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Teach me about rifling...
Colt Super
Member Posts: 31,007 ✭
I know how rifling works, but there are different techniques, and I'm unclear about each one's effect on accuracy and velocity.
1)Button rifling
2)Cut rifling
3)Gain twist rifling
4)Back boring
5)Free boring
I'm sure that there are some that I've missed, and I'd appreciate anything you can tell me about all of it.
I know that I could hunt this information down on the 'net, but my confidence in receiving good data from the GB Experts is as high as confidence gets.
Thank you.
Doug
* clarity
1)Button rifling
2)Cut rifling
3)Gain twist rifling
4)Back boring
5)Free boring
I'm sure that there are some that I've missed, and I'd appreciate anything you can tell me about all of it.
I know that I could hunt this information down on the 'net, but my confidence in receiving good data from the GB Experts is as high as confidence gets.
Thank you.
Doug
* clarity
Comments
Cut rifling uses a cutter to make several passes through the bore removing the metal in the grooves.
Gain twist is what it implies. the barrel actually increases the rate of twist the further down the bore the bullet travels. The idea is to equalize pressures so the bullet does not have all the forces applied to it at the throat.
Free bore is an area in front of the case mouth with no rifling. It allows the bullet to get moving before encountering the resistance of the rifling. Weatherby rifles were famous for free bore barrels.
Back boring is used in shotgun barrels, it is an area of expanded bore diameter after about a foot of travel, the bore actually gets bigger then again tapers down to the final choke area. My Browning Citori has back bored barrels, it does help with recoil and some claim improved patterns.
Hammer forging- the blank has a mandral inserted into it with the shape and rifling rate desired and is subjected to tons of pressure during the hammering process. The mandral makes the internal dimensions of the barrel since it is basicaly a "negative" of the photograph you are trying to create (most stresses imparted in the steel using this method) Most factory barrels are hammer forged,....cheap and fast.
Cut rifling- the machine is programmed to produce a land height, groove dia, and twist rate. The single point cutting surface makes hundreds of passes removing very very little steel at a time until all the lands and grooves have been formed through the cutting away process. Steel that is subjected to this method is harder (since no metal was displaced, but rather removed) and this method imparts the least amount of stresses into the steel.
Gain twist- simply put, the rifling twist rate increases as it gets closer to the muzzle.
back boring-- is simply removing rifling from a certain distance of the barrel from the muzzle toward the chamber. Sometimes done after magnaporting a barrel or is done on some muzzle loaders to get the saboted slug to start into the barrel easier
free bore- is extra distance in front of the chamber leade that allows the bullet to get a running start at the rifling. This makes for a greater muzzle velocity and is also why weatherby ammunition is quite often a max load. Since it leaves the neck before engraving into the rifling, there is far less chance of a dangerous pressure spike.
While your confidence is deeply appreciated, this answer really could fill a lengthy book. There is some very good information on the internet which is accurate. Also, the barrel makers have good information free of charge.
This is a good, simple primer with photos and illustrations:
http://www.firearmsid.com/A_bulletIDrifling.htm
and:
http://findarticles.com/p/articles/mi_m0BQY/is_12_47/ai_79586229
This is a brief list of articles written by Dan Lilja:
http://www.riflebarrels.com/articles/barrel_making.htm
I think you'll find a great deal of interest in the various shapes of rifling available these days. Again, the barrel makers have this information. The biggest area is polygonal as this covers the multitude of the shapes currently being used for rifling.
Shilen uses 'Ratchet' rifling.
Broughten uses '5C' or Canted rifling.
Obermeyer uses the '5R' which has become the most famous, now copied by several companies including Remington.
Along with these, some older forms are still around; Metford, Ballard and Whitworth being three that I recall instantly.
All of this makes for great research and hours of satisfying, contemplative reading and study.
Best.
I will be furthering my education about rifling through the sources y'all have pointed out.
I plan to learn a lot more on this subject.
Doug
[img][/img]
Conventional eight groove rifling on the left, and octagonal polygonal rifling on the right.Polygonal rifling is a type of rifling wherein the traditional lands and grooves are replaced by "hills and valleys" in a rounded polygonal pattern, usually a hexagon or octagon.
