GUNS AND AMMUNITION. 



THE THEORY OF DRIFT. 



W. E. CARLIN. 



While there are yet many problems to be 

 solved, in the science of rifle shooting, the 

 theory of drift is well understood among 

 experts. Drift is the term applied to any 

 deviation from -the vertical or horizontal 

 planes of fire, other than that caused direct- 

 ly by the force of gravity. It is a complex 

 affair and not easily disposed of. It may 

 be divided under several heads, and the 

 reasoning that would apply to one given 

 set of conditions may not at all apply to 

 another. 



We say, for instance, that a bullet drifts 

 in the direction of the grooves in the barrel. 

 This is true for several forms of projectiles, 

 but not true in the case of a solid cylinder 

 having its flat head opposed to the resist- 

 ance of the air. The resistance of the air is 

 the cause of drift, but not, as explained in 

 February Recreation, because of the 

 slightly greater resistance offered beneath; 

 owing to the bullet falling under the influ- 

 ence of gravity. The bullet is constantly 

 entering undisturbed air, and the slight dif- 

 ference in resistance could account for but 

 a small amount of the actual drift of elon- 

 gated projectiles. Your correspondent is 

 also in error on several other points. The 

 axis of the bullet fired does not remain 

 parallel to the axis of the bore; and there 

 is no such cushion of air for it to roll on. 

 The form of air waves set up by the bullet, 

 when its velocity exceeds that of sound, is 

 beautifully illustrated in the photographs 

 taken by Professor Boys, R. S., and others. 



Finally the curve described by the bullet 

 is not a parabola, and the higher the veloc- 

 ity the more the trajectory departs from the 

 parabolic curve. 



I am aware that in many text books which 

 treat of trajectory in vacuo the curve is 

 called a parabola, for simplicity's sake; but 

 if all the factors are considered, the result- 

 ing equation is that of an ellipse and the 

 greatest range is not obtained in vacuo 

 at an angle of 45 , but slightly less. 



I shall here consider only the usual form 

 of bullet, i.e., an elongated projectile having 

 a more or less sharp pointed head and hav- 

 ing its center of gravity situated behind its 

 center of figure, and fired at an angle of 

 elevation to the horizontal. In substance 

 it is taken from Professor Bashforth's ex- 

 planation, which has since been proven cor- 

 rect on the experimental range, but is 

 abridged and simplified as far as possible, 

 and with some additions. 



To begin with, it may be stated that our 

 elongated projectile is never absolutely 

 steady in its flight. The drift is in every 

 direction as viewed from the rifle; the devia- 



tion being greatest in the direction toward 

 which the projectile rotates, and that the 

 beginning of all drift (under the conditions 

 assumed) is a drift upward. 



Fig. :. 



Let us first consider our projectile fired 

 from a smooth bore barrel, at an angle of 

 elevation as already stated. We may sup- 

 pose (Fig. 1) the center of gravity, G, a 

 pivot. The resistance of the air will raise 

 the point, which will assume the positions 

 A, A', etc , until the projectile has been 

 turned completely over and the heaviest 

 part, the base, will travel foremost. Now 

 if the same bullet be given a very rapid mo- 

 tion of rotation about its longer axis, by 

 means of the grooves of the barrel, the re- 

 sistance of the air will tend to raise the 

 point as before, while the rotary motion 

 about its longer axis will resist this and 

 tend to keep the projectile point foremost. 



The projectile is therefore acted upon by 

 2 rotary forces, the rotation about its longer 

 axis and the resistance of the air which 

 tends to give it a motion of rotation about 

 its shorter axis passing throilgh the cen- 

 ter of gravity. It is easily seen that the 

 axis of progression is not tangent to the 

 trajectory, it being deflected from it by the 

 action of gravity; so that the resistance of 

 the air does not pass through the center of 

 inertia, but above it or below it, depending 

 on the shape — in this case, above it. This 

 forms a couple tending to overturn it. It 

 is easily proved that a projectile acted on 

 in this manner will not yield fully to either 

 force, but will move slowly up and to the 

 right or left, as proved long ago by Pro- 

 fessor Magnus, depending on the relative 

 direction of the 2 forces. Captain Ingalls 

 considers that the axis of the projectile will 

 describe a cone about an instantaneous 

 axis; that is, an axis passing through the 

 center of inertia of the projectile and paral- 

 lel to the resistance of the air. This would 

 lead to a slight motion of mutation, or the 

 cone described would be slightly corru- 

 gated. 



This conical, or spiral drift, was gone 

 into experimentally by my friend, Mr. E. A 



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