- What Makes A Barrel Accurate?
- Is Your Barrel Shot Out? click here
There are some obvious mechanical attributes that a barrel should possess if it is going to be a "tack-driver". These include the straightness of the hole, uniformity of the rifling geometry, and the twist rate. If the width of the lands or the depth of the grooves varies, that barrel is never going to shoot accurately. Likewise, if the interior finish of the barrel is poor then the barrel lacks the end-to-end uniformity required of an accurate barrel. A rough finish will also foul badly, causing accuracy trouble. And along with surface finish, the direction of the finish is very important too. Ideally the "lay" of the finish will be parallel to the bullet travel. If there are reamer marks left inside the barrel on the tops of the lands they will be perpendicular to the bullet travel and tend to foul.
Along with the above, it is also imperative that the barrel is chambered, threaded and fit to the action, and crowned by a competent gunsmith. A poor fitting and chambering job can force the best barrel to shoot under its potential. I've also seen the throat area of barrels polished with horizontal marks left from a chamber polishing that got too deep. It is unnecessary to polish a chamber beyond the shoulder. The lack of a shoulder makes it easier to get too deep with rimfire chambers.
One of the most important pieces to the puzzle is a straight and uniform throat that is concentric to the bore. The bullet must get started into the rifling straight. If it doesn't; it will become somewhat deformed and will not be centered in the barrel. Upon exiting the muzzle it becomes free to rotate around its new center-of-gravity, departing from the centerline of the barrel. In effect it will make a corkscrew-type path around the barrel centerline.
Gunsmiths like to argue. One of their favorite topics concerns the best method for chambering a barrel: holding the barrel through the headstock of the lathe, or running it in a steady rest. I've used both methods and don't think either one is better than the other. The important factor is getting the barrel to run true in the lathe. The chamber is then cut true to the bore, allowing the throat to be cut uniformly with the bore and grooves. And it also means the case head will contact the bolt face evenly. If the contact is uneven, then an undesirable vibration can be set up in the entire rifle, causing inaccuracy. Often this shows up as vertical stringing in a group. The same situation can develop with poor bolt lug engagement against the action. A crooked barrel can prevent a chambering reamer from cutting an accurate throat.
The diameter of a throat should not be more than .0005" over the nominal bullet diameter for cartridges of .338" or smaller. This is the freebore section. If it is too large the bullet can start to wobble in the throat before it becomes fully engaged in the rifling. And again, accuracy will deteriorate. And nearly as important, the throat angle should be a close match for the bullets being used. For most 7 caliber tangent bullets this is an angle of about 1.5 degrees per side. We have another article on this subject. Throat angles.
As mentioned above, a uniform twist rate is also crucial, so is matching the proper twist rate for the bullets to be used. Ideally a barrel would have a twist with no variation. If the twist rate decreases, accuracy will suffer, but a slight increase in twist is not detrimental. The explanation for this is fairly simple. If we look at a recovered bullet we'll notice that the rifling cuts a partial helix in the bearing surface of the bullet for each land. The helix is on an angle matched by the twist in the barrel. If the twist rate decreased, the angle of this helix would decrease also, and would effectively cut a wider groove into the bullet. This condition is undesirable because the bullet could then yaw while still inside the barrel. It would lack the full support of the barrel, especially on the driving side of the land. The yaw would be demonstrated by poor grouping on a target once it was released from the barrel.
Conversely, an increase in rifling pitch would tighten the angle. While this is no better than a constant twist on the bullet, it is certainly better than a decrease in twist, as we first discussed. And as we mentioned, the rate of twist must be matched to the bullet. This subject is covered in some of our other articles.
The above drawing is a somewhat crude attempt to demonstrate the decrease in rifling pitch mentioned in the above paragraph. Imagine the above drawing to be the bearing surface of a bullet, with the heel of the bullet on the right. The parallel black lines represent the pitch angle and engraving on a bullet for a .308 caliber 10" RH twist. The helix angle for this combination is about 5.6 degrees. The red lines represent the angle for 16" twist, a helix angle of about 3.5 degrees. If the pitch angle decreases, as it would if the twist rate slowed down, the width of the engraved grooves on the bullet would have to become wider. In a right hand twist barrel, the right hand side of the lands (looking from the chamber end) is the driving side. If the angle decreases, as it would if the twist decreases, the groove width on the bullet widens and the left side of the lands try to become the driving side. If the twist rate increases or remains constant, the right hand side of the rifling is always the driving side.