Contents
1 History
2 Advantages
3 Lead bullets and polygonal rifling
4 See also
5 External links
History
While polygonal rifling has been around since the earliest days of rifled barrels, it had faded out of use by the time of the early cordite cartridges. The last common rifle to use polygonal rifling was the Lee-Metford rifle, named after the Metford rifling, a seven sided polygonal type rifling. The switch to cordite from black powder proved too much for the shallow rifling in the relatively soft barrels of the time, and the Lee-Metford became the Lee-Enfield when the Metford rifling was dropped. Heckler & Koch was the first manufacturer to begin using polygonal rifling in modern arms. Companies that utilize this method today include Heckler & Koch, Glock, Magnum Research, and Kahr Arms. Polygonal rifling is usually found only in pistol barrels, and is less common in rifles, However some extremely high end rifles like the PSG-1 or the FX-05 Mexican assault rifle use polygonal bores.
The term "polygonal rifling" is fairly general, and different manufacturers employ varying polygonal rifling profiles. H&K and Glock use a female type of polygonal rifling similar to the bore shown above right, this type has a smaller bore area than the male type of polygonal rifling designed and used by Lothar Walther. Other companies such as Noveske(Pac Nor) and LWRC use a rifling more like the conventional rifling with both of the lands sides being sloped but has a flat top and defined corners, this type of rifling is more of a canted land type of rifling than polygonal rifling.
Advantages
A number of advantages are claimed by the supporters of polygonal rifling. These include:
Less bullet deformation, resulting in reduced drag on the bullet which helps to increase range, accuracy, and velocity
Increased barrel life and reduced buildup of copper or lead within the barrel
However, precision target pistols such as those used in bullseye and IHMSA almost universally use traditional rifling, as do target rifles. The debate among target shooters is almost always one of cut vs. button rifled barrels, as traditional rifling is dominant. Polygonal rifled barrels are used competitively in pistol action shooting, such as IDPA and IPSC competitions.
Part of the difference may be that most polygonal rifling is produced by hammer forging the barrel around a mandrel containing a reverse impression of the rifling. Hammer forging machines are tremendously expensive, far out of the reach of custom gunsmiths (unless they buy pre-rifled blanks), and so are generally only used for production barrels by large companies. The main advantage of a hammer forging process is that it can rifle, chamber, and contour a bored barrel blank in one step. First applied to gun barrel rifling in Germany in 1939, hammer forging has remained popular in Europe, but was only later used by gunmakers in the United States. The hammer forging process produces large amounts of stress in the barrel that must be relieved by careful heat treatment, a process that is less necessary in a traditionally cut or button rifled barrel. Due to the potential for residual stress causing accuracy problems, precision shooters tend to avoid hammer forged barrels, and this limits them in the type of available rifling. From a practical standpoint, any accuracy issues resulting from the residual stresses of hammer forging are extremely unlikely to be an issue in a service pistol or a typical hunting rifle.
Lead bullets and polygonal rifling
The manufacturer Glock advises against using lead bullets (meaning bullets not covered by a copper jacket) in their polygonally rifled barrels, which has led to a widespread belief that polygonal rifling is not compatible with lead bullets. Firearms expert and barrel maker, the late Gale McMillan, has also commented that lead bullets and polygonal rifling are not a good mix. Some have made a point of the fact that neither H&K nor Kahr explicitly recommend against lead bullets in their polygonal rifled barrels, and feel that it is probable that there is an additional factor involved in Glock's warning. However, Kahr's FAQ does include a warning that lead bullets can cause additional fouling and recommends special attention to cleaning after using them. In addition, while H&K doesn't warn against the use of lead, at least one well-documented catastrophic incident in an H&K pistol appears to be related to this issue. Furthermore, Dave Spaulding, well-known gun writer, reported in the February/March 2008 issue of Handguns Magazine that when he queried H&K about their polygonally rifled barrels that they commented: "It has been their experience that polygonal rifling will foul with lead at a greater rate than will conventional rifling."
One suggestion of what the "additional factor involved in Glock's warning" might be is that Glock barrels have a fairly sharp transition between the chamber and the rifling, and this area is prone to lead buildup if lead bullets are used. This buildup may result in failures to fully return to battery, allowing the gun to fire with the case not fully supported by the chamber, leading to a potentially dangerous case failure. However, since this sharp transition is found on most autopistols this speculation is of limited value. The sharp transition or "lip" at the front of the chamber is required to "headspace" the cartridge in most autopistols.
Another possible explanation is that there are different "species" of polygonal rifle and perhaps Glock's peculiar style of polygonal rifling may be more prone to leading than the particular styles employed in the H&K and Kahr barrels.
Leading is the buildup of lead in the bore that happens in nearly all firearms firing high velocity lead bullets. This lead buildup must be cleaned out regularly, or the barrel will gradually become constricted resulting in higher than normal discharge pressures. In the extreme case, increased discharge pressures can result in a catastrophic incident.