From the Border Barrels ashes, GB Barrels Phoenix rises...?

[meles meles]

Terms such as 416 and 420 are really far too generic to be used as adequate descriptors and are effectively only an indicator of the basic steel type. Each manufacturer has their own variation upon what such a steel is made from, and how it is processed. A bar of '420' bought from one source will almost certainly be different from a bar of '420' bought elsewhere. It's the subtle compositional differences, and the associated thermomechanical processing, that makes one better than the other. We don't doubt that some barrel makers prefer a 416 with added sulphur for their products: it makes the steel easier to machine. However, that doesn't mean its a better barrel steel, only that their machining practices are perhaps not as good as those of makers who specify reduced sulphur steels for barrels with better long term performance. It's a case of paying the money and making a choice, which brings us back to centering the thread on specific products. If we were to be in the market for a premium priced barrel, we'd like to think that it was manufactured from an appropriate material, by a well controlled and understood process, by someone who understood what they were doing. Under Dr Kolbe, Border Barrels used to produce excellent barrels, we hope that the new regime still does.

[Dennisgrm]

Barrel Manufacture

I’m not a metallurgist and as such we employ the experience and skill of our steel specialists. Every batch of steel undergoes a hardening and tempering process and is shot blasted, reeled and stress relieved. It also has a chemical composition analysis and mechanical property tests carried out.

One of the primary factors in producing an accurate rifle barrel is finishing with a ‘stress free’ product. So let’s think about the term ‘Stress’. The structure of steel is not a true bonded Chrystal structure. By this I mean the steel exhibits no shared electron pairs like true crystals. Instead steel is like a mass of bonded crystals and can therefore move without breaking the crystal lattice. Defects in the steel can take on many forms however we can think of these defects as tensions and compression's in the lattice.

The steel we use in our barrel manufacture is martensitic. To achieve this state the steel has to be austenitized, quenched and tempered. Quenching the steel makes it hard but too brittle so the tempering stage has to be carried out to make the steel have a useable tensile and impact strength. During the 300-600 deg region there is significant relocation of dislocations (the defects as tensions and compressions.) within the lattice relieving ‘stresses’ from the steel. I should point out that it is usual to expect some minor residual stresses to be present even after stress relieving.

“The 416 type stainless steel used in barrels is one of a group of martensitic steels which can be hardened by heat treating like regular Carbon steels. 416 stainless is more accurately described as a "free machining, rust resistant" steel having a high Chrome content, around 10%, but with sulphur added to give it good machining qualities. It is widely considered that stainless barrels will have a longer life and are more accurate than Chrome Moly barrels. If stainless barrels are "shot in" using the prescribed procedure, the barrel aquires a burnishing which almost eliminates fouling, so making stainless barrels very easy to clean.” Dr Geoffrey Kolbe

Although cut rifling is a relatively slow and high skill based method of rifling it is still regarded as the method which produces the most consistently accurate barrels.

The first observation we can make is that all the material removed in cut rifling a barrel is ‘cut’. At no stage is a disruptive method used unlike other manufacturing methods, for example button rifling or hammer forging. As this is the case correctly tempered steel should not have stresses induced during the manufacturing process.

After the barrel is deep bore drilled we then profiled (contour) the barrel. It is then reamed or honed as this routine eliminates the possibility of any residual stress affecting the bore. The barrel is then ready to be rifled.

The rifling head contains a single point cutting tool, much like that used in broaching. This is an incremental cutter and after it has indexed through all the grooves required for the given barrel it is bumped up in the order of 0.0001” and the cycle repeats until the grooves reach the desired depth. While CIP specification allow for groove dimensions to be over the nominal calibre size it is usual for custom barrel manufacturers to make the grooves nominal or less as this gives greater accuracy results.

“This method produces almost perfect concentricity between bore and groove, a very uniform twist rate, and induces no stress into the steel that later has to be relieved. Along the same lines, we do absolutely no straightening of our barrels as this would only put stress right back into the steel.” Krieger

To further ensure uniformity and finalise the surface finish of the bore the barrels are hand lapped. The lapping process removes the last few tenths of a thou and any machining marks or surface imperfections that may exist.

