2 Февраля 2015

How to rifle a barrel

Seasoned hunters and shooters are aware that a quality barrel for a good rifle is almost everything

What is the reason of rifled barrel impact scattering? There are quite a number of reasons. Too tightly fixed stock screws, scope mounting ring slackening, maverick pressure process inside the barrel, wrong combination of a powder charge and a bullet. Nevertheless, there are factors which influence on the accuracy deadly.

CARTRIDGE CHAMBER

A cartridge chamber should be manufactured very precisely to get the optimum accuracy. The cartridge chamber produced with minimum clearances ensures better accuracy as a cartridge is less bulged in this case. Radial and angle deviations of the cartridge chamber from the barrel's axis are not acceptable. The reason of such deviations goes back to the cheap production technology.

If a gunsmith is hand-making a cartridge chamber out of the blank barrel by a reamer which lasts 1.5-2 hours then in line production this operation takes 40 seconds only. Nevertheless, scattering of impact points may be caused by the significant deflection of the cartridge chamber and the barrel axially. It is quite easy to figure out such a flaw. Take a suitable cartridge, smoke its bullet and push it into the cartridge chamber. If you find the traces of the grooves printed on the bullet rather one-sided and you need a high-accuracy barrel, then you may discard such a barrel without hesitation.

BARREL WEAR

Following a certain number of shots fired any barrel will show decline in accuracy. There are no completely similar barrels therefore their life expectancy may differ. As for the standard hunting calibers like .308 or .30-06 where the progressive-burning propellant powder types are deployed which are not influencing much on the barrel bore erosion, its life expectancy may reach 8000 shots without any noticeable shot pattern. Whereas the barrels for such calibers like .30-378 or 7.82 Warbid are hardly bearing 800 shots in total. The wear is showing up in the firearms by its statistic dispersion ratio increase. Primarily a transition cone of a barrel is subjected to wear – this is that part of the bore where the bullet is forcing into the grooves, and a muzzle face – where the bullet departs the barrel bore. Slowly the transition cone is burning out under the flame and exhaust gases influence. That peculiar erosion net picture is well-noticeable in the transition cone of a wear-affected barrel with the chrome-plated bore. As an erosion consequence the so called seating depth is enlarged and the cartridge loaded into the chamber is not touching the grooves already. Therefore, the pre-shot bullet attains certain free travel before being forced into the grooves. As far as the muzzle face is affected, it is easily wear off even in the cleaning process. It might be easily damaged while wire brush or cleaning rod handling without due carefulness. Attentive cleaning and barrel temperature balance maintaining while shooting extend its life expectancy and preserve specifications as well. However, if the barrel would be cleaned by the ammonium/ chlorate-based agents they would evaporate too long out of steel which might have undesirable consequences. Many rifles made of stainless steel are suffering more from the negligent cleaning rather than anything else. The temperature and bullet friction are influencing barrel wear most. And the highest temperature and friction are reached in the transition area from the cartridge chamber to the rifled barrel bore. The “magnum" rounds are accelerating the barrel wear due to the high velocity of the bullet, increased friction and exhaust gases temperature.

All aforementioned faults – lack of cartridge chamber and barrel bore alignment, transition cone erosion, muzzle face wear off plus action assembly flaws – will cause the high-frequency barrel vibrations while shooting that would influence shot accuracy adversely.

BARREL MANUFACTURING TECHNIQUE AND ITS QUALITY

This factor influence the accuracy most to compare with the other ones. The barrel quality is evaluated by the strict standards of manufacturer and metal quality rather than the rifling method.

It is known that the rifled barrels are made by three ways as a rule: cut-rifled, button-swaged and hammer forging. Although not every tough shooter might be able to explain the point - what is the superiority of one method over the other. A quality barrel could be manufactured out of any blank with well-eliminated internal stresses and produced of a good-quality barrel-grade steel whether it is stainless or molybdenum type, having plain surface and honed finishing.

There is one of three common steel grades applied to manufacture a barrel depending on the caliber and pressure developed in the cartridge chamber. 1137 steel grade (domestic analogue – 35Г2 steel grade) is used for the .22 rim-fired cartridges barrels. The standard steel for the barrels intended for calibers from .243 to .30-06 is 4140 CrMo grade (domestic analogue – 42XM and 38XMA grades). The match-grade barrels recently are used to be made of 416R grade patented alloy (domestic analogue – 20X13 grade).

The cut-rifling is the oldest method to manufacture the rifled barrels which means the multiple pass of the rifling cutter completely through the barrel bore wherein only one groove is processed in one pass. The grooves are made by a special cutting fixture as its cutting head edge is moving by one groove only to complete the pass in one full rotation of the blank. Each pass of the cutter removes 5 microns. As the pass is completed, the cutting head is released and returned into the cartridge chamber to restart the operation. It is required to make approximately 25-30 full passes in order to reach the necessary groove's depth therefore this process is rather a long matter. Although the cut-rifling is a laborious method and a pricey one besides but it is implemented to ensure an accurate shot.

The button-swaging method is the most simple and cheap one to manufacture a rifled barrel. As the blank is divided to normalized rods they are drilled, then reamed and honed. The honing is a finishing process for internal bore surface by the very fine polishing abrasive bars mounted on the head of the honing device which is rotating and simultaneously effecting reciprocal movement. Actually, the button swaging starts following this process only.

