Mayer, Scott E

Newton’s third law tells us that for every action there is an opposite and equal reaction.

That leads to the principle of conservation of momentum. How that principle applies to firearms was best stated by our Ballistics Editor, William C. Davis, Jr., as “the momentum imparted to the gun upon firing must always be equal in magnitude, and opposite in direction, to the momentum imparted to the bullet and the powder gases that are expelled from the bore.”

Because firearms comply with physical laws, we’re sentenced to recoil. From the fundamental standpoint, recoil is divided into three categories: recoil impulse, free-recoil velocity and free-recoil energy. Recoil impulse relies on the performance of the ammunition regardless of the gun. Free-recoil velocity has to do with whether the sensation of recoil is a sharp slap or a robust shove. Free-recoil energy is the amount of “kick” expressed in ft.-lbs.

From the shooter’s standpoint, recoil can be divided into two categories: actual and perceived. Actual recoil can be measured while perceived recoil is felt. Both can be changed to increase or reduce recoil. We recently built a recoil pendulum at NRA for evaluating recoil reduction methods and testing such claims as “short magnums feel like they kick less than full-length magnums.” Look for future reports on such things as the measured effectiveness of various muzzle brakes and if the short magnums really do kick less.

Forum Updates: Greenhill’s Formula in the Jan. 2003 “Shooters Forum” generated several responses including ones from members in Paoli, Pa., and Walker, Minn., who had both come up with a formula that didn’t require converting from calibers to inches. Calibers are used very often in ballistic formulas because they allow greater flexibility in using the data. But for the purpose of finding only a particular rate of twist for a particular bullet, the simpler formula is T-150 (d^sup 2^)/1 where d is the bullet diameter in inches and 1 is the bullet length in inches. The example in the Jan. article computed the optimum rate of twist for Sierra’s. 30-cal., 200-gr. MatchKing bullet. Using the simpler formula, the twist is calculated as:

T= 150(0308^sup 2^)/1.405 s

T=150(.094864)/1.405

T=14.2296/1.405

T=10.1

(The same as Greenhill’s Formula.)

Regarding bullet stability, another member in Mt. Vernon, Ohio, wanted to know what would happen to light, short bullets if they’re overstabilized by too fast a rifling twist, I have never personally experienced a problem when firing overstabilized bullets. That said, however, others have reported problems such as over-stabilized bullets at very high velocity flying apart in mid-air as the centrifugal force exceeds the strength of the bullet.

Another member from Florida read the Sept. 2002 “Forum”on rifle accuracy and argued that the increase in accuracy from 2.78″ to 2.35″ was 39 percent, not the 16 percent that I reported. In this instance, the smaller 2.35″ group is 84 percent the size of the larger 2.78” group, which is where I get the reported 16percent improvement.

Scott E. Mayer, Shooting Editor

Copyright National Rifle Association of America Jun 2003

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