September 23, 2010
Without the barrel, all the other parts of a rifle are useless. In addition to allowing the propellant inside a cartridge to transfer its energy to the bullet, the barrel also is responsible for sending it on an accurate path and in the desired direction.
Most centerfire barrels made today are either type 416 stainless steel or type 4140 chrome-moly steel. All else being equal, the accuracy potential of the two types of barrels is the same. Both do, however, have their advantages and disadvantages.
Chrome-moly is a bit easier to machine because it is less susceptible to galling, but this presents no problem with stainless steel as long as it is properly lubricated during the machining process.
Stainless steel is better at resisting oxidation, but since it does contain a bit of carbon it will rust under severe conditions. Stainless is also a bit more resistant to bore erosion, so accuracy life can be a bit longer--how much longer depends on several factors.
Most barrel makers will tell you that when both types of barrels are properly maintained, the stainless barrel will withstand the firing of 5 to 10 percent more rounds before losing its accuracy. This is more important in a varmint rifle because it usually is fired much more than a rifle used for hunting big game. The bigger the cartridge for a particular bore size, the shorter the accuracy life of a barrel.
Given proper care, a good barrel will last longer than some shooters believe. Based on my experience, I'd say a top-quality stainless barrel in .233 Remington should digest upwards of 6,000 rounds before losing its prairie shooting accuracy, and I've known a few that lasted longer than that. A good barrel chambered for hotter cartridges such as the .22-250 and .220 Swift should deliver long-range accuracy to at least the 4,000-round mark, while a barrel in .257 Weatherby Magnum, 7mm STW or .300 Weatherby Magnum should still be delivering minute-of-whitetail accuracy at 2,500 rounds.
Several years ago I discussed barrel life with Earl Chronister who for several years held the world's record for the smallest 10-shot group of 4.375 inches fired from benchrest at 1,000 yards. A member of the Original Pennsylvania 1000 Yard Benchrest Club, Earl shot a rifle in .30-378 Weatherby Magnum and usually switched barrels every ,2000 rounds.
All you have just read on barrel life assumes three things: the barrel is not heated up excessively by a lot of rapid-fire shooting, its bore is properly maintained, and the barrel is a good one to begin with.
When it comes to determining accuracy potential, a close match between bore and bullet diameter along with end-to-end bore and groove dimensional uniformity are by far the most important. The surface quality of the bore is also important because bullet jacket fouling, which will eventually spoil accuracy, accumulates more slowly in a smooth barrel than in a rough barrel. But regardless of how smooth its bore surface is, a barrel will not deliver top-level accuracy if bore and/or groove diameter varies a great deal from chamber to muzzle.
By the same token, a barrel is not likely to be accurate if its bore diameter does not closely match that of the diameter of the bullet being fired. This is important with any type of bullet but more so with those of monolithic design such as the X-Bullet from Barnes simply because it does not obturate as freely to fill an oversized bore as a conventional lead core/jacketed bullet is prone to do.
Aftermarket barrels are manufactured in several grades, but to keep it simple I'll divide them into three groups. The Utility grade is for the rifleman who wants to build a good shooter but is satisfied with less than tackhole accuracy. Several shops offer this grade of barrel and the two that come to mind are McGowen and E.R. Shaw.
Other shops offer Match Grade along with what I refer to as Benchrest Grade barrels, with Shilen being a good example.
As far as accuracy goes, the primary differences between the three grades of barrels are how close bore and groove dimensional tolerances are held during their manufacture and how smooth their bores are. Benchrest Grade barrels (and some Match Grade barrels) are bored and rifled slightly undersize and then a lead lap coated with a mild abrasive is used to arrive at the finished diameters.
In addition to reducing any diameter variations left from the boring and rifling processes, lapping also removes bullet jacket-grabbing tool marks that run across the lands and grooves. The fe
llows who hand-lap rifle barrels have arms like Popeye and usually finish no more than a half-dozen barrels during a hard day's work. In large part, this explains the higher cost of a super-accurate barrel.
The bore and groove diameter of a Shilen Match Grade barrel will be no more than .0005 inch larger or smaller than is standard for a particular bullet diameter and bore, and groove diameter will vary no more than .0003 inch from one end of the barrel to the other.
Moving up a bit in precision, bore and groove diameter of a Benchrest Grade barrel from Shilen will be within .0003 inch of bullet diameter standard and uniformity will be within .0001 inch from chamber to muzzle.
To put those tolerances into perspective, a single page of the wonderful publication you now hold in your hands is .002 inch thick or 20 times greater than the end-to-end bore and groove diameter variation of a Benchrest Grade barrel. Several other shops offer Benchrest Grade barrels--Lilja, Schneider, Hart, Pac-Nor and Krieger to name a few.
So which grade do you choose? Thousands of hunters and shooters are quite satisfied with Utility Grade barrels, which is what the majority of factory-built rifles wear. A barrel of this grade capable of delivering accuracy as good as an aftermarket Match Grade barrel does occasionally come along, but only rarely is one as accurate as a Benchrest Grade barrel.
Serious benchrest competitors always use the most accurate barrels available simply because in order for a rifle to win at that game it must be capable of consistently shooting five bullets inside .200 inch at 100 yards.
I usually stick with Benchrest Grade regardless of what the rifle will be used for. It is usually not all that much more expensive than Match Grade, and even though I may not actually need its accuracy in the field, it gives me confidence and the more confident I am in a rifle the more accurately I shoot it.
And then there is twist rate. The longer a bullet of a particular caliber is, the faster it must rotated in order to maintain its stability during flight. Bullet rotation is, of course, governed by rifling twist rate of the barrel and bullet velocity.
An increase in either one will increase rotational velocity, but it takes a rather substantial increase in speed to equal a small increase in rifling pitch. When, for example, a .224 caliber 90-grain MatchKing exits an AR-15 rifle barrel with a 1:6.5-inch twist (one turn of the rifling in 6.5 inches) at a velocity of 2,700 fps, it is spinning at a rate of 4,985 revolutions per second. Keep velocity the same but increase twist rate to 1:5.5 inches and rotational speed will increase to 5,891 rps. In order to increase bullet spin by that much in a 1:6.5 twist barrel, velocity would have to be increased to 3,200 fps, which isn't possible with the .223 Remington at acceptable chamber pressures.
Choosing Twist Rates
How does one choose twist rate? One way is to pick the rate the manufacturers use, although that's not always correct. Barrels for the .257 Roberts, for example, use a 1:10 twist, which worked fine for the 100-grain bullets originally popular for the caliber but sometimes doesn't match up well with 117-grain pointed bullets. Anyone who decides to buy a barrel for the .257 Roberts today is wise to choose a 1:9 twist as it will stabilize pointed bullets weighing up to 120 grains and still deliver good accuracy with those as light as 75 grains.
The correct rifling twist rate for a particular bullet can also be calculated by using a formula originated by Sir Alfred Greenhill during the late 1800s. Simply divide a constant of 150 by the length of the bullet (in calibers) and then multiply by the diameter of the bullet (in inches).
For example, using the Roberts again, the Speer 120-grain spitzer has a diameter of .257 inch and a length of 1.125 inches. Dividing that bullet's length by its diameter gives us 4.38. Divide 150 by 4.38 and multiply the result by .257 and you arrive at a rifling twist rate of 8.80 inches. Rounding off to the next highest number gives us a twist rate of 1:9 inches which, as I said before, is correct for the .257 Roberts when it is loaded with a 120-grain spitzer.
Another and perhaps the safest way to determine the correct rifling twist rate for a new barrel is to tell the maker of the barrel what cartridge it will be chambered for and which bullets you intend to shoot and let him decide (some barrel makers post this information on their websites).
Benchrest shooters choose relatively slow rifling twist rates that border on bullet instability, but they get by with it because they usually stick with a single bullet weight. When choosing a twist rate for a rifle to be used for varminting or big game hunting, I prefer more flexibility in bullet choice and tend to err on the quick side. The accompanying sidebar contains some recommendations from Shilen.
Just as there are different twist rates, there are different types of rifling. Over the centuries, different types have been tried, but the simple land-and-groove style has proven most practical. Today, four- and six-groove barrels are the most popular.
The rifling itself is formed by four common methods. The oldest (and the slowest) is the use of a single steel scraper attached to a long rod to cut each groove individually. The broaching system used by some European manufacturers utilizes a series of cutters that cut all the grooves at once.
In the button process, a short carbide slug with its surface containing a reversed version of the rifling is pulled or pushed through the barrel and the rifling is actually ironed into the bore. In the hammer-forging process, a tungsten-carbide mandrel wearing the rifling in reverse is placed inside the barrel and a very large (and extremely noisy) machine containing multiple hammers capable of delivering tremendous force with each blow pound the barrel into shape both inside and out.
Most shops and companies offer only one type of rifling but there are exceptions. The famous Mark V rifle sold by Weatherby has a button-rifled barrel while the barrel of the Vanguard from the same company is hammer-forged. The barrel of the Remington Model 700 is hammer-forged, but the 40-X target rifle from Remington's
custom shop wears a button-rifled barrel.
Savage barrels are button-rifled while Sako barrels are hammer-forged. As barrels made by some of the smaller shops go, those from Shilen, Lilja, Schneider and Hart are button-rifled while Krieger offers both button-rifled and cut-rifled barrels.
I prefer button-rifled barrels because I have had such good luck with them through the years, but I'm quick to admit that the rifling in some of the most accurate barrels in the world is formed by other methods.
The quality of a rifle barrel is dependent, not on the type of rifling it has, but rather on the quality of the steel used, the condition of the tools used to manufacture it and the skill of the person who is operating the machinery
|RECOMMENDED* RIFLING TWIST RATES |
1:7 -- heavier than 80 gr.
1:8 -- up to 80 gr.
1:9 -- up to 70 gr.
1:12 -- up to 63 gr.
1:14 -- up to 55 gr.
1:15 -- up to 55 gr. at 4,100 fps or faster
1:16 -- up to 45 gr. or up to 55 gr. at 4,300 fps or faster
1:8 -- VLD bullets over 100 gr.
1:10 -- up to 100 gr. and VLD bullets lighter than 100 gr.
1:12 -- up to 85 gr.
1:13 -- up to 75 gr.
1:12 -- up to 70 gr.
1:9 -- up to 120 gr.
1:10 -- up to 100 gr.
1:12 -- up to 90 gr.
1:13 -- up to 80 gr.
1:14 -- up to 70 gr.
1:8 -- up to 180-gr. VLD
1:9 -- heavier than 140 gr.
1:10 -- up to 140 gr.
1:8 -- heavier than 220 gr.
1:10 -- up to 220 gr.
1:12 -- up to 170 gr.
1:14 -- up to 168 gr.
1:15 -- up to 150 gr.
1:17 -- up to 125 gr.
*Partial listing of recommendations for pointed bullets from Shilen Rifles, Inc. Abbreviation: VLD, very low drag