All About Barrels
September 23, 2010
Ask 10 different makers how to come up with the most accurate rifle barrel and you'll get 10 different answers. And the more you press them for details, the more you come to realize just how divergent-and even conflicting-those opinions can be.
What makes it all so confusing-yet at the same time comforting-is the fact that there are so many barrel makers out there using different methods, procedures and equipment, yet all are capable of producing superbly accurate barrels. Think about that for a moment.
A barrel can be rifled in a matter of seconds with a single pass of a broach or button; or in two hours by the incremental cutting of one groove at a time using a solitary cutting tool. Or, if you've got the big bucks of a major firearms company, you can shell out a million-plus dollars for a hammer forging machine and literally beat the rifling into the barrel, while at the same time shape it to its finished contour. Some manufacturers even hammer-forge the chamber at the same time by using a mandrel that looks like a cartridge case stuck on the end of a rifled steel rod.
Accuracy isn't partial to any particular steel, either. A terrific barrel can be made of either a 4140-type chrome-moly or 416-type stainless. The former is less expensive and a little easier to machine; the latter will have longer accuracy life and is more corrosion resistant. In either case, however, it is of a special‚'barrel quality'' that has special requirements for straightness and metallurgical specs that are roughly twice as stringent as the industry standard.
Some makers stress-relieve their barrel steel by heating it up, then slowly cooling it; others cool it to near absolute zero, then warm it up (How's that for two divergent schools of thought!) Because there seems to be no "right" way to make a barrel, it's interesting to take a closer look at the ways in which this most critical component to a rifle's performance can be made. Of course we can only scratch the surface in the limited space we have here, but at least we can get a feel for the various procedures as practiced by a few of their better-known proponents.
If there's a thread of consistency evident anywhere, it's that every aspect of the barrel-making process is critical; it seems there are no unimportant steps in the transformation of a bar of steel into a gun barrel. And those steps begin with the selection of the steel itself. The ideal steel type and hardness varies somewhat depending on the rifling process to be used. Beyond the initial choice between stainless or chrome-moly, the metallurgical specs best suited for cut rifling are not the same as they would be for button rifling or hammer forging.
Whatever the steel specs happen to be, the first thing any barrel maker does is check to be sure the steel is exactly what he ordered. Most makers order steel that has already been stress relieved, because the suppliers are better equipped to do that sort of thing and can do it more economically. That goes for either method, heat or cryo, though some are installing their own equipment and doing it themselves. Generally speaking, stress relieving is done twice, first by the steel supplier or barrel maker prior to the deep-hole drilling and reaming of the bar, and a second time after the barrel has been rifled and contoured.
Some makers, like John Krieger, for example, who produces both cut- and button-rifled barrels, have embraced cryogenic stress relieving, but he makes no accuracy claims for it. Others-mostly folks who are not barrel makers but who are in the cryo business, do claim accuracy benefits for freezing the bejesus out of steel. Krieger is convinced that cryo produces a steel that is easier on tools and machines better. Those who use it all agree, of course, and those who don't say it doesn't.
Before cryoing, Krieger told me in a recent conversation, he would often scrap three or four barrels out of 10 because the deep-hole drilling operation would produce blanks having more than .005-inch run-out when turned on centers. It's hard to believe that you can start drilling a hole smack in the middle of a 11⁄4-inch-diameter steel bar and after boring 28-30 inches, actually expect to come out within .005 inch of dead center at the other end!
But good barrel makers do, and now that he's cryoing, John tells me his scrap rate because of excessive run-out is virtually nil. As with all aspects of barrel-making, the methods used to first drill the hole in the bar stock, then ream it for smoothness, vary with the maker. Some drill and/or ream on the horizontal, some on the vertical. Some rotate only the drill and/or reamer, some rotate only the barrel, others still rotate both-obviously in opposite directions.
As much attention as the rifling process gets, the reaming of the bore following the deep-hole drilling is an equally critical operation. After all, once the rifling is done-by whatever method-the bore accounts for roughly 50 percent of the internal surface of the barrel. Therefore, the smoothness and uniformity of the land surface from chamber to muzzle is just as important as the groove surface.
As you can well imagine, stress can be introduced... or relieved, by the deep-hole drilling process, because it essentially removes the central core-or "spine" if you will-of the steel bar, hence some of its resistance to take a set once that metal has been removed.
The initial hole that's drilled is several thousandths under the desired finished diameter. For a .30-caliber barrel, for example, the hole might be as small as .285 inch. The progressive reaming operations that follow enlarge the hole a few thousandths per pass, while at the same time smooth the walls to a higher degree of polish each time. The final ream will bring the bore to a high state of polish and to the desired .300-inch diameter. Some makers, like E.R. Shaw, stop a tad short of .300 inch and pull a sizing button that has no rifling through the bore after the final reaming to further smooth it and bring it to the final diameter. More stress, or the relief of same, is again encountered when the bore is rifled by buttoning or hammer forging. One would think that the latter, especially, would put all kinds of strain on a barrel. I mean, we're taking about taking a nearly 2-inch-diameter bar of steel about 12 inches long with a hole through its center, and by hammering the living hell out of it, elongate it into a 24-inch-long tube having a muzzle diameter less than one-third of what it was originally. I mean, we're talking major stress and realignment of the steel's molecular structure here; that's why the stress-relieving process is so critical.
The fact that hammer forging works is attested to by the fact that most of the major firearms manufacturers use it-Remington, Winchester, Ruger, Sako and Steyr, to name a few. Indeed, Savage may be the only major company that uses button rifling. Obviously, either method is capable of producing an accurate barrel.
The button rifling process also introduces stress, but there's less of it and it's more localized to the area immediately surrounding the bore. Buttoning is the most common method in use today by specialty companies who make barrels exclusively. There's the larger ones like E.R. Shaw, Douglas, Wilson and McGowan who cater primarily to those gunsmiths rebarreling or building hunting rifles. Then there's the smaller, lower-volume companies who cater more to competitive shooters and accuracy nuts-firms like Lilja, Schneider, Shilen, Pac-Nor and Hart, to name but a few. Buttoning is fast; it can produce superbly accurate barrels, and the machinery required is relatively inexpensive. Indeed, it is generally war surplus, albeit highly modified and well maintained.
Conceptually, the button rifling process is quite simple. A highly-polished carbide "button," about half an inch long, tapered at both ends and containing the rifling in reverse, is silver-soldered to the end of a smaller diameter rod and pulled through the bore. The grooves are literally pressed or "ironed" into the steel. Despite the fact that the button is pressing grooves that are only .0025 to .004-inch deep, depending on caliber, it obviously has to introduce some degree of stress to the steel.
The button must be made slightly larger than the desired bore and groove dimensions because there is a certain resilience to steel that makes it spring back when it's bent or pushed aside. It's the same with a brass cartridge case; if it didn't spring back slightly after being expanded against the chamber walls at 60,000 PSI pressure, we'd never be able to extract it. To end up with a .300-inch bore and .308-inch groove, then, the button must be slightly larger by a precise amount-somewhere around .002 inch.
Another tricky aspect of buttoning is that it must be done when the barrel is in the blank state, i.e., a straight cylinder, so that its resistance to the button passing through it is uniform the entire length. However, when the barrel is contoured to its final shape and all that metal surrounding the bore has been machined away, there's that "memory" causing problems again. In other words, when the barrel was rifled it was a 11⁄4-inch-diameter steel bar of uniform diameter. Now we've removed as much as a half or more of the material surrounding the bore. Moreover, we've tapered it so that the barrel's much thicker at the rear than at the muzzle. It's not hard to imagine, then, that when all that surrounding metal is machined away to form, say, a lightweight sporter contour having a .550-inch muzzle diameter, there's a tendency for the bore to "bell" the closer it gets to the muzzle, because the steel's resistance to latent expansion stress from the button having passed through is now diminished.
Obviously, the metallurgical specs of the steel must be highly uniform from batch to batch; otherwise it will have more or less spring-back, which would have to be compensated for with new buttons. Obvious, too, is that stress relieving after the barrel is shaped and chambered is extremely important, as it minimizes these tendencies and "relaxes" the steel.
Which brings us to cut or "hook" rifling. The major advantage of cutting the grooves rather than hammering or ironing them into the bore is that there is no stress introduced. Perhaps the best-known proponent of cut rifling is Tom Houghton of H-S Precision. According to Houghton, they stress relieve only once, and it's done to H-S's specs by the steel supplier prior to delivery. "We've done a lot of experimentation over the years with a second stress relief after contouring and rifling," says Houghton, "but have found no difference what-soever in accuracy or hot-barrel-group stability. And yes, we've tried cryo several times."
Some of the other advantages for cut rifling is that special twist rates and groove depths present no more of a problem than standard specs. One can truly have a "custom" barrel using this method.
Of course, cut rifling is not without its downside; it is very time-consuming and, therefore, more expensive than buttoning. A barrel is buttoned in about 15 seconds, whereas for an H-S Precision 10X barrel it takes more than two hours just to cut the rifling. To my knowledge, H-S Precision is the only barrel maker who guarantees half MOA in writing in all calibers of .30 and under.
John Krieger is another one of the relatively few to do cut rifling, as does Rocky Mountain Arms. Earlier I mentioned that the other type of cut rifling was the broach method. A broach cuts all the grooves at once and does it in a single pass. Broaches, however, are very expensive-too much so for the independent barrel makers. Only governments can afford to use 'em, and indeed that was the way it was done for wartime production. To my knowledge, no custom barrel maker uses broaches for rifle barrels, but a few use them for pistol pipes.
The latest trend in barrels, beside cryogenic treatment either during the manufacturing stages or as an aftermarket process, is fluting. There's really no controversy about fluting; it works. Fluting a barrel definitely lightens it, stiffens it and cools it faster because it exposes so much more surface area. Because a certain wall thickness must be maintained for safety reasons, however, a barrel must be of a heavier contour than normal and the flutes can only be so deep. Depending on the length, width and depth of the flutes, the trade-off in terms of weight come close to being a wash. The net result is a thicker barrel than normal, but one that weighs about the same as a thinner, non-fluted one. How-ever, you still have the advantages that flutes provide. While on the subject of flutes, E.R. Shaw, Inc., was granted a patent recently for helical flutes. Because the flutes spiral around the outside of the barrel, they are longer for any given barrel length, and therefore lighten it to a greater degree and expose more surface area than a straight-fluted barrel. For the time being, Shaw plans to offer their helical-fluted barrels only in a non-tapered configuration, but hope to perfect the process to include tapered sporter barrels as well.
So what can we conclude from this brief overview of the barrel-making process? Not too much, I'm afraid, other than that there are several ways to make a good barrel. Besides, how good does a barrel have to be? For general hunting, 1 MOA is considered superb accuracy, assuming you can shoot that well under field conditions. Most of us cannot, yet the average barrel today is capable of that kind of accuracy, given proper bedding and handloading techniques.
Competitive shooting is something else again. Whether it be benchrest, 1,000-yard, silhouette or position shooting, any accuracy edge, real or imagined, is beyond price. I stress "imagined" because in competition you must believe that you're shooting the best barrel that science and skill are capable of producing. Shelling out $750 or more to have a "name brand" match-quality barrel installed by an accuracy gunsmith is peanuts to pay for the confidence a competitor must have if he expects to win. A competitive shooter will grasp at any straw, including the burning of incense and incantation recitation if he believes it will improve his shooting by one percent. If he believes, for example, that the cryo treatment he had done to his barrel has helped accuracy, it probably will because he'll perform better with that mindset.
But again, one has to find comfort in the fact that, regardless of what production methods are employed, today's barrels are far superior to those of just a generation ago. Whether you're about to touch off the tenth round of a group that might be a new world's record, or a shot on a 16-inch pronghorn standing 350 yards away, that's good to know!