John Krieger, February 2009

A person once said to me, “You think single-point cut-rifling is the best because that’s the method you employ to rifle.” He had the right words but in the wrong order. I employ single-point cut-rifling because I think it is the best method to rifle. Perhaps, to explain why I think it is the best method, a brief explanation of the other methods of rifling a barrel is in order. The other methods we will discuss are hammer forging, button rifling, and broach cut.

In Hammer Forging, the blank, with an oversize hole in it, is placed in the machine over an undersized carbide mandrel that is the negative of the rifling. The barrel blank is shorter than the finished barrel length. It will “grow” in length as the blank is worked down over the carbide mandrel. Two sets of opposing “hammers” then cold work the steel onto the carbide mandrel, thus forming the rifling twist form, size, and barrel length. At the same time, the outside contour can be formed and even the chamber can be put in; although that requires an even bigger starting hole leading to even more cold working of, and induced stress into, the barrel.

Once set up, hammer forging produces very high production rates. The downsides are its lack of flexibility for smaller quantities, tremendous stress induced into the barrel’s now uniform twist, and spalling of the metal in the bore.

Spalling is microscopic flakes of metal breaking off the bore surface and leaving a void or depression. Oddly, in the bore of the barrel itself, this may not affect much, but I feel it is the “kiss of death” at the crown as far as accuracy is concerned. It does not necessarily happen right away or it may never happen. But when it does, it is sudden and severe. It is characterized by the rifle shooting to its normal level of accuracy; and then in one shot, it will never shoot again. It can be recrowned, of course, and accuracy may return; but the problem can return as well. It is for this reason I feel hammer forging produces a barrel that is unsuitable for serious work; i.e. a police sniper facing a hostage situation. The spalling is caused by the excessive cold working of the steel and is exacerbated by a poor finish in the bore before hammering.

In Button Rifling, the blank is of the proper length to begin with. It has a hole drilled and reamed in it. The blank is a uniform diameter on the outside from end to end. For example, the breech is 1.250 all the way to the muzzle of 1.250. As in hammer forging, the button is again a carbide negative of the rifling. Only, unlike hammer forging, it is bigger than the reamed hole. The button and the barrel bore are lubricated with an extreme pressure lubricant, and the button is pushed or pulled through the bore.

The passage of the button through the bore with the negative of the rifling, cold forms the rifling into the barrel. The outside of the barrel has to be a uniform diameter to resist the extreme pressure of the button uniformly, or else the rifling will not fill out properly. This extreme pressure involved induces stress into the barrel that now must be stress relieved in a furnace at 1000 – 1100 degrees F. After the stress relief process, the barrel can be turned to its final contour. It is now and only now that the actual bore and groove dimensions can be determined. Even though the barrels were stress relieved, all of the stress can’t be taken out without the risk of drawing too much hardness out of the steel. This results in an opening up of the bore as the barrel is turned. The more steel removed from the outside, the more the bore opens up. Thus, a sporter barrel, for example, will have the bore at the muzzle slightly larger than at the breech. Premium barrel makers will lap the barrel back to a uniform condition.

The many different factors that can be involved in what will be the button-rifled barrel’s final size and quality are as follows: the particular run of steel involved, its hardness, its outside diameter, the size and finish of the reamed hole, the size of the button, how fast the button goes through the barrel, how much the barrel springs back after the button has passed through, how much the bore closes up with stress relieving, and how much it opens up and/or bows in final turning. As bad as this sounds, the button-rifle barrel maker’s considerable skill and expertise allows him to play most of these factors to achieve the desired result. Nonetheless, he still won’t know exactly where he stands until the barrel is completely finished. This is why some barrel makers offer different grades of finished button-rifle barrels.

One other factor pertaining to button-rifled barrels worth mentioning is twist uniformity. All methods of rifling a barrel uniformly depend to some degree on the homogeneity and uniformity of the barrel steel but possibly none more so than button rifling. The button has the desired twist ground on it. Some makers depend on this alone to rotate the twist into the barrel. Others use this plus, they “leader” the button with a machine device to help control the twist. The problem occurs if the button encounters different grain structure or harder or softer areas. Then the button will tend to slow down the twist or increase it leading to a lack of uniformity. It is felt that this, more than any other factor is why, on the average, more button-rifled barrels do not fall into the middle of the bell-shaped curve where the very accurate barrels are found.

None of this is to say that button-rifled barrels are not good barrels. They are. To say otherwise is ridiculous. They have been winning and setting records from the early 1950’s to the present day. Krieger Barrels actually has a button rifled OEM barrel line called Criterion Barrels where we produce production barrels for various manufacturers. I have a few Criterion barrels on my own rifles, and they shoot extremely well.

The third method, Broach Cutting, is used mostly for pistol barrels, but a number of manufacturers produce rifle barrels with this method as did the arsenals during WWII.

Unlike the previously described methods, broach cutting cuts metal away to produce rifling rather than cold forming it by moving metal. Thus, it induces no unwanted stress into the barrel.

The broach itself is a long cutting tool that has a series of cutter bands that have the twist, the number of lands and grooves, and the rifling form ground on them. All in all, it employs hundreds of cutters on the broach tool to rifle the barrel. Each band of cutters is approximately .0002 of an inch larger than the previous band. Thus, as the broach is pulled into the barrel, each band progressively cuts the grooves deeper until final size is reached. Depending on the length of the machine used, several roughers and a finisher broach may be employed. Broach-cut barrels make good production barrels. The down side is the high up-front cost of the broach tool and its caliber, and that the number of bands and grooves and twist can’t be changed without going to a different tool. Also, due to the tremendous force needed to pull the broach through, there is a tendency of the broach tool to stretch as each additional band of cutters enter the barrel. This makes the actual twist slow down as the broach is entering the barrel and speed up again as it exits the barrel. This also makes the groove width vary. All in all, if made properly, they shoot well and have no induced stress to cause trouble.

Single-Point Cut Rifling employs one single cutter to rifle all the grooves. The cutter is ground to the width of the groove and the top is ground with the radius of the bullet. The cutter is pulled through a groove removing approximately .00009 of an inch from the groove per pass. The cutter riding in the rifling head returns to the front of the machine and exits the barrel. The barrel then indexes to the next groove and is pulled through the barrel again removing .00009 of an inch from the groove. The process continues until the barrel has indexed for the proper number of grooves; i.e., 4, 6, 8 etc. For example, after the fourth cut on a four-groove barrel when the rifling head returns to the front of the machine, it enters a device called the feeder. The feeder turns a screw in the rifling head a slight amount which in turn pushes a wedge under the cutter a slight amount which raises the cutter the .00009 of an inch to make its next set of four passes through our four- groove barrel. This continues on for hundreds of passes until the proper groove depth is reached.

Obviously, this method induces no stress into the barrel. But there are numerous other benefits to single-point rifling as well. Unlike a button-rifled barrel that has to have a uniform outside diameter before buttoning, the cut-rifled barrel can have the severe outside turning even fluting done before any of the critical inside work; i.e. the reaming, rifling, lapping, etc. is performed. The irony in this is a button-rifled barrel should be pre-turned but can’t be; a cut-rifled barrel can be but (because of no stress) doesn’t have to be. Still, because of the personalities of steel, it is a safer route to do so. Another advantage of single-point cut-rifling is its versatility. One cutter can often cut a number of different twists, although the cutter life is shortened by the different grinds involved. Groove diameters can be easily varied and only require a regrinding of the top radius to the new groove diameter. Bores, however, can only be appreciably changed by retooling as a new reamer would be required (and sometimes a drill) and also a rifling head to fit the new reamed hole size. The rifling head and reamer are married to each other by their size. However, no other rifling method can vary the bore diameter without a tooling change either. Twists in single-point cut-rifled barrels are very uniform as the rifling head is leadered by a precision bar with the twist on it. Also, single-point cut- rifling is the least sensitive to variations in the steel.

Speaking of Steel, all barrel makers are at the mercy of the steel. Good steel – homogenous, uniform & stress free – gives good barrels. Bad steel – all other things being equal – gives bad barrels. And, unfortunately, there is usually no way to tell completely ahead of time. This is why we cryogenically treat all our barrel steel before we begin to make a barrel from it. This is the time to do it as it makes the steel better, and we can make a better barrel with better steel. No step in making an accurate barrel is unimportant so cryogenic treatment must be done first. Cryogenic is not a cure-all. It just makes the steel a few percentage points better on the average to bring more barrels into the center of the bell-shaped curve. Cryogenically treating the steel does two things for us. First, it makes the grain structure of the steel more homogenous and uniform. Second, it gets rid of the last bit of stress. It does this by the following. The metallurgist tells us that at the steel mill when our steel is quenched and tempered there are two types of grain structures present – austenite and martensite. They both have stress. When the steel goes into the oven at 1150 degrees F for stress relieving, the martensite is stress relieved but the austenite does not respond to the heat. But by treating the austenite to cycles of extreme cold (-300 degrees F), the austenite is converted into a martensite and can be and has to be stress relieved afterward by heat. This is because the new martensite is brittle just as it was when originally quenched.

Interestingly, over time austenite will convert into martensite on its own, but it is brittle and should be tempered. Perhaps this is why old steel is sometimes brittle. So what are the advantages of single-point cut-rifling that make me like it so much? It is the most stress-free process. The barrels can be contoured before reaming and rifling. Twists are the most uniform. Bore and groove diameters are the most uniform. The dimensions are also very predictable. No surprises at the end. And cutting tool angles can be matched to the steel to produce the best finishes.

In everything there is a trade off. Get something here, give up something there. So what is the trade-off with single-point cut-rifling? Simply the two hours or more it takes to rifle the barrel along with the very high tool maintenance. So why do we do it? Because I believe overall it is the preferred method to produce the best barrels possible.