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Author Topic: Rifle Barrel Making (Part 2)  (Read 68 times)

Offline Dutch-Hunter

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Rifle Barrel Making (Part 2)
« on: November 21, 2017, 01:34:51 PM »
Rifle Barrel Making (Pt 2)

LAPPING

Lead lapping the barrel is done to polish the bore and remove machining marks and also to remove any tight spots in the barrel and make it dimensionally uniform end to end. If you were to sit down and write a top ten list of barrel makers, past or present, almost all would lap their barrels.

This process is usually done by hand, though the process is mechanized in larger shops. It also acts an inspection process for the barrel maker who can “feel” what is going on up the barrel.

First, the lapping rod (an old cleaning rod) is passed up the barrel to within about four inches of the end and then, with the barrel held vertical, molten lead is poured into the barrel. The lead freezes onto the end of the lapping rod forming a cast which precisely matches the inside form and dimensions of the barrel. The lead lap is then pushed out and smeared with lapping paste like that used for valve grinding. The lead lap is then pulled and pushed up and down the length of the barrel for several hundred strokes occasionally adding more paste or oil. Because the big particles of grit are embedded more deeply in the lead than the small particles there is an even bearing pressure from all the particles of grit onto the steel of the barrel. The net effect is to polish the barrel rather than scratch it which would leave a mat finish.
Lapping the barrel adds between one and three tenths of a thousandths to the bore and groove diameters of the barrel and is used by most small custom barrel makers as the finishing process on the inside of the barrel. Lapping a barrel will improve the performance of almost any barrel.

Generally, lapped barrels will shoot well from the word go where as the same barrel not lapped may take a thousand rounds or so until it starts performing at its best.

WHAT MAKES A BARREL ACCURATE?

Some people want their barrel dimensions accurate to the nearest tenth of a thousandth. But in truth, the golden rule seems to be that the groove diameter must be same or less than the bullet diameter to get good accuracy. It does not seem to matter how much less you make it, one tenth of a thousandths or one thousandth so long as it is less.

As a general rule, barrels with shallow grooves are better than those with deep grooves because the bullet will be distorted less. Barrels with shallow grooves reach their best quicker than deep grooved barrels,- but they certainly do not last as long.

Some people say that the finish in the barrel is important and the better the finish, the better the barrel. What seems to be true is that if the finish is too good, less than 10 microinch roughness, then metal fouling tends to become a problem. It seems that in a "rough" barrel, the bullet rides on the high spots and so there is relatively little friction. But the smoother the barrel gets then the greater the surface contact with the bullet. The friction goes up and more bullet jacket gets left behind. Of course, if the barrel is too rough, then this just picks up metal and acts like sandpaper on the bullet. But there is a band of surface roughness where metal fouling is minimized which is between 10 and 20 microinches.

There have been many claims over the years that different forms of rifling profile will give better results. But so far, there is no conclusive proof that the so called concentric form almost universally used these days is any worse than any other - or any better!

What is important is that the bore and groove dimensions are uniform down the length of the barrel, that the twist rate is uniform and that the groove circle is concentric with the bore. Many think a slight choke at the muzzle end of a ten thousandth or so will improve accuracy. But bench rest shooters have shown that parallel barrels seem to win more matches than choked ones. The exception is barrels which shoot lead bullets, like .22 Rimfire barrels and air rifle barrels. These barrels definitely shoot well if there is a slight choking in the barrel. The barrel should also be completely free of stress so that as it warms up during a course of fire the barrel does not bend so leading to group shifting.

WHICH METHOD MAKES THE BEST RIFLE BARRELS?

Like all old technologies, cut rifling is slow and requires a high level of skill to make and maintain the tooling. This makes it expensive. The other side of the coin is that the tooling is readily made with simple machine tools and is very flexible - by which I mean that a large range of twists and bore dimensions and numbers of grooves are readily achieved using the same cutter and cutter box. This makes cut rifling an efficient way to go if you are making custom barrels in one's and two's to a customer’s specific requirements and you can charge a hefty sum for your efforts. But cut rifling machines are expensive to buy and expensive to operate.

The button rifling machine is relatively simple and cheap to build, (compared to a cut rifling machine), and the buttons are also available and very cheap - in terms of tooling cost per barrel. Also, no real skill is required to pull a button down a barrel.

If it is so easy to rifle barrels using a button, why do some barrel makers persist in the difficult, time consuming art of cut rifling? As outlined above it is critical in an accurate barrel that bore and groove dimensions are uniform end to end. With buttoning a barrel it is critical to have the steel be very homogeneous and of uniform hardness along the bar. If not then the button will engrave deeper into the soft parts than the hard parts so giving varying dimensions down the barrel. The button rifled barrel maker is very much in the hands of his steel mill to supply him with uniform, homogeneous steel.