The button is a very hard rod made of tungsten carbide having recesses for the rifling lands and lugs for the grooves. It is pushed through the barrel with a force of about 80000 Newtons which leads to grooves engraving with technical data required (quantity, depth, twist rate). Metal stresses set up in the process are quite minor as the internal surface of the blank bore only is subjected to operation. To eliminate them the blanks are normalized once again in the vacuum furnace filled with nitridic agent.

The button-swaged method made a revolution in the post-war weapons industry changing tradition in rifled arms manufacturing.

A skilled gunsmith having high-quality steel billets can manufacture hundreds of barrels with almost identical inner diameter bores per day. The most important point of this rifled barrel manufacturing method is a drastic drop in price to produce a quality barrel.

Hammer-forging which is the method of forging on the mandrel was developed in Germany back in the 1930-ies. The process itself is quite costly therefore it is implemented primarily by the major manufacturers of weapons. As it goes from the title of this method, the hammer pounds the steel in the process of rotary-swaging. As the billet is forged, the mandrel is rotating and moving forward. The procedure takes three minutes and eventually the barrel produced requires minimum machining and finishing up to size.

In the process of hammer-forging on the mandrel the steel molecular structure is compacted therefore the barrel bore tended to taper in the muzzle. It is considered that this tapering diminishes bullet deformation, increases muzzle velocity and accuracy, but reduces the life expectancy of a barrel. An advantage offered by the method of manufacturing with the punch and cold rotary-swaging on the mandrel is that the barrel bore comes more even and smooth than to compare with the cut-rifled one. The further thermal treatment intents to relief internal stresses in the barrel which are adversely influencing mean impact point deviations as this barrel is warming-up. The real core of any advertising around these or those technologies advantages lies in the fact of such barrel stresses absence.

Theoretically, it is considered that the cutting allows producing the most accurate barrels and the buttoning – less accurate ones. Eventually, it is not that simple. Therefore, it will be more accurate to assume that it is possible to manufacture a sound quality barrel and a loose one as well by any of those aforementioned technologies. It is fair to keep in mind that there is no such thing like perfectly straight and flawless barrel. What is important - it is to realize what kind of accuracy level you are willing to achieve.

CARTRIDGES SELECTION

Cartridges selection comes down to find the most predictable barrel vibration that the bullet leaves the barrel at the invariable muzzle face position. The bullet bottom damage will lead to drastic accuracy decrease.

MUZZLE FACE

A bullet shall leave the barrel correctly and the muzzle face is responsible for that. If the muzzle face is made skewed to the barrel axis than the exhaust gases following the bullet create jet action which deflect the bullet and cause accuracy drop. The same effect appears in case of the muzzle face having dents or the rifled part of the barrel close to the muzzle face is damaged. The muzzle face should be even around the entire circumference and any aberration in it will mighty affect the accuracy adversely.

ACTION ASSEMBLY ADJUSTMENT AND STOCK SCREWS TIGHTENING

These works may improve the quality barrel specifications, but they are unable to make a faulty barrel shoot accurately. The screws tightening should not be excessive but it is necessary to check it regularly.

The action assembly being displaced towards the barrel axially may decrease the rifle accuracy partly, yet the quality of barrel manufacturing influences accuracy much more. Generally, even the high-accuracy barrel acquisition from the specialized dealer always represents a lottery. It is required to use endoscope to check the barrel bore, and there is such a flaw like decreasing twist rate which means transition to the lengthier twist especially in the barrel part close to the muzzle face and it is not possible to define it at all.

It is fair to point out that high-quality rifle manufacturing is a very laborious process which is based mostly on experience selection of all parts and a cartridge.

But along with it this question is understudied as there are quite a number of rifles production models which are distinguished by their excellent accuracy despite of the modest quality of their component parts.



YOUTH, FAREWELL! The part of the barrel near the cartridge chamber is subjected to the maximum temperature and pressure influence. Therefore, the barrel is subjected to thermal erosion in this particular place most of all. Especially, it shows up in case of considerable rounds per gun (rpg) ratio. In due course even in the chase a bell muzzle is forming of such dimensions sometimes that the cartridge inserted into it falls down up to the very case shoulder.



Photo by Alexey Klishin


Photo by the “Promtechnologia" LLC - Recent realities in high-accuracy arms manufacturing require high-tech equipment


Photo by the “Promtechnologia" LLC


Photo by Anton Juravkov – Contemporary equipment allow to process four barrel blanks simultaneously


Photo by Anton Juravkov – While cutting process vibrations are unacceptable and this coin standing firmly on its rib shows their absence


Action assemblies' production for high-accuracy arms and their adjustment require modern high-precision equipment and thorough observance of technological process


Photo by Anton Juravkolv – An example of modern CNC hub work – such a billet is processed by a single operation


Photo by Anton Juravkov – Weapons-grade laminate is a new word in the weapons design sphere


Оружейная ложа. Возможность регулирования под конкретного стрелка делает ее универ­сальной.


Photo by Anton Juravkov – POLYMERS FOR THE STOCK – Current realities are forwarding new requirements as new materials and technologies appear. Polymer stocks are too durable and tough to compare with their classic wooden counterparts. Even concerning aesthetics they are almost equal to classics. Given historical background, walnut wood was estimated as the best material to curve stocks. The stock manufactured out of walnut had sound durability and survivability and was rather lightweight. It was usually impregnated with special oil, coated by paraffin, varnish and painted sometimes. But even such protection was not an ultimate cure. Even provided the most careful handling of arm its wooden parts are lacking their properties in due course unlike polymers.