GUNSMITHING: A rifle buyer’s guide to twist rates
Jonny continues his rifle series with this in-depth article looking at twist rates and improving accuracy. Be warned, there is some high-level maths thrown in, you know, just for fun!
To maximise the effects of rifling, the spirals must be tailored to the specific calibre, type of shooting and type of bullet. This tailoring can be done using twist rate, which refers to the tightness of rifling in the barrel.
The numbers involved are describing one revolution per number of inches. For example: ‘1 in 8’ refers to 1 complete revolution in 8" of barrel. This can be checked easily on any rifle by pushing a cleaning rod down the barrel, marking the rod, and allowing the rifling to naturally twist it.
Once the rod has turned 360°, the length of rod can be marked again, and the distance can be measured. Some guns will have it printed on the side and most manufacturers will have gun specs on their website.
The general rule of how high the twist rate should be is dependent upon the size and weight of the bullet. The smaller/lighter the bullet, the more quickly it needs to twist, while bigger/ heavier bullets need less twist or they will over-stabilise and start to tumble. See page 50 for the average twist rates on some popular calibres.
Some rifles come with a standard twist rate based on this average, others give options of a few, and it is possible to have rifle barrels custom-made to ensure the perfect twist rate for bullet stabilisation. For most people, the twist rate a factory rifle is supplied in will be more than adequate, given that the effects of a non-perfect twist rate to bullet match can rarely be seen in distances of up to 100m.
So, how is this all calculated? Well, a British mathematician named Alfred George Greenhill (1847-1927) developed a formula to work this out using a mixture of known factors. Here is his equation, plus an example using Hornady .243 Win 58gr V-Max.
ALFRED GEORGE GREENHILL
Twist Rate = CD² SG L x 10.9 Where: C = 150 (180 when muzzle velocity > 2,800 f/s) D = Bullet diameter in inches L = Bullet length in inches SG = Specific Gravity = 10.9 for lead-core bullets
EXAMPLE Hornady .243 Win, 58gr V-MAX = 11.17
150x0.243² 10.9 0.793* x 10.9 = 11.17
This was later disputed by another chap named Don Miller. Miller believed that the formula should be more precise, and designed a further formula with the assumption that the velocity is 2,800fps, standard temperature is 59°F, and pressure is 750mm Hg with 78% humidity.
These assumptions technically should change based on exact measurements, but the difference is so little that it can be counted as insignificant and therefore ignored. His equation and an example using the same ammunition as before can be seen on the next page.
Assuming that twist rates will always use whole numbers, Hornady 243 58gr V-max would work best through a 1 in 10 or 1 in 11 twist rifle. Therefore, a shooter who knows that this would be the best bullet for their activity would be best to own a rifle with this twist rate.
This can also help to explain why rifles have bullet preferences, and can be reverseengineered to find an adequate bullet for a specific gun. See, it was worth reading this far through all that maths!
For those of us who are maths shy, there are many online calculators ready to work out the perfect twist rate without showing the formulas used. Alternatively, like me and many others, buy an average twist rate (and therefore more versatile) rifle, buy some average weight for calibre ammunition, and go and use it!
The minimum legal barrel length for a Section 1 firearm in the UK is 30cm (11.811"), so a shorter barrel than this is not only impractical, as you shall see, but illegal!
There is a long-standing debate about the perfect barrel length among rifle shooters. Many factors, including calibre and intended use, are often deciding factors in the exact length suitable for individual shooters.
Manoeuvrability and handling
When hunting, manoeuvrability is vital; avoiding being noticed by prey is difficult enough without trying not to hit trees and bushes with a cumbersome 28" rifle. Shorter barrels are both easier to control when walking or crawling, and more lightweight so you don’t wear out your shoulder; they are also easier to free hand or take ‘mildly supported’ shots.
The barrel is traditionally the heaviest part of a rifle so, as length increases, the centre of mass will move forward dramatically. For high seat, prone or bench shooting, the longer barrel and associated weight can be an advantage, making the rifle less prone to outside movement.
Some powders burn faster than others. Slower burning powders tend to produce more velocity at a more predictable rate, while faster burning powders reach a high velocity quickly before rapidly decreasing.
Predictability of the bullet’s velocity, course and trajectory is essential for accurately aiming over long distances. A longer barrel has more time and distance for a powder to burn entirely, making a slower powder the preferable choice.
However, a faster powder will burn more efficiently in a shorter barrel, which has less distance for the powder to fully burn. Factory bullets do not cater for lengths of barrels or burn rates; this is important, as when unburnt powder is left in a barrel repeatedly, the barrel’s lifetime will decrease. It is possible that a shorter barrelled rifle will burn out faster than a longer barrel. This matters little if you don’t homeload, though!
The length of the rifle barrel has a positive correlation with velocity obtained from the bullet used. Pressurised gas in the barrel forces the bullet forwards, causing acceleration for the entire time that it is inside the barrel. Upon exiting the barrel, the expansion of the gas is no longer under pressure, and the velocity of the bullet stabilises. The longer the barrel is, the more time the bullet has to accelerate in. Therefore, as shown in the graph, longer barrels produce higher velocities.
While there is no known point in which barrel length no longer affects velocity, it always begins to even out, making a less significant difference to velocity, and illustrating a sweet point for compromising. In the graph, the ascent can be seen flattening at 24.75".
Most velocities out of a 16" barrel are still very capable – but make sure you put the rifle over the chronograph as with certain calibres/loads you can fall below 1,700ft/lb muzzle energy, making it not legal for deer in England.
It is important to note that you should be able to outshoot your rifle before worrying about making it more accurate. That said, there are three main ways in which longer barrels are known to improve accuracy.
The first is less relevant today, but throughout history, iron sights were the primary method of aiming a weapon. Longer barrelled muskets would therefore be more accurate as the distance between the fore sight and the rear sight were marginally longer, and therefore more precise. This is still a consideration for some rifles, although the vast majority of rifles are now able to hold telescopic sights that are far superior in accuracy than iron sights!
The second factor is heat transfer. As a barrel becomes too hot from successive shooting, it can cause warpage in the barrel. Even slight warpage throughout a shooting session can be detrimental to consistency (and therefore accuracy). A longer barrel has more mass available to transfer heat in between shots, thereby keeping the barrels cooler and less likely to warp. Gunsmiths have been trying to find other solutions for heat transfer. Varmint barrels are thick barrels designed to stop warpage and don’t have to be particularly long. Be careful with fluting, though, as some fluting can actually accelerate warping.
Lastly, a longer barrel provides a longer path for the bullet to stabilize prior to exiting the barrel.
However, with the above theories in mind, a large proportion of barrel accuracy can be put down to the stiffness or rigidity of the barrel. When
a shot is fired, the barrel vibrates, starting at the action and increasing towards the muzzle. The vibrations, though small, make a difference at the muzzle to the direction in which the bullet will continue its path. A more rigid barrel will vibrate less and will be more accurate.
While all these theories on accuracy are conceptually sound, little evidence has been found to show any difference in accuracy with regards to barrel length – determining factors of accuracy in rifle shooting are more likely to be barrel quality, trigger weight, bullet quality and wind strength.
Effectively, the argument comes down to velocity vs manoeuvrability. Velocity necessary for target shooting at extreme distances; manoeuvrability necessary to move around the wilderness when stalking.
The bottom line in my mind is simple – shooters that mostly go stalking and lamping shoot standard rifles with standard ammo, because the average set-up will easily shoot 1 to 1.5 MOA, which is enough for shooting deer and fox at 100-200m.
For long-range target shooters, twist rate and barrel length is vastly more important, but these guys are shooting at targets I can’t even see. And before I accidentally insult anyone else, I shall stop. Thanks for reading!
Choosing a long or short barrel is not as simple as you might think
The weight, length and construction of the bullet you use will affect which twist rate is best
The length of the rifle barrel has a positive correlation with the velocity, which you can test using a chronograph
Fluted barrels can dissipate heat quicker