Firearm Forums - Arms Locker banner

1 - 7 of 7 Posts

·
Registered
Joined
·
1,690 Posts
Discussion Starter · #1 ·
When we think of sound we usually think of our voices carrying over a distance or the daily sounds of our neighborhood. The sound we hear is the result of the compression and rarefaction, the coming together and spreading apart, of air molecules. But sound is much more. Sound, like waves in the ocean, is vibration. As the air molecues vibrate with our speach, so too does a rifle barrel vibrate when the cartridge within is fired.

Sound travels through air at sea level about eleven times as fast as the speed limit on an Interstate highway. In water sound can travel four times as fast as it can in air. That's why sound works so well for fish. But in steel, sound can travel 15 times as fast as it can in air. In case you were wondering, the speed of sound in a material can be estimated by dividing the material's bulk modulus by its density and taking the square root of the quotient. Fast rifle bullets usually reach muzzle velocities of three times the speed of sound in air and they average perhaps two times the speed of sound in air while in the barrel. Because of the faster sound speed in steel, the barrel will be vibrating before the bullet has left.

If you hold a fishing rod or plastic water pipe straight out (horizontal) it droops. Barrels are the same way. When the bullet moves down the barrel, the barrel starts to whip up and down. As the bullet moves down the barrel, the rifling forces the bullet to rotate and the barrel's muzzle begins to whip about in a circle. The strong force of the gas pushing on the base of the bullet makes it try to fatten, bulging out sideways. This creates a pig-in-a-python effect in the barrel.

So how can we steady the barrel? The obvious answer is to make it thicker. Unfortunately this adds weight. If we are seeking a lightweight rifle, then this is clearly not good. A popular approach is to free float the barrel. This allows the barrel to vibrate at will in the hope that it will at least send the bullet to the same spot each time. In some lightweight rifles a small amount of pressure, 12-15 pounds, is intentionally exerted against the barrel near the forearm tip. This pressure serves to lessen the magnitude of the barrel whip. Despite criticism of tip pressure by some folks, I have found that rifles using forearm tip pressure can often be made to shoot 1.5 MOA three shot groups. The original pre-64 Winchester M70's used adjustable tip pressure screws.

An interesting apprioach is that taken by Browning with their B.O.S.S. system. A special elastic bedding material is used and the muzzle is fitted with an adjustable weight. Turning the weight on the screw threads moves the weight fore or aft to tune the barrel's vibrations. The goal in the tuning is to make the muzzle into an anti-node or point of maximum movement. While this seems counter-intuitive, it minimized the angle change of the muzzle during firing. Thus the bullet leaves the barrel in a more predictable manner.

There is one barrel vibration tidbit that strikes me as funny. If the amplitude of the barrel's motion during firing were magnified, the barrel would look remarkably like Elmer Fudd's gun in the Bugs Bunny cartoons.
 

·
Registered
Joined
·
1,690 Posts
Discussion Starter · #2 ·
Let me add:

Barrels have been constructed with reverse taper contours to dampen vibration. These barrels tapered down from the receiver until about midway along the barrel. From midway forward to the muzzle the taper reversed, leaving the narrowest part of the barrel near the middle. This was designed to produce a "B.O.S.S.-like" effect. Although it met with some success, it never caught on with the shooting public.
 

·
Registered
Joined
·
10,279 Posts
If you want to add pressure under the barrel, what material should you use? Would something like a business card or plastic playing card be good? One of the gun writers suggested the business card.

RIKA
 

·
Registered
Joined
·
1,690 Posts
Discussion Starter · #4 ·
RIKA, the original Winchester M70's have a barrel contour that looks like a barrel band where the rear sight attaches. The tip pressure screw presses directly against the underside of that "band". In other words, it has metal on metal contact.

A business card will absorb moisture and vary the pressure. Regardless, I would try placing a small piece of a business card between the barrel and stock just to see if tip pressure works for your rifle. The card can always be replaced later.
 

·
Registered
Joined
·
10,279 Posts
That smart stock tuner looks interesting. Wonder if there are any reviews yet.

RIKA
 
1 - 7 of 7 Posts
Top