“Our barrels are lapped after reaming to remove the tool marks, and then hand-lapped again after rifling. It has been said that if a barrel is cut-rifled correctly, it does not have to be finish lapped, and to some extent this is true. It should not have to be lapped to obtain uniformity of dimensions. This should come from the tooling and procedures used. But there is a slight improvement to the finish achieved by finish lapping, and the lay of the finish is now in the direction of the bullet travel so fouling is greatly reduced, and cleaning is made easier. It takes longer to finish lap, but we do it because it makes a better barrel” Krieger

One major benefit to cut rifling is the ability to infinitely change the twist rate of the rifling and to govern exactly the bore and groove dimensions. The cut rifling method also ensures the groove’s are cut concentric to the bore. This is a problem seen most commonly in button rifling where the button does not follow the bore exactly and produces eccentric rifling. The main problem here is the bullet is engraved eccentrically and once unsupported by the barrel in flight it is unbalanced. For this reason among others most button rifle manufacturers grade their barrels.

Button rifling is an economical method of rifling which can produce a good end product when done correctly. However there are many hurdles to overcome.

When a barrel is button rifled it is drilled and reamed or honed in preparation. It is also important that the parent steel is of a uniform diameter, hardness and the bore finish is free from machining marks as these will be pressed into the rifling form. Then a button is passed through a barrel it displaced the steel to form the rifling. No material is removed. This inherently causes problems for the button rifle manufacturer as stresses are formed in the steel.

“conditions include warping of the barrel during other machining operations, an increase in the bore diameter towards the muzzle end of the barrel during the contouring phase, and in the extreme, lengthwise splitting of the barrel. Also, if there are stresses remaining in the barrel they can be slowly released as a barrel warms up during firing. This causes the barrel to actually move during the course of shooting, causing inaccuracy." Dan Lilja

The buttons used in most operations creates a burr at the edges of the lands, this is essentially the excess steel trying to escape the pressure exerted as the button passes. To overcome this many buttons have a follower to press these burrs back into the rifling thus reforming a uniform bore hole. Of course this is all done ‘cold’ so the metal has the potential to fold. Unfortunately this does occur and small flakes or chips can separate through the life of the barrel.

It’s not all doom and gloom for button barrels though. Firstly from a manufacturing point of view it is far more productive, with the average button barrel taking about one minute to rifle compared to a cut barrel at over an hour. Also the required skill is often said to be much less, I personally think it is just a different skill set required as many processes take year of experience to learn fully. The majority of the negative effects on the steel can be rectified by a final stress relieving process. This has to be carried out before the profiling of the barrel as the dimensions of the bore are changed as the crystal structure of the steel is altered. If this is done in an oxygen free environment say an argon oven then little to no surface oxidisation or tarnishing should occur making the lapping of the barrel much easier and a good final conditioning to the bore.

There is very little to no difference to the actual ‘overall’ life expectancy of either barrel manufacturing process. However defects could be the most prominent cause of early problems in button barrels. Numerous studies have shown that a barrels life expectancy is directly related to erosion caused from the ignition and high burn temperature of the powder, along with the shot blasting effect from the projected powder.

“It was found that over the course of tens of thousands of rounds the actual groove diameter of the barrel was only increased by a few ten thousandths of an inch. It is this type of wear that moly might prevent or slow down. But in this test the throat area grew progressively longer and larger in diameter from gas erosion, not friction between the bullet and barrel."

I hope my take on this helps you see the benefits and hindrances to these rifling processes and makes it easy for you to appreciate the work that goes into both methods.

This is not intended to create friction or arguments just to give an understanding of the method for both practices. Again I hope it helps give an overview.

[TattooedGun]

Thanks for your very informative reply Dennis.

A great asset to have on the boards and a wealth of expertise and knowledge.

[mag41uk]

Fantastic! Thank you.

[DW58]

Most informative Dennis - many thanks.

[R.G.C]

Dennis,

Thank you fpr this extremely well documented (and passionated) clarification.

I was preparing a reply to place ‘Le Tachon’ in front of he facts, but you have done it much better thann what I could have done.

Steels are processed and released under strict specifications and prpcedure from reliable sources and with complete tracability. Challenging that is nonsense…I presume you use the same sources as Dr Kolbe did?.

Sorry to be pedant, but Cr cpntenr of the 416 is 12%, not 10%.

I simply disagree slightly with what you say about carbpn steels: In my modest opinion; most of barrel wear is caused by mechanical action, not the thermal one and then, a few more points in Brinell hardness certainly does not harm…

R.G.C

[Dennisgrm]

That is true. Our potion runs at 12%. That was a quote from an old article by Geoff.

