Rifle Stocks - Barrel Vibration and Accuracy

From the moment that trigger is pressed and the firing pin strikes the primer until the bullet leaves the muzzle, a series of many vibrational impulses begin in the rifle, all of which are transmitted to the barrel in various magnitudes. These include such minor things as; the trigger sear releasing the firing pin, the firing pin moving forward, striking the primer, and the cartridge being moved forward. The powder then begins to ignite, and the bullet starts moving forward and engages the rifling. Because of the twist of the rifling, the bullet while it is being propelled forward, begins to spin imparting a small but measurable torque, but more importantly as it traverses the barrel it also sets up a circular vibrational pattern, or arc. The heat of the burning powder along with the pressure wave generated by the expanding gasses start another vibrational pattern that is induced into the barrel. All of these movements cause the barrel to stress and vibrate with a number of different harmonic patterns which if not controlled by some means cause each projectile to leave the muzzle at a slightly different point in the vibrational arc. Some people speak about the "whip" of the barrel, which would imply to some, that the barrel simply vibrates up and down like a buggy whip. Although there are some of the vibrations that are traveling in this direction, the main vibrations are circular. If this were not true, then a 3 shot group from a rifle would always be in a vertical string. This would be because, one would leave at the bottom of the "whipping action", one would leave from the center and one from the upper travel of the "whip". As we all know this seldom occurs, and if it does, it is usually caused by the barrel being under a heavy stress, such as way to much pressure exerted against it, caused by improper bedding, usually of the barrel. Most 3 shot groups you will see will be virtually triangular in shape, this is caused because as the barrel vibrates through its "circular arc" one bullet leaves the muzzle at say 12 o’clock, another at say 4 o’clock and the third at maybe 8 o’clock. The larger the arc of the barrel, the less accurate the rifle will be, and the larger the triangle. As a rule the less mass a barrel has, (the thinner) the more it is affected by the vibrations, this is the reason that a "heavy" barrel seems to shoot more consistently than a sporter barrel, and is also easier to tune. A shorter barrel of the same diameter will also have less amplitude to its arc of movement. As you tune the barrel with the Accurizer, what you are doing is changing the vibrational length of the free floated forward end of it, causing the vibrational arc, or circular vibrations of the barrel to get smaller and smaller. As the arc gets smaller the groups get tighter. Even though the bullets may still be leaving the muzzle at 12, 4, and 8 o’clock, the diameter of the arc has been lessened, so the triangle gets smaller. Until the past couple of years, when reliable barrel vibrational control devices came upon the scene, virtually the only way to control these barrel vibrations that affected the rifles’ accuracy was to custom load the ammunition until a correct combination of cases, bullets, primers, powders, seating depths and etc. could be found that would allow that particular rifle to shoot good tight consistent groups. Now however with the use of the SmartStock, it is no longer necessary for the average shooter to go to these lengths to have the accuracy previously attributed only to hand loaded ammunition. As long as you are using well made ammunition, that is consistent, whether it is factory or handloads, you can simply tune your rifles’ barrel to the ammunition of your choice.

Below is a table which was kindly provided by Varmint Al. It shows the relationship of the amount of vertical movement of a fluted barrel compared to a standard barrel. It also shows how shortening the free floated length a barrel with the SmartStock's tuner, greatly reduces the amplitude of the vibrations, and therefore reduces the dispersion of the bullets in a group.

Note that the Projected Exit Angle of the bullet at 100 yards in a 22" non-fluted barrel is 0.6069". Reducing the free-floated length of the forward end of the barrel to 14.6829" as is shown in the chart below, reduces the angle by 70% or nearly 3 times less vertical movement. This coincides nearly exactly with the independent barrel sag and accuracy heat testing done by me previously. You can see those results at:

http://www.rifle-accuracy.com/barrel-movement.htm

Table 1. Results for six different 416 stainless steel 6mm rifle barrels 22" long

Barrel Weight

(lb) Barrel OD

(in) Flute Root Dia

(in) Flute Depth

(in) First Mode Natural Freq

(Hz) Section Moment of

Inertia (in^4) Barrel End Sag

(in) Barrel Exit Angle Projected

to 100 Yard Target (in) 7.222 1.250 none none 73.783 0.11916 0.00278 0.6069 7.222 1.3687 1.1187 0.125 75.238 0.12390 0.00268 0.5837 7.222 1.4747 0.9747 0.250 79.278 0.13757 0.00241 0.5257 7.222 1.5674 0.8174 0.375 85.075 0.15842 0.00209 0.4565 4.820 1.2500 0.7500 0.250 67.724 0.06700 0.00330 0.7204 5.871 1.2500 1.0000 0.125 68.619 0.08378 0.00322 0.7017 4.820* 1.2500 none none 165.646 0.11916 0.00052 0.1804 5.871** 1.2500 none none 111.648 0.11916 0.00122 0.3261