In this drawing the angles are exaggerated to help show our point. And it also shows just one single groove being engraved into the bullet rather than the normal 6 grooves. The helix angle for any caliber and twist rate can be easily computed using this formula: Angle = inverse tangent (Pi * bullet dia. / twist rate). The bullet diameter and twist rate are both expressed in inches. Inverse tangent is also shown as Tan^-l on some calculators or ATN. Pi is the constant; 3.1416.
A rough or burred crown will cause big problems, too. It could cause the bullet to yaw upon exiting the barrel, as well as strip jacket material from the bullet.
We've highlighted the obvious features an accurate barrel should have above. But there are some others that might not seem as clear-cut or be a surprise by their lack of importance - from my point-of-view.
Some may wonder why I left bore and groove diameters out of the first part of this article. Mainly because I don't feel that there is an exact size that a barrel must be for a nominal caliber. For example, a .30 caliber barrel does not have to have a groove diameter of exactly .30800" to be super accurate. While we do hold close tolerances on groove diameters (within about .0003") that actual diameter is not as important as the uniformity end-to-end. Actual bore diameter is less important than groove diameter too, but uniformity is still a necessity.
Jacketed lead-core bullets and waxed lead bullets in a .22 rimfire are actually quite soft. Under the pressures of firing and initial engagement into the throat, the bullet is either squeezed down in size to conform to the barrel diameters or is obturated up in size to fill the barrel. A variation of several ten thousands in size between the bullet and barrel in either direction does not seem to have any meaningful effect on accuracy. But, as we mentioned, once the bullet conforms to the barrel size, it is very important that the barrel does not change size. This is especially true of an increase in internal barrel diameters as the bullet travels towards the muzzle. This situation is similar to the decreasing twist rate we already talked about. And a decrease in diameter away from the chamber is akin to a tightening of the twist rate.
Residual stress in a barrel is another possible cause of inaccuracy. Stress can be caused by the rifling process, as it is in button rifling (our method), or can exist in the steel bar as it was received from the steel mill. We stress relieve our barrels with elevated temperatures. As discussed elsewhere (FAQ) we have not found that the deep cyrogenic process is very effective at removing stress in the steels commonly used in rifle barrel manufacture.
Stress in steel will tend to come out when the steel is either heated or machined. The heating of a barrel through firing it is often enough to allow some stress movement to occur. In effect what happens is the barrel warps and is no longer "looking" where we thought it was. The bullets don't hit the target where they were intended. If a bar of steel is machined while it contains stress, it will move. This can be noticed in an increase in bore and groove diameters as the outside diameter is reduced. This can mean an increase of these diameters as a barrel is contoured smaller in diameter towards the muzzle. Or an enlargement in the internal dimensions under the flutes of a fluted barrel.
All of the steel used in the manufacture of our barrels is stress relieved by the steel mill as their last operation and again by us in our shop following the rifling operation. In both cases this stress relieving is done through the application of heat.
Rifle barrels made using the hammer forge process contain a tremendous amount of stress. This explains why some barrels on mass-produced factory rifles will walk their shots as those barrels heat from firing.
In another article on barrel stiffness, we outlined the mechanics and math behind barrel rigidity. The simplified version of that article is that a barrel too long for its diameter is not rigid. It can be whippy and accuracy will not be at its best. Generally speaking, a short and fat barrel is more accurate.
Some cartridges are inherently more accurate than others. Very few benchrest shooters would deny that the 6PPC or variations of the same cartridge in .224 caliber are the most accurate rounds ever developed for shooting out to 200 and 300 yards. The flip side is that some cartridges are not as accurate.
It seems as though some experienced shooters and gunsmiths tend to place too much emphasis on one single characteristic of barrel as it relates to accuracy. For example, some gunsmiths look at not much more than how straight a barrel is in evaluating its potential before chambering it. Others look at internal finish or bore diameter. We had experience with one customer that paid a business that checked twist rate deviation to examine all of his barrels.
In our opinion this is putting the blinders on, a case of being myopic. All of these properties are important and a serious problem with any single one of them could cause accuracy trouble. But the point is they re all important To sum up the critical factors we've mentioned, they include: a straight hole of uniform diameter and correct size for the intended caliber; a smooth and uniform surface finish that lays parallel to the rifling; a uniform rifling pitch; stress-free steel; adequate stiffness for the type of shooting it will be used for; and a first-rate installation job with special attention paid to the throat. An accurate barrel is the result of a happy marriage of all of these.
