People that don’t know a huge amount about guns might not realise that a barrel’s rifling goes far beyond some grooves down the barrel. Some people might not even understand what rifling is or what it does! Here’s a quick little guide to help you understand everything about barrel rifling that you could ever need to know.
What is barrel rifling?
First things first, what is barrel rifling? Bullets are not very stable in flight. Whilst they are aerodynamic in shape, the smallest amount of force can cause it to begin tumbling through the air – this is terrible for range and accuracy. In order to better stabilise it in flight, the bullet is given spin along its long axis to stabilise it. Think about how an American football or rugby ball is thrown.
So, how is this spin given to a fired bullet?
A gun’s barrel is manufactured with spiralling grooves down its length that grab the bullet as it’s forced down the barrel and cause it to spin.
These little ridges and grooves (called lands and valleys respectively), are what is known commonly as ‘barrel rifling’ and is present in practically all modern weapons systems with the exception of most shotguns.
Barrel rifling types
The most common type of barrel rifling that you will find on most gun barrels is conventional rifling. This rifling features defined sharp lands and valleys. The bullet is slightly larger than the bore and is therefore forced into this shape producing marks on the bullet known as ‘rifling marks’.
These lands and valleys can vary in number, depth, shape, direction of twist (left or right) and twist rate, depending on manufacturer.
Polygonal barrel rifling has a much less defined set of lands and valleys, and is generally smoother in shape. This results in less resistance for the bullet when travelling down the barrel, higher bullet velocities and cleaner operation. The smoother bore deforms the bullet less than conventional rifling and supposedly leaves it more aerodynamically stable.
According to users, since the barrel rifling is still tight there is no little to difference in range or accuracy between the two. Although, polygonal rifling is reported to be easier to clean, as it’s less destructive to bullets and has less ‘corners’ for deposits to form.
Barrel rifling variables
Barrel twist rate is the most common variable that you will need to be aware of. The twist rate denotes the distance that it takes for the rifling to complete 1 revolution and therefore how long it takes a fired bullet to rotate 360 degrees.
Twist rate is measured as 1:X, where X is the distance for the bullet to complete a revolution. A common twist rate for rifles, for instance, is 1:8inch (1-in-8 inches) where the rifling completes 1 twist every 8 inches of barrel.
The shorter the distance for 1 revolution the faster the bullet will be spinning when it leaves the barrel.
The twist rate needs to be carefully paired with the barrel length, ammunition and calibre, as a bullet spinning too fast can be unstable and deteriorate before impact. A bullet not spinning fast enough will be less accurate and potentially tumble through the air.
Gain-Twist Rifling / Progressive Twist
Gain-twist rifling, often referred to as Progressive Twist rifling, is a progressively increasing rate of twist towards the end of the barrel. The twist rate of the barrel starts off relatively shallow but begins to increase as the bullet exits.
This is done for a number of reasons but primarily allows the bullet to fire initially with little disturbance and then introduces angular momentum. It has the added benefit of lowering the initial torque stress on the barrel and spreads it across its length, supposedly elongating the life of the rifling (especially in the throat of the barrel).
Do you need to worry about barrel rifling? Unless you’re manufacturing or modifying your own firearms, or using ammunition outside the rifle’s original specifications you don’t really. However, understanding the inner workings of a firearm goes a long way to establishing safe firearm practices and improved performance.