The Daystate Guide To Airgun Barrels – Part One

This is the first of a series of three posts about airgun barrels by Daystate’s Tony Belas.

As many HAM readers will know, Tony has many years experience in the airgun industry. Before that, he served in the British army. In addition, he has always had a strong interest in historic airguns and firearms. He knows a lot about airgun barrels!

You will find aspects of all this experience in his provocative, interesting thoughts. Take it away, Tony…


 

A lot is being said about barrels recently. They are, after all, the key link to accuracy of any air rifle. So the aim of this short series is to go through barrels, trying as far as possible to stay with the facts as I understand them.

Airgun Barrels – Caliber

The measurement of the internal diameter of the barrel is known as the caliber. In a rifled barrel, the measurement is made across the diameter between the grooves.

What's Inside Your Airgun Barrel? First Borescope Results

Caliber can usually be measured at the muzzle unless the barrel is reduced at that point. If so, that is called a choke.
Caliber is defined in a few different ways. Archaically in a shotgun, it is called a BORE and 12-bore is an 18.5mm barrel. The term 12 bore is gained by how many same sized balls made from 1 pound of lead can fit into the diameter of the tube…
If you didn’t know this before you know now!
There is still a lot of imperial measurement around in barrels, .30 .306, .45 of an Inch etc. Equally the metric equivalent, 5.56, 7.62 these are in millimeters.
Not everything translates exactly, for example a .22 airgun has a bore of 5.5mm. A .22 rimfire is a ‘true’ .22 and in metric measures 5.588 mm.
Older UK airguns also were a true “.22” and this fact alone can explain why they mysteriously do not seem to be that accurate with much modern 5.5 mm ammunition.
Going the other way, 5.56 is .2189 mm not the .223 its usually related to and is in fact a different cartridge.

Types Of Barrels

Most airgun barrels start of as a piece of steel rod or tube. Types of steel can vary enormously to include stainless steels. Most people think stainless is the best material, but not always the case, as it’s harder to work than plain steel.

Unless starting with a tube, a hole is drilled centrally into the rod, using progