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RIFLING
RIFLING
I J Larivers
You can have the biggest and best calibres out there, and the latest state-of-the-art magic bullets, but shot placement is everything. Craig Boddington once said “Mistakes tend to compound themselves and the first mistake usually occurs with the first shot.” He was writing in Bob Forker’s excellent reference book Ammo and Ballistics II, and he was specifically referring to shots on heavy, dangerous game, but if a thing is worth doing, it’s worth doing right. Accuracy - being able to place that magic bullet just where you want it - is a factor of both the ballistics of the combination of rifle and ammunition, and the skill of the shooter. The skill comes from lots of hours spent on the range, and then even more hours spent on the range. It comes easily to some and, to be honest, never to others. So let’s look at the firearm.
Muzzle energy, sectional density, weight and velocity - they all have their time and place, but a barrel’s rifling is all too often just regarded as an afterthought.
Rifling consists of a series of helical grooves or cuts running the length of a firearm’s barrel which put a spin on the bullet which gyroscopically stabilises it and improves its accuracy. In theory, as long as the bullet is right for the rifling. The concept has been around for half a millennium.
Rifling is described as turns in inches, e.g. how many inches of rifle barrel the bullet has to traverse before it completes one complete rotation on its axis. The shorter the distance the faster the spin. What we have to look at when selecting the correct bullet for a particular rifle are firstly the twist rate, and secondly the characteristics of the bullet itself: shape or design, length, and weight (which of course affects length).
Going back to the old muzzle loaders, where large calibre, short round balls were the projectiles, a very low twist - 1 in 40-50 inches - worked just fine. One of the reasons the old Kentucky rifles had such long barrels perhaps. As bullet design progressed through the Minié ball (named for Claude-Étienne Minié and in no way descriptive of the bullet) to the plethora of modern projectiles available today, the aerodynamic characteristics of bullets improved markedly. As bullet designs became more diverse in terms of weight, shape and bearing surface, so too did bore rifling have to adapt.
The general rule is that the longer the projectile, the faster the spin required to achieve stability. In the world of commercial ammunition manufacturers, the application is also important. If we take the 5.56mm NATO round as an example, its longer lighter projectiles require a rifling twist of around 1 in 7 or 8 to stabilize - generically but more on that later. A relatively fast spin. I say “around” because a bench-rest rifle shooter or a varminter using the calibre is going to require a greater degree of accuracy and consistency than a soldier. Larger calibre, shorter bullets can make do with, say, a 1 in 16 twist, which imparts a much slower spin. And this, of course, is where handloaders have all the advantage.
In flight the bullet is subjected to the same vectors as, say, an aircraft in flight. If it does not stabilise, it will begin to pitch and yaw. This obviates accuracy, and occurs if the bullet is not spun quickly enough, or too quickly. On a paper target at close range, this can manifest as “keyholing” where the strike diameter is greater than the bullet diameter and indicates that the bullet was not flying true at the point of impact (note, other factors, such as an improper bullet-to-bore fit, can also cause keyholing). If the spin is too fast, it also adversely affects accuracy and can cause core-jacket separation in bullets and excessive wear on the bore.
Early shooters using muzzle loaders would select a slightly under-sized ball and make use of a fabric patch to achieve the necessary seal. The patch also afforded another layer of adjustment, insofar as different materials could be used, and “reading” a spent patch to see whether it has achieved the correct fit to the rifling is in itself an art. With the advent of the hollow-based Minié ball, the soft lead bullet base would bell and form the required tight seal in the bore, and pick up the rifling. But one thing the shooter had to know, if he was going for any degree of accuracy, was the correct diameter of his bore so the most suitable projectile could be used. Even today, when the chamber’s throat and freebore - that portion of the throat which is un-rifled - perform the task of seating the bullet into the rifling, the shooter still needs to know his bore diameter.
Rifled barrels have two diameters - the bore diameter which is the distance between two opposing lands (the high points) and the groover diameter which is the distance between two opposing troughs (the low points). Why is the .303 British of a larger diameter than the .308 Winchester? Because the .303 refers to the bore diameter and .308 to the groove diameter. Both are generically .30 calibre bullets.
To be effective in improving a bullet’s accuracy, the rifling must be consistent along the bore’s length, it should be polished or cut to a high degree of smoothness so it does not malform the projectile, it must be crowned at the muzzle end so as not to introduce inaccuracy as the projectile exits the barrel, and it must be fitted to the bullet, which must swage into a proper, tight and consistent fit. Given modern engineering technology, this isn’t really a tall order for most manufacturers.
There are different manufacturing techniques to rifle modern gun barrels. Cut rifling is where a machine tool is used to either cut one groove at a time or all grooves at once down the length of the barrel. Button rifling can be pressed into the bore with a tool called a button, or the rifling can be hammer-forged or flow-forged over a mandrel. (A number of modern manufacturers - especially of handguns - have returned to the concept of “polygonal” rifling that was originally put forward by Sir Joseph Whitworth in the middle of the 19th century. Polygonal rifling is much shallower and less distinct than cut rifling, and is supposed to convey higher velocities, greater accuracy, longer barrel life and whiter teeth. Having extensive experience with a number of Glock pistols before building up another Colt 1911 I was pushed into abandoning cast bullets which lead excessively in polygonally-rifled bores in favour of CMJs; as to greater accuracy, well, I have to fall back on “operator error” for the ones that didn’t go exactly where I wanted them to!
One of the rifles that hung above the fireplace when I was growing up was an Enfield .303. One of my jobs as the junior member of the family was to make sure the stuff hanging on the wall was cleaned and oiled from time to time. I remember being curious about this old relic that my father had brought home after WWII, because it only had two grooves. It was an Enfield No 4, Mk I. Around three million of these were produced in the US by Savage Arms, whose engineers found the rifles would shoot just as well with a two-groove 1 in 10 twist as with the conventional five-groove 1 in 10 configuration and the two-groove models, marked “US Property”, were easier and cheaper to produce. Who knows whether that was just spin – no pun intended - or not? After the war, the two-groove concept fell away.
An 1858 Enfield Tower musket I once owned also had two-groove rifling - where any was left. And even with eroded rifling in places it was “good enough for government work” as a casual plinking rifle with a 480gr Minié mould I imported. So yes, the number of grooves is also variable. The be-all and end-all of accuracy is probably achieved by bench-rest rifle shooting, and when I put the question “What say you?” to Zimbabwe’s champion bench-rest shooter Roscoe Dickinson, he replied:
“The facts, as best I know them, are that nobody produces a two-groove barrel any more. It would appear that the barrel industry has standardised on a set of grooving for the most part. In smaller calibres this is four-groove going up to eight-groove for the .50 calibres. Within this there are various manufactures that produce alternate rifling patterns other than the "standard" square rifling such as 5R and polygonal. In respect of bench-rest Hart Barrels do produce three-groove 6mm barrels in 14:1" twist rate. The "best" barrels, as currently regarded, are all 4 groove in 6mm.”
So the number of grooves would seem to be more about production processes and less about accuracy, as I thought.
In order to keep productions costs in line, the majority of rifle barrels have a constant twist rate along their length. There is, however, a process called gain-twist rifling, whereby the spin rate is increased along the length of the barrel, after the bullet has engaged the rifling. This means that the projectile’s torque is more evenly distributed along the barrel’s length. It supposedly improves velocity and accuracy, but its main benefit is probably in prolonging barrel life by reducing erosion in the throat area. Like the concept of rifling itself, gain-twist rifling is nothing new; it dates from the US Civil War, and possibly earlier. It requires more complex manufacturing processes, and is consequently more expensive - and in any case in today’s world of extreme precision in machining techniques and exacting tolerances, the concept of gain-twist rifling is probably a complicated and expensive solution to a no longer existent problem.
Now, you have to ask yourself whether you will move heaven and earth to ensure that you always, until the end of time, will have exactly the same components for your particular rifle once you’ve found your optimum load - as a bench-rest shooter would - or whether, like a hunter, you are going to experiment with different bullets and powders. You can optimise either for a certain projectile or for a barrel that will give good performance with a wide range of bullets. I’m guessing most folks - apart from long-range varminters and bench-rest afficionados - will choose door number two. The true cognoscenti will, of course, have a perfectly good excuse to rush out and buy a lot more guns!
One of the reasons I have selected the 5.56mm NATO calibre as an illustration here is because of its popularity both as a military and a sporting cartridge.
A note here - the 5.56x45mm NATO is not exactly the same thing as the .223 Remington. 5.56mm ammo can generate higher pressures in .223 chambers (and therefore malfunctions), and the length of the throat (or leade) of the 5.56 chamber is longer than for the .223 and if the bullet comes in contact with the rifling too soon pressure spikes can result. In a nutshell, milspec 5.56mm ammo is best shot through a 5.56mm rifle, which of course can also fire the .223 safely. If you are only going to work with .223 Remington, then that may well be the correct choice for you.
Lumping them together for the sake of my purpose here, the calibre is in widespread use as a military cartridge, in International Practical Shooting Confederation 3-gun matches, and as a light hunting rifle. So what application are you looking at? In a heavier and more powerful military configuration like the 77gr MK262 you would want a fast twist like a 1 in 7. The lighter (and shorter) 55gr commercial loads can work well with a 1 in 9, and if you go for a really light varmint bullet like Hornady’s 35gr NTX, you’ll possibly be quite happy with a 1 in 12"twist.
“Can work well”? “Possibly be quite happy”? Well, we haven’t looked at all the other factors that go into making up a truly accurate round, so you’ll just have to sit down at the loading bench and make a few forays out to the range to see what works best in your rifle for your intended application.
Without wanting to sound overly condescending, given the IPSC rifle matches I’ve seen, it’s all Formula 1 stuff and based on kit more than application; many competitors, charging as close as they can to the targets, don’t seem to grasp the fundamental difference between a rifle and a pistol. The late Col Cooper would no doubt be nonplussed. With an “A” zone on the target the size of a small pizza box, the term “good enough for government work” truly comes into its own. Bench-rest shooters and varminters will demand more precision, and, as Edd Woslum, owner of Evolution, USA, one of America’s finest custom rifle manufacturers puts it, repeatability, from their rounds.
You have to consider what your specific requirements are. Perhaps you can’t sit down like Edd and manufacture a fine custom rifle for every bullet offering on the shelf, but an appreciation of rifling and how it works - together with all the other concepts that make up handloading - will help you get the very best performance out of what is available to you.
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