rifling for pistol

How Pistol Barrel Rifling Affects Handgun Accuracy

The interior of a handgun barrel isn't just a smooth tube that spits out bullets that go fast — there's a bit more to it than that, and it affects just how accurate the bullet is, the velocity and the path it takes from magazine to target.

Conventional rifling is a topographical manipulation of what's inside the barrel, a number of spiralled "grooves" cut into the metal with "lands" separating them, while polygonal barreling is similar but, as opposed to the cut grooves of conventional rifling, these are smoother, molded bulging sections that are less impactful on the ammunition.

There's a reason behind these grooves that involves measuring how and why they cause the bullet to spin as well as how that improves stability.

Bullet Stability is a Result of Barrel Rifling


Before stability and barrel rifling is more thoroughly defined, it's important to review how a bullet is even fired. Just considering the bullet and thinking about it in basic terms, think of it as a small chain reaction.

The hammer strikes the primer, which ignites the propellant that causes a rapid expansion of gas, heat and energy. The projectile (bullet) is then launched from the top of the casing that holds all these components together as a cartridge.

As the bullet travels down the length of the barrel, this is where rifling comes into play and twists the bullet. Now, twist rate is the measurement of how many inches it takes a bullet to rotate completely within the barrel. A 1:7 twist rate means a complete rotation happens within seven inches, for example.

Matching the twist rate to the right heavy or light grain ammunition can optimize spin rate, but note that manufacturers these days typically make their handguns compatible to most common grain ammunition for specific calibers.

Think of the spin of a bullet like throwing a football. As it twists through the air, its trajectory is much more accurate and reliable than tossing it willy-nilly and hoping for the best.

This is because of gyroscopic and dynamic stability within ballistics technology. Gyroscopic stability is a culmination of the spin, velocity, weight and shape of the bullet, as well as atmospheric effects on the bullet. As the bullet spins, it's more resistant to being blown off course, and its impact with the target is improved.

There are specific mathematical formulas, like the Miller twist rule, that measure optimal twist rates, with variables for bullet mass in grains, gyroscopic stability factor, bullet diameter, bullet length and twist in calibers per turn. Miller sought to improve British mathematician Alfred George Greenhill's formula.

Twist rate is just one factor. The type of rifling affects bullet choice, potential barrel fouling and optimization of the gases behind the bullet as it travels down the barrel and runs against those grooves.

Barrel Rifling in Firearms: Conventional vs. Polygonal

polygonal vs conventional rifling

The proliferation of barrel rifling smothered smoothbore rifles — these were the firearms where a load of black powder was dropped in and ignited to launch a ball of metal at a target.

Their accuracy was inconsistent.

Conventional barrel rifling, through various advancements, was a way to rotate the ammunition as it traveled down the barrel. The idea is to cut grooves in a spiral down the bore of a firearm to a certain extent. The area between each of these grooves is called a land. The sharpness of these grooves may be controlled through steep or ramped edges.

As the bullet travels the barrel, these grooves cut into the soft, unprotected metal (like lead). A bullet's diameter is just slightly larger than the barrel, and that excess metal is what's fed into those spiraled serrations, naturally twisting the matter being propelled forward.

Does that metal get stuck in there over time with conventional rifling? You bet. That's why cleaning is important. This is also why cheaper unprotected lead bullets can be used with conventional barreling.

Polygonal barreling came about in the middle of the 19th century, but really picked up around the Second World War to make a more efficient machine gun.

Heckler & Koch is said to be one of the first companies to implement this method of rifling, but companies like Glock, Walther and Kahr Arms also implement it today. It's often found in handguns, and it tends to have a longer service life.

Jacketed ammunition is useful when in conjunction with the smoother polygonal barreling, because those smaller grooves can become dangerous with buildup of soft metal left behind, which squeezes the gas into a tighter space. With unprotected lead sliding along along polygonal rifling, it rubs fractions of the soft metal onto the bore.

As the bullet travels through the barrel, there's less drag than in conventional rifling that cuts into the bullet. It more or less guides the bullet, and better traps the gas behind it for greater velocity — which could minutely affect accuracy.

There are a few common ways rifling is implemented, and there are variations in how many grooves there are. Rifling can be created by cutting grooves one at a time, cutting all grooves at once, pressing precisely with a tool called a "button" that has the inverse of the rifling on a tool inserted into a bore and pressed to form congruent grooves, hammer forged by inserting a barrel length mandrel with the reverse image of the grooves and forcing the rifling into the bore, and also flow formed rifling in some military applications that uses a mandrel and rollers under heavy pressure.

Rifling is just one way firearms have historically improved accuracy. Let others know how you've improved shot placement and grouping at the range in the comments below, or explain more about your experience with rifling.

Jake Smith  

About The Author

Jake Smith (@notjakesmith) is a copywriter and photographer based in the Pacific Northwest who enjoys shooting pictures and ammunition outdoors.

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