[meles meles]

Dennis, many thanks for the overview of your process. A similar assessment can be found here by Dr Kolbe:

http://www.firearmsid.com/feature%20art ... acture.htm

We agree on many points, and certainly that any of the three processes, correctly performed on excellent steels by good equipment and operatives will produce an acceptable barrel. However, a forging process, despite being more difficult than a cutting process, ultimately has the capacity to produce a better barrel. It takes more skill to achieve with button rifling, and more costly equipment to achieve by hammer forging, and so most of the high quality barrels used by civilian shooters in high end competition these days are cut rifled. They can offer excellent value for money. However, if performance matters above all else, a forged barrel can be better, and need not cost more, though many do as the market ultimately determines the price that can be borne.

The most accurate barrels we have worked with were made from ESR steels and cold hammer forged. They are capable of a first round hit at ranges out to 4000 metres, and subsequent shots from a warmed barrel have less than 0.25mills total dispersion over the next 5 shots. They are, unfortunately, rather expensive. The (large) compressive stress this forging process generated in the bore was beneficial and very significantly enhanced the life. Contrary to what many believe, residual stress is not always a bad thing. When a round is fired, the barrel is pressurised and seeks to expand, potentially placing the steel under tension and increasing its chance of failure. Expansion also changes the bore dimensions, an unwelcome phenomenon, and one which can lead to many problems such as uneven wear, gas cutting and nutation of the bullet. Furthermore, if the steel seeks to expand, it is easier for any imperfections at the surface of the bore to scab off. If, however, there is a residual compressive stress in the steel then that stress must be overcome before the steel can begin to experience tensile forces and expand. Button rifling achieves many of the benefits of cold hammer forging, albeit to a lesser degree, but at lower cost. Properly done, button rifling can provide the benefits of a cold forged barrel at or below the price of a cut rifled barrel. Quality control though matters: it is usually easier to make a bad button rifled barrel than a bad cut barrel.

[R.G.C]

Dennis, many thanks for the overview of your process. A similar assessment can be found here by Dr Kolbe:

http://www.firearmsid.com/feature%20art ... acture.htm

We agree on many points, and certainly that any of the three processes, correctly performed on excellent steels by good equipment and operatives will produce an acceptable barrel. However, a forging process, despite being more difficult than a cutting process, ultimately has the capacity to produce a better barrel. It takes more skill to achieve with button rifling, and more costly equipment to achieve by hammer forging, and so most of the high quality barrels used by civilian shooters in high end competition these days are cut rifled. They can offer excellent value for money. However, if performance matters above all else, a forged barrel can be better, and need not cost more, though many do as the market ultimately determines the price that can be borne.

The most accurate barrels we have worked with were made from ESR steels and cold hammer forged. They are capable of a first round hit at ranges out to 4000 metres, and subsequent shots from a warmed barrel have less than 0.25mills total dispersion over the next 5 shots. They are, unfortunately, rather expensive. The (large) compressive stress this forging process generated in the bore was beneficial and very significantly enhanced the life. Contrary to what many believe, residual stress is not always a bad thing. When a round is fired, the barrel is pressurised and seeks to expand, potentially placing the steel under tension and increasing its chance of failure. Expansion also changes the bore dimensions, an unwelcome phenomenon, and one which can lead to many problems such as uneven wear, gas cutting and nutation of the bullet. Furthermore, if the steel seeks to expand, it is easier for any imperfections at the surface of the bore to scab off. If, however, there is a residual compressive stress in the steel then that stress must be overcome before the steel can begin to experience tensile forces and expand. Button rifling achieves many of the benefits of cold hammer forging, albeit to a lesser degree, but at lower cost. Properly done, button rifling can provide the benefits of a cold forged barrel at or below the price of a cut rifled barrel. Quality control though matters: it is usually easier to make a bad button rifled barrel than a bad cut barrel.

I have had direct knowledge pf 100 barrels in Palma config. made by Suhl Kombinat just after the re-unigication when they were looking for markets. At the time they only used CrMo.

The chambers were semi-finished and it was necessary to adapt reamers pilots to tje bore variations from a barrel to an other. Those variation are understandable for spmeone who know a little about metalworking. The Brinnel hardness was approaching the 280 due to work hardening, as compared to the 190 of tje 416.

Extremely hard to machine and gunolumbers already reluctant to thread and chamber a LW (220HB) would find a challenge here....

They were very cheap and no one of them revealed to be a real shooter.

The saga ended when Merkel was taken over ....

I mention this by personal experience as at the time I barelled a good third of the batch....

R.G.C