The depth of engraving of the button depends also on the thickness of the bar at the point where the button is. If the bar is thin then the metal can then just expand elastically allowing the button the pass without doing much engraving. If the bar is thick there is more resistance to the button and so it will engrave deeper. To hope to get uniform bore dimensions it is critical to pull the button through a blank which has the form of a parallel cylinder. A button rifled barrel must be rifled and then stress relieved before it is profiled. There is always the problem that any residual stresses are going to make the barrel move when profiling thus leaving you with a barrel that is bent or bell-mouthed.

The tendency of the bore dimensions of non-stress relieved buttoned barrels to expand on profiling the outside had been used by some manufactures to advantage. Barrels having bore dimensions that vary over the length can be made to shoot well if the muzzle end is the tightest part of the barrel. The bullet will then make a clean exit without gas leaking around the sides to destabilize it. When mass producing barrels it is then possible to allow a fair degree of variation in the bore and groove dimensions down the barrel, provided some choking at the muzzle is included in the manufacturing process. If the buttoned barrel blank is profiled so that the last inch or so at the muzzle end is left at a larger diameter than the rest of the barrel, the expansion of the bore dimensions will be least at the muzzle so leaving a slight choke. That is why the barrels on almost all .22 target rifles look the way they do.

Cut rifling a barrel puts no stress in the steel and so it is possible to profile the barrel after the drilling stage. Any moving around the barrel is going to do will be done and the barrel can be reamed and rifled after profiling. The problem of stress induced changes in bore dimensions during manufacture can be eliminated.

When making fluted barrels, the fear is always there that putting six flutes on the outside of the barrel will leave the bore shaped like a hexagon instead of round. With cut rifling, the barrel can be completely profiled and fluted after the drilling stage and then reamed and rifled. All the niggling doubts as to whether fluting the barrel will ruin it can be dispelled if it is done this way.

The exact twist of a buttoned barrel is also unpredictable. The button tends to slip in the barrel so what set out to be a 12 inch twist may end up as a 12.5 inch twist. This is not a problem if the twist is uniform, but if it varies down the barrel.  If the twist rate slows then more than likely it will not shoot zccuzrately at all.

Another problem, is that the button may not engrave as deep on one side as on the other so leaving a groove circle that is not concentric with the bore. The result is bullets leaving the barrel which are not balanced and so unstable. This becomes less of a problem if you have lots of shallow grooves instead of a few deep ones.

None of these problems arise in traditional single point cut rifling a barrel. Groove circle and bore are always very concentric. Because very little work is being done on each passing cut the twist rate is very consistent and very uniform. As a general rule button rifled barrels are not as uniform in bore dimensions straight off the machine as a cut rifled barrel. Button rifled barrels usually need a good deal more lapping than a cut rifled barrel due to this problem.

Exact dimensions are easier to achieve by cut rifling and are not dependent on the hardness or thickness or type of steel as they are in buttoning. No stress is put into the barrel by cut rifling so no stress relieving is needed.

Many people think that the superior finish in a button rifled barrel must mean it will shoot better. In the first place, the surface finish that counts is the longitudinal surface finish, down the length of the grooves. The tool marks in cut rifled barrels go in just this direction, so causing mini-lands which are quite uniform down the length of the barrel. The transverse "roughness" caused by the lands are never viewed as an impediment to accuracy, so the mini-lands left by the tool marks in cut rifled barrels will also not affect accuracy. In the second place, the final finish in the barrel is determined by the lapping. If the cut rifling barrel maker has paid attention and kept his cutter sharp to minimize tool marks, then after lapping only a very experienced eye will be able to tell the difference between the cut rifled and the button rifled barrel.

In my opinion you are more likely to get a top of the line tack driver by cut rifling a barrel than by any other method.
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Online Madgomer

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Re: Rifle Barrel Making (Part 2)
« Reply #1 on: November 21, 2017, 06:02:18 PM »
Dutch, in the button rifling process, is the "button" simultaneously cutting the grooves as well as the top surface of the lands, or are the lands defined by the boring process prior to the button rifling of the grooves?
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Offline Dutch-Hunter

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Re: Rifle Barrel Making (Part 2)
« Reply #2 on: November 21, 2017, 07:38:52 PM »
Mad, the ID of the bore (lands) is set by the ream. The cutting of the grooves is done by both the button and cutter process.
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