1. Every shooter is familiar with the recoil of a rifle. Simply stated, recoil occurs because expanding powder gases propel the bullet and the cartridge case in opposite directions. The bullet travels forward through the bore. The force applied to the base of the cartridge case is transmitted to the rear against the bolt.
2. If the rear of the bolt action were placed squarely against a solid support, the recoil of the barrel and action would be straight to the rear. However, the rifle barrel and bolt action normally rest on top of a wooden stock. The butt of the Btock drops considerably below the axis of the barrel and bolt. Consequently, the rearward^ motion of the barrel and action tends to exert a force above the axis of the stock, causing the muzzle to be pivoted upward. This upward movement in recoil is called jump. The degrees of an arc described by the muzzle is called the angle of jump. For example, if the barrel jumped from the exactly horizontal to an absolute vertical plane, the angle of jump would be 90 degrees.
3. The angle and the direction of jump can be affected by external pressures on the rifle. A tight sling exerts a downward force on the muzzle of the rifle and lessens the angle of jump. In addition, the jump can be deflected to one side by a pressure exerted against the opposite side of the stock.
4. There are many technical considerations that should be accounted for in recoil. However, for purposes of this discussion, it is sufficient to say that recoil commences as the bullet starts to move. The gas pressure that causes recoil cease the instant the bullet leaves the barrel. No account is taken here of gas pressures on the muzzle face, or of the effects of inertia. It may be seen that since the bullet is moving in the barrel during recoil, a change in the angle of jump will place the muzzle at a different point as the bullet emerges. This would thus alter trajectory (and point of impact) of the bullet. This occurs even though the sights are perfectly aligned on the target at the time the weapon fires. A shooter should be concerned, then, that the angle of jump of the rifle remains constant throughout a shooting string. Otherwise, he will experience a change in zero.
5. In the standing position, slight changes in the angle of jump can be brought about by changes in the adjustment of the palm rest. These changes are usually very minor. However, in the other positions, changes in sling tension or changes in the position of the left hand can have a noticeable effect upon the angle of jump, and cause considerable changes in zero.
6. In all positions, a change in the position of the butt plate against the shoulder can cause a major change in zero.
7. Cheek pressure is also critical in affecting the angle of jump. If the cheek is moved forward or back on the stock, or if the cfieek pressure is increased or decreased on the stock, there will be a noticeable change in the point of impact. This can occur in any position, and it occurs even though each shot may break with the sights in perfect alignment on the target. The shooter should take care to apply the exact same check pressure to the exact same point on the stock throughout a series of shots. Most good shooters rest the head naturally against the stock. They do not attempt to increase or decrease this natural pressure by use of the muscles. Experience will teach the shooter to sense unintentional changes in head position or cheek pressure. The shooter should attempt to hold the rifle in exactly the same way each time he fires during a string.
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