2. Shot Out Barrel?
Just what effects does continuous shooting have on a barrels accuracy?
A new study reveals some startling information.
In truth, every shot fired takes a toll on every barrel, regardless of case or caliber. When you pull that trigger, you're going to remove surface metal from the bore. That wear is most notable in the area forward of where the case mouth sits in the barrel, immediately surrounding the bullet. This area is called the throat, and gets the hottest flame and heaviest pressure on firing. The barrel forward of that area through the initial hard and hot start has a relatively smooth ride down bore and out the muzzle.
It's the throat that takes the beating, and once it has eroded far enough, bullets will tip as they exit the case neck, gasses pushing along the bullet's base move erratically favoring the worn out area as an easy way out. All loads do this. At some point, if you do a lot of shooting, your throat will wear. What matters then is, how many rounds will it take before your throat has worn enough to affect accuracy? With the introduction of the .223 WSSM, Winchester chose to cut off the naysayers and actually test barrel wear. They shot a standard steel barrel in .223 WSSM, a chrome-lined barrel in the same caliber, and a conventional steel barrel in .22-250. Factory loads were fired in all three and throughout the test, throat erosion was carefully measured and accuracy at 100 yards monitored. The result are real eye opener, not just for evaluation the .223 WSSM, but in illustrating what actually goes on inside every bore.
For shots 1 through about 500, erosion at the throat in both standard barrels was ongoing, but stayed under .003. Accuracy stayed at 1 inch or less. After that point, erosion increased rapidly. Accuracy between 750 and 1,000 round fell coinciding with erosion measurements between .003 and .0035. For chrome-lined bores, this level of erosion didn't occur until over 1,000 round had been fired (nearly twice as much).
In that short span, groups went from sub-1 inch to between 4 and 6 inches. For the chrome-lined bores, accuracy held to MOA or less to over 1,500 rounds before it too fell off to over 6 inches, but interestingly, it only took wear of .002 to begin the loss of accuracy in the chrome-lined barrel. But - it took a lot more shots to realize that erosion in the chrome. Both the unchromed .223 WSSM and .22-250 behaved exactly the same. I'm convinced, as would a Swift, TTH or the likes of a .223 Ackely Improved.
What is happening is clear. As the throat area heats, the metal in the area is actually hardening or carbonizing. To some degree, this is an asset, as can be seen by the actual improvement in accuracy in all tests as the bore approached 750 rounds. Then, the metal hardens to a point off surface metal easily. From that point, erosion is rapid and accuracy falls off fast. The chrome-lined bores were able to prevent this until about 1,500 rounds, when they too gave out.
The magic number seems to be that once you have about .003 of throat wear on a standard barrel or .002 on chrome-lined, the barrels shot. That has been my experience over decades of testing with a multitude of varmint calibers.
This erosion can't be avoided, but it can be delayed. Rapid shooting, as found in rifles used to waylay endless target in a prairie dog town, get throat hot fast. Hot metal expands and is more vulnerable to wear. Let them cool between shots and you will greatly increase the life span of any barrel.
Shoot 40 or 50 rounds of any high-pressure cartridge through any rifle as fast as you can load and pull the trigger, and you will quickly kiss the barrel goodbye.
The Fixes
A new barrel is the best choice, but some may opt to have the barrel removed, turned back one turn, and rechambered. This cuts away the worn out section and moves the throat further up the barrel into new, unworn territory. It works, but your gunsmith may not be very happy about it. The problem is the work hardened or carbonized area is the shot out throat can be very tough to cut through, often ruining an expensive reamer. Given this data replacing it with a chrome-lined barrel seems like a good idea for a rifle that will see extensive shooting.
No, the .223 WSSM isn't any harder on barrels than any cartridge, nor is a Swift or lowly .222. They all wear out. How fast is up to you. Throat wear is a fact of shooting life, but it's an easy fix. It is clear the .223 WSSM is no tougher on barrels than a standard .22-250. With a little care, most shooters will never reach this point. For the average hunter, even 500 rounds is a lot of shooting, but the varmint hunter or those who spend a lot of time and rounds in load work up may experience a shot out barrel. If it happens, change the barrel. It's no big deal and as predictable as changing brakes or tires on your vehicle.