The How and Why of
Long Shots and Straight Shots

Cornhill Magazine, April 1860

Let us try to understand the effect of this rotation. A bullet in moving rapidly through the air, separates it; and if its velocity is at all greater than the velocity with which the air can refill the space from which it has been cleared behind it, it must create a more or less complete vacuum. Now when the barometer stands at thirty inches, air will rush into a vacuum at the rate of l,344 feet per second; and if the bullet is moving at a greater velocity than this there will be a total vacuum behind it. But it can be easily understood that even when moving with a less velocity, there will be a greater density of air before than behind. If the bullet be rotating on a vertical axis - that is, spinning like a top, point downwards, as in the diagram No. 1, from left to right, in the direction indicated by the crooked arrow, at the same time that it is moving forward (sideways it would be in the top) as indicated by the straight arrow, - it is evident that the left half rotates with the general motion of translation of the bullet, and the right half backwards against this motion, and therefore that on the left side it is moving quicker relatively to the air through which it is passing than on the right side. And its rough surface preventing the air escaping round it on that side, while it, as it were, assists it on the other side, the air becomes denser where shown by the dark lines, and tends to deflect the bullet in the other direction, that is, in the direction in which the anterior or front surface is moving. (2)

If the bullet rotate on a horizontal axis at right angles to the direction of its motion of translation (that is, like a top thrown spinning with its point sideways, when it would strike the object thrown at with its side), shown in the diagram No.2; if the anterior portion be moving, as shown by the arrow, from above downwards, it is evident, for the same reasons, that the air will become denser, as shown, and assist the action of gravity in bringing the ball to the ground - that is, decrease the range. A spherical bullet resting on the bottom of the bore of a gun would always have a greater tendency to rotate in this manner than in a contrary direction; for the friction against the bore would be augmented by the weight of the ball in striking against the bottom, and diminished by it when striking against the top.

Shot were constructed in 1851 to try the effect of rotation in the above-mentioned and in the opposite directions. They were made excentric, that is, lop-sided, by taking out a portion of the metal on one side, and replacing it either with a heavier or lighter body. The manner in which they would rotate was, therefore, known; for, not to use too scientific language, the light side moved first, and according to the relative positions of the heavy and light side when placed against the charge so the rotation took place. Thus, when the light side was resting against the bore of the gun, the rotation was exactly contrary to the direction shown in diagram No.2; and a range of 5,560 yards was obtained from a 10-inch gun, being 910 yards farther than with a concentric shot from the same gun. The deflections to the right and left were proportionately large, according as the light side was placed to the left or right.

We need not specify further; this will be sufficient to show, the reason why the smooth bore with a spherical bullet never made a straight long shot, for it was not only that the bullet did not go in the direction in which it was aimed, but it did not even follow the direction in which it started. This was well shown by Mr. Robins in the experiment we commenced with. He bent the end of a gun barrel to the left, and aimed by the straight part. As would be naturally expected, the shot passed through the first tissue-paper screen 1 1/2 inches to the left of the track of a bullet, which had been previously fired from a straight barrel in the same line with which the crooked barrel had been aimed, and 3 inches to the left on the second screen; but as he had predicted, and as the company could hardly have expected, on the wall which was behind, the bullet struck 14 inches to the right of the track, showing that though it had gone at first as directed by the bent portion of the barrel, yet as the bullet in being turned had rolled against the right-hand side of this portion of the barrel, it had a rotatory motion impressed upon it, by which the anterior portion moved from left to right, and the bullet, after moving away from, turned back and crossed the track of the other bullet again, or was incurvated to the right.

We now see why spherical bullets from a smooth bore, though they may fly almost perfectly accurately a short distance, cannot be depended on in the least for a long distance, as the bullet which might strike within 1 inch at 100 yards would not strike within 2 inches at 200 yards, and still less within 3 inches at 300 yards of the mark at which it was tired.

The cause of these deflections, we have seen is almost wholly rotation or spin. The object of the rifle is to place this rotation under our control, and if the bullet must spin, to make it spin always in the same direction, and in the way which will suit our purpose best. With this object the interior of the cylindrical bore which we have been considering as smooth, is scored or indented with spiral grooves or furrows. As we are merely concerned with the principles, and not with the constructive details, we need only mention that the number of these grooves varies in different rifles from two to forty; that their shape and size, though dependent on certain conditions, is, we might almost say, a matter of fashion; and that Mr Whitworth, in his almost perfect rifle, uses a hexagonal bore, and Mr. Lancaster makes a smooth oval-bored rifle; but that in all, the deviations from the circle of the interior cylinder do not pass straight from end to end of the barrel, but spirally, and constitute, in fact, a female screw. The bullet, fitting tight and entering the grooves, is constrained to rotate while being forced out of the barrel by the gunpowder, in the same manner that a screw is necessarily twisted while being drawn out of a hole or nut; and this rotation or spin being impressed upon it by the same force which projects it from the barrel, continues during the flight. This spin different in direction from those we have been considering previously; it is like the spin of a top thrown point foremost, the axis of rotation coincident with the line of flight. While it remains in this position (coinciding with the line of flight) none of the deflecting effects of the air we have mentioned can come into operation, as the resistance is equal on all sides; and not only that, but if there are any irregularities on the surface of the ball, as they are brought rapidly first on one side and then on the other of the point or pole of rotation, they can have no effect in deflecting it to one side more than to the other. Hence the accuracy, or straight shooting, of our modern gun, the rifle.

Notes

2. This tendency is found in practice to overcome the tendency that there is for the ball to be deflected in the opposite direction, from the greater friction arising from the greater density of the air pressing against the anterior surface than against the posterior surface.

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