Bradley Wright



Bradley Wright

Mr. Louie Ninni

English III

25 March 2012

Factors that Govern Long Range Rifle Accuracy

Rifles and smooth bore cannons are complex instruments that can be modified to enhance accuracy, velocity, and range. There are many factors and components that contribute to this complexity. A shooter can increase his accuracy, velocity, and range by investigating and modifying barrel length, barrel design, weapon stock, and the trigger.

It is good to have the correct length barrel for the task an individual needs to compete with firearms or pneumatic guns. This is true for match competition, hunting or just plain target shooting. Some people think the longer a barrel, the better it is. Due to this, some problems can commonly happen that people are unaware of. When the barrel is made too long, extreme problems happen that affect range, velocity, and the quality of the weapon.

To illustrate this point, a test was completed with a smooth bore air cannon that shoots a three inch projectile at a consistent pressure per square inch. This test was completed with different barrel lengths that all shoot the same three inch round. The barrel lengths consisted of one, two, three, and seven feet. From the data gathered, after the two foot barrel length is exceeded, the velocity decreases. Due to the decrease in velocity, the range distance and accuracy also decreased. Comparing the data from the two foot and the seven foot long barrels, there was approximately nine meters a second change in velocity. On an average, there was also approximately an 18 meter drop in range distance.

There are two reasons why the velocity and distance is lower with the longer barrel. First, the friction between the projectile and the barrel. Second, the air released has expanded as much as it can. This occurs when the compressed air is released, and it goes to the path of least resistance which happens to be forward out of the barrel. This speeds the projectile up and sends it down the barrel. Once the air pressure has decreased close to normal, the projectiles friction within the barrel slows it down before it passes out the end of the muzzle.

A long barrel not only can decrease the range and velocity but it can also cause harm to the gun itself. For many years, Mr. Steven Shelp has shot in 1000 yard competition matches (Shelp). From his experiences with a 338 caliber rifle, he has seen that a 37 inch barrel shot approximately 3350 feet per second (fps). One of his opponents found out that with his 46 inch barrel, due to extreme copper fowling in the last six inches of the barrel, it could only shoot approximately 3250 fps. The effect of this fowling was detrimental to not only velocity, range distance, and accuracy, but also to the structure of the barrel. With the last six inches sawed off the barrel, the new 40 inch barrel shot approximately 3290 fps, and with greater accuracy. This is better than the 46 inch barrel length, but not as good as the 37 inch barrel.

In an article written by David E. Petzel, he stated that before there was a chronograph lurking under every load bench, gun makers used barrels of sensible lengths (Petzel). If a .30/06 or a .270 is purchased, it most likely had a 22-inch barrel. If a magnum was purchased, it was probably 24 inches. Over the years, however, barrels have been getting longer, possibly because manufacturers are afraid their guns/ammo won’t deliver advertised velocities. Mr. Petzel never feared shorter barrels. They are handier to use, weigh less, and often are more accurate than the longer ones. As a rule, when steel is lopped off, very little velocity is lost. As proof of this, ace Texas rifle maker Charley Sisk recently published an experiment where he barreled six rifles with a 27-inch barrel, and chronographed them while cutting each one back an inch at a time (Zwoll). His work is published in the book, “Hunter’s Guide to Long-Range Shooting,” authored by Wayne Van Zwoll. Space doesn’t allow listing all the figures; however, a condensed table of figures is shown below.

.22/250, 27 to 24 inches, from 3,469 to 3,407 fps

.270 Winchester, 27 to 21 inches, from 3,115 to 3001 fps

.300 Win Mag, 27 to 22 inches, from 3,055 to 2,960 fps

.340 Weatherby, 27 to 22 inches, 2,834 to 2,755 fps

.338 Win Mag, 27 to 20 inches, 2,806 to 2,656 fps

.257 Roberts, 27 to 20 inches, 2,860 to 2,717 fps

There are a fair number of variables involved, mostly involving bullet weight and the burning rate of the powder, but this tallies very closely with what Mr. Petzel has seen. A few years ago, he had the barrel of his .338 RUM chopped from 26 inches to 24 inches. Accuracy increased to an astounding degree, and the velocity loss with 250-grain bullets was only 38 fps.

Whether an air cannon or a high powered rifle is being shot, remember, just because the barrel is long doesn’t mean it is good. The barrel can cause the slowing of the projectile, and in some cases extreme fowling causes reduced performance. This proves that if a barrel is too long, that it isn’t good for range, velocity, or the gun itself.

Another important item in accuracy is the fit of the ammunition to the barrel. The chamber should be concentric, and the throat sized just slightly larger than the bullet diameter. The cartridge must headspace correctly, be held firmly in place, concentric to the bore, and the bullet must fit the bore and be guided to engage the rifling cleanly. Once the bullet is engaged in the rifling, with a good seal and coaxial fit, it must stay that way. It is often possible to shorten a barrel slightly by removing material from the breech end and recutting (reboring) the chamber, which can correct many problems.

A chamber can be rebored to increase its size. A popular resized chamber is the Ackley improved. This resizing procedure provides a straight side cartridge with the shoulder moved forward. It generally yields 5% extra case volume for more powder and up to 100 fps faster velocities.

A cartridge must fit in the chamber properly with the bullet seated in the cartridge at the best depth for top accuracy. This usually is a depth where the ogive (spot where top of bullet equals the exact diameter for a particular caliber) is ¼” or less from the rifling, but not touching.

Most experienced shooters like to have the tip of the bullet .001 of an inch from the rifling. Figure 1 shows the ogive ¼” from the rifling (see Figure 1).

[pic]

Figure 1. A cartridge sitting in the rifle chamber with the bullet ogive ¼” from the barrel rifling ().

When a gun’s rifling is of the wrong twist rate for a particular bullet or velocity, if the rifling was damaged or absent, or if the bullet’s diameter doesn’t properly match the gun’s bore diameter, the bullet is likely to tumble in flight. This results in poor accuracy, unpredictable bullet flight and unpredictable bullet performance when it reaches the target or game, and can result in “keyholing”.

Keyhole refers to the hole which a tumbling bullet makes in a target, or it may describe the tumbling which the bullet was doing when it struck the target (Chaistain). When a conical bullet strikes a surface in any manner other than truly stabilized flight, the bullet has keyholed. Evidence of keyholing most often comes in the form of oblong holes in a paper target. Sometimes the holes are just a bit longer in one direction, and at other times the bullet will strike the target sideways. The causes of keyholing may vary, but in all cases the bullet’s flight was not properly stabilized when it struck the target.

In ideal conditions, a conical bullet is given some spin by the rifling in a gun’s barrel as it travels through the barrel after being fired. That spin will stabilize the bullet’s flight so that its nose is always forward, and it moves through the air in a nice straight manner. Consequentially, the alignment of the barrel in the rifle must be exact and pressure point free to allow for optimum accuracy. Free floating a barrel is the solution to this.

The problem with firing accurately at distance is that even the slightest changes in the alignment of the barrel can throw the round way off target. Even though barrels are typically made of hardened steel, the fact that they’re rather long allows even minute forces to act on them like a lever. When using a barrel that hasn’t been free floated, each time the shooter changes the forces working on the barrel, it will bend and the bullets won’t necessarily go where the shooter anticipated. When the barrel and the stock heats up, the stock will touch the barrel, and increase the pressure on the barrel, and throw the rounds off even farther. This means if the shooter never changed how the gun rests in the stock; the rounds are going to spread out even worse.

On a traditional rifle a sufficiently rigid stock must be selected and material must be removed from the stock forend (around the barrel) until the barrel no longer touches the stock. The standard test for checking to see if a barrel is free floated is the ability to run a dollar bill between the stock and the barrel all the way to the receiver. When this is accomplished, there will be no pressure point so when the trigger is pulled, and the barrel heats up and swells, it will not touch the stock. Free floated barrels do in fact lead to smaller shot placement groups.

The dynamics of the trigger, known as its pull, are one of the most important aspects of accuracy since any movement of the rifle caused by pulling the trigger can have an impact on shot placement. No matter what the potential accuracy of a rifle is, it can not be fired accurately unless the shooter can do so. A rifle with an easy and consistent pull trigger can provide the shooter with the means for accomplishing this.

Most serious hunters and bench rest shooters have special triggers installed in their rifles which will eliminate pre-travel and weight. Mr. Overton personally likes a Jewell Trigger set at 1 ½ lbs pressure (Overton). This allows for an easy, crisp, and fast movement of the trigger for firing the rifle (SeeFigure 2).

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Figure 2. Fully adjustable, finest quality Jewel Trigger that is used to eliminate rifle movement and enhance accuracy ().

Rifles and air cannons must be refined to achieve maximum range and accuracy. There are numerous areas that can be changed to increase the performance of the weapon, and several have been examined. When a shooter makes the changes that have been examined, his ability in firing weapons will greatly improve.

Works Cited

1. Chastain, Russ. "Definition of Keyhole, Key Hole, Keyholed Bullet, or Bullet Keyholing." Hunting / Shooting. Web. 12 Mar. 2012. .

2. .

3. Leghorn, Nick. "Ask Foghorn: What Does a “Free Floatâ€? Barrel Do?" Ask Foghorn: What Does a “Free Floatâ Barrel Do? The Truth about . Web. 12 Mar. 2012. .

4. Overton, Charles W. Personal interview. 8 Mar. 2012.

5. Petzal, David E. "Why Shorter Rifle Barrels May Be Better | Field & Stream." Field & Stream. 9 Feb. 2012. Web. 12 Mar. 2012. .

6. Shelp, Mr. Steven. Personal interview. 20 Feb. 2012.

7. "The Effects of Barrel Length on the Range of a Projectile Fired From A Pneumatic Cannon." TuHS Physics Home Page 1.1. Web. 21 Feb. 2012. .

8. Zwoll, Wayne Van. “Hunter’s Guide to Long-Range Shooting,” (2006): 43-60. Print.

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