The hunter has always demanded the best accuracy from his rifle and ammunition. After scope sights became standard equipment over 60 years ago, his accuracy requirement increased considerably. Sighting in and testing accuracy over a benchrest , the aiming mark can be seen very clearly, since it is magnified through the scope, and if we hold the rifle rock steady and squeeze off our shots without disrupting our aim, we are capable of shooting tight groups. Having fired the usual three or five rounds, he examines the target hoping the bullets have cut a ragged hole. Conditions were just right, not a breath of wind and full daylight. The rifle and scope are both high quality products. The ammunition carefully handloaded.
If he views the target with such high expectations, he is almost certainly going to be bitterly disappointed. Unless the rifle is a heavy varminter, the group from a light sporter seldom forms a tight group, the bullet holes are more likely to be scattered about a bit. As a rule the dispersion is relatively small, so the result is more than sufficent to meet the requirements of the practical hunter. However, many shooters will question that since their hold for each shot was so good, why aren't the bullet holes a lot closer to each other? Why are three close to each other and one high and to the left and one slightly lower?
Accuracy - An Equation With Many Unknown Factors
The unavoidable dispersion in rifle shooting is seen as a problem caused by a number of unknown factors, each of which contributes to give a series of a smaller or larger group size on the target. If the dispersion is abnormally large, it indicates that there is an obvious and identifiable problem which can be usually be corrected. If the dispersion is more normal we can only try the reduce it as much as possible. 'Zero dispersion! i.e a ragged one-hole group not much larger than the bullet's diameter is only wishful thinking where a light sporter is concerned; as a rule it falls only within the capability of a simonpure benchrest rifle.
The Shooter's Ability
One of the many unknowns of the equation is first and foremost the shooter himself. Obviously, the amount of skill of the average shooter varies a great deal. As a shooting platform, man is clearly deficient. 'Faults!, or variations in experience and varying degrees of shooting ability small enough not to be obvious to ourselves, can cause considerable dispersion at distances less than 100 metres.
The rifle is the other large factor of dispersion. Even the most excellent and well maintained precision arm can be the main factor in the same way as a poor marksman can.
More often than not the excuse for missing is blamed on the rifle's barrel. Most of the time this is not so. Even a rifle barrel with minor flaws can deliver acceptable accuracy. This is particularly the case with the larger bore sizes. Good examples of this were old ex-military Martini .450-577s. Many of them had, by today's standards, badly eroded and pitted bores, but there are many well documented cases where good shots were able to achieve respectably high scores on the range at 300-500 yards. The barrels were thick and heavy and the breech pressure relatively low. The fact that the barrel was crooked or had other flaws did not seem to matter all that much after the rifle had been sighted in.
With the modern bolt action, it is the total stability of the gun and chiefly the quality of the barrel itself and its chamber which determines the degree of accuracy. Also, the relationship of the barrel to the bedding and recoil absorbing properties of the stock. During the short period of time from the ignition of the powder, and violent build-up of pressure until the bullet leaves the barrel, a wave of tensions and vibrations move through the barrel. These cause what the experts call barrel oscillations. Heavily exaggerated, the barrel has been described as being : like a shirt tail flapping about in the breeze.! A better description is that of a suspended hose squirting water under high pressure when the bullet is being driven through the barrel. The muzzle moves more or less in a rotary pattern and the point of impact depends on where in this pattern of movement the muzzle was pointing in the very instant when the bullet left the barrel.
The need for stability in the gun as a total unit is therefore considerable. The most insignificant variation in support and varying tension in any given area of the bedding results in a corresponding variation in the direction of the bullet. Even very minute variations in the way in which the action shifts or the stock absorbs recoil can produce such variations. The progressive warming of the barrel is also another reason for increased dispersion. The more stable the gun is as regards action stiffness, exact barrel thread, constant recoil absorption, quality of barrel manufacture and material, etc, the less are the variations in the direction of bullets leaving the muzzle.
Scopes And Mounts
Today, most rifles are equipped with scope at the time they are bought. Many shooters are not aware that the scope, its mounts and its lenses can have a built-in dispersion factor. Small for a high quality scope, larger for one of lesser quality. Often unacceptably large for a cheap sight.
Modern ammunition is manufactured using exceptionally sophisticated methods and with very high demands on precision and uniformity. Factory loaded ammunition for hunting very rarely gives unsatisfactory accuracy, and there's a bullet for literally every purpose and size of game. High quality ammunition like Winchester's Supreme and hyper-velocity stuff like Hornady's Light Magnum are worth the extra they cost.
Factors contributing to the dispersion of ammunition are variations in the powder and priming compounds, case volumes and variations in the weight and dimensions of the bullets. Also, the quality of the powder and its reaction to changes in temperature, as well as variations in cartridge length and bullet pull will affect accuracy.
Ammunition variation affects accuracy in two ways. Firstly, it affects the barrel's harmonics causing erratic barrel oscillations, secondly it causes the bullets to have different barrel times. Barrel time is the time it takes for the bullet to travel the full length of the barrel.
Considering the barrel oscillation, it is obvious that the uniformity of barrel time from round to round is important to reduce dispersion. Uniformity of barrel time is also important inasmuch as during the time the bullet travels through the barrel, the recoil has begun. As most shooters are aware, recoil causes the barrel to rise.
This brings us back to the shooter as a factor in dispersion. His ability to absorb the recoil from round to round and to do so in the same manner also affects the size of the group. As a conclusion of the dispersion factors of the shooter, the gun and ammunition, we have now learnt that there are several factors which cause the bullet to deviate from the desired point of impact. We have also learnt that many of these factors affect and interact with each other. If a bullet's jacket is only marginally thicker than the previous one, or is not uniform in thickness, the pattern of barrel oscillations is affected. But also the barrel time. In turn both factors affect the point of impact.
Accuracy then, is not only an equation with many unknown factors as we initially pointed out. These factors can also be functions of each other. Ammuniton manufacturers have learned how to control, all these factors from cartridge to cartridge. This is why modern premium grade ammo delivers accuracy that's hard to equal let alone better even with the most carefully assembled handloads.
This article was first published in Sporting Shooter, July 2009.