the comparatively low price of rifles and ammunition in this country is dependent upon quantity production of a standard article. Hand work, or special work is expensive. With the present price of labor a reliable hand made rifle, made by a firm prepared for such an output, would cost $200, at least. The rifles and ammunition presented for sale by American manufacturers are all made by the system of quantity production, and this means that certain tolerances or variations in size and dimension of certain parts must be permitted as the direct result of manufacture with speed machinery. These tolerances are kept to the lowest limit by careful inspection, but they are bound to exist because the only way to eliminate them is to finally finish the product by slow and expensive hand work. Thus the bore of a rifle must have a certain maximum or minimum dimension to pass the inspector. If it falls within this limit it is passed.
Cartridges also have tolerances. The bullet has to have a minimum and maximum diameter and weight. There is a certain minimum and maximum standard in measuring the powder charge because there is no time to weigh it on accurate scales. Quantity production necessitates that the shell be drawn in a die, and not accurately turned on a lathe, and thus in practice it is never exactly the same diameter and size, nor even the same thickness on any two sides.
When a small bullet enters a large barrel it expands to fill the barrel, quickly if a lead bullet in front of black powder, slowly if a jacketed bullet propelled by smokeless powder. In being expanded it is deformed to a certain extent, and this, added to the fact that no two bullets can be made in quantity exactly alike, results in inaccuracies in flight. When a small cartridge is inserted in a small chamber it lies therein at the bottom of the chamber, with the axis of the bullet out of line with the axis of the bore, and thus a certain inaccuracy is introduced even before the cartridge is fired. The next cartridge may be larger, and it may fit better in the chamber, and therefore it will not start out in exactly the same way as does the first cartridge.
For the purpose of this article we may define the word “accurate” as meaning that the rifle and ammunition shall shoot as accurately as the skilled rifleman can aim and hold at any specified range. This means that the radius of its shot group at any range must not exceed 1 ½ inches per each one hundred yards of range—for example, 3 inch radius or 6 inch group at 200 yards.
With a clear understanding of the above, we are ready to consider those factors, or that peculiarity of design, by which we can determine in advance what the accuracy of any given rifle and cartridge will be within very close limits. These factors are as follows:
1. Short bullets fly accurately only at low velocity, and then only for a short distance. Long bullets fly accurately to a much longer range than short bullets, but require high velocity, and a twist of rifling in proportion to the velocity. For example, speeding up a bullet like the .44-40-200 results in inaccuracy, and at any velocity such a bullet cannot be made to fly accurately to a distance much greater than 150 yards. A long bullet like the .25-35-117 requires a twist of about one turn in ten inches to shoot it accurately at a velocity of 2,000 feet per second. If the muzzle velocity be lowered much it will keyhole. If the bullet be speeded up considerably the twist may be reduced to one turn in about 14 inches. Where the twist and velocity are correct such a bullet is accurate to 600 yards at least.
2. Cartridges having bullets which project a considerable distance beyond the mouth of the shell shoot more accurately than those which have only a short, blunt point extending only a short distance from the mouth of the shell, because the lead in front of the chamber straightens up the bullet which projects a consider able distance from the shell, and causes its axis to lie more nearly in coincidence with the axis of the bore. The lead has practically no such straightening effect on a bullet which projects only a short distance from the shell. For this reason we would expect a cartridge like the .25-35-117 to be more accurate than one like the .44-40-200, and it is, or we would expect a cartridge like the .22 Long Rifle to be more accurate than the .22 Winchester Rim Fire, and it is.
3. Given smokeless powder as the propellant, jacketed bullets will shoot more accurately when the bullet is the full size of the groove diameter of the bore than when it is smaller than groove diameter. A small jacketed bullet does not expand in the bore at once to make a perfect gas dam. Until it does expand the powder gasses rush past it, cutting and deforming it, and also cutting and deforming the bore, and causing the dam age to the bore near the breech known as erosion. In being finally expanded to full groove diameter the bullet also receives a certain deformation which tends toward inaccuracy of flight.
4. A large chamber or a small cartridge means that the cartridge lies in the bottom of the chamber before discharge, and that the bullet is out of line with the axis of the bore. When the rifle is fired the bullet has to jump forward out of line and be straightened up by the lead and bore, and in thus being straightened it is deformed. The comparative size of a chamber and cartridge may be determined roughly by measuring a new cartridge and a shell that has been fired in the chamber.
Taking these factors into consideration we may make certain deductions as to accuracy. In practically every case with in my knowledge, deductions made in this way have agreed with the results obtained from many years of carefully conducted accuracy tests.
a. The most accurate commercial rifles and ammunition are those which follow the modern military type of long, jacketed bullet projecting a considerable distance from the mouth of the shell, bullet full groove diameter, and fairly tight chamber. Such a combination should be accurate as defined above to 1,000 yards at least.
b. If the chamber of the rifle be cut with considerable tolerance, or if the groove diameter of the bore be larger than the diameter of the bullet, the loss in accuracy will be almost in direct proportion to these tolerances, even in rifles which otherwise follow the specifications given in (a) above. Thus a military type of rifle having a large “chamber and a bore, say, about .002-inch larger than the bullet, can be estimated to give groups about twice the size of the rifle and ammunition described in ( a) above.
c. Cartridges with only a short point projecting from the shell, like the .44-40-200, the .45-00-300, or even the .405 automatic, all also having short, blunt bullets, will also give groups averaging about twice the size as stated in the definition of accuracy above. But they will only give this poor accuracy up to about 150 yards, most of them greatly increasing their dispersion at 200 yards. If we attempt to speed up these cartridges over the normal black powder velocity they become even more inaccurate.
Now keep these things in mind, consider the length of the bullet, its size and the size of the bore, how far it projects from the mouth of the shell, and the size of the chamber. You can then estimate very closely the accuracy that a given rifle and ammunition will give.
In fact, since I recognized these principles I have never gone wrong in an estimate. A subsequent accuracy test has always borne out the estimate. I therefore believe that the application of these principles will be of considerable assistance to those about to select a new rifle and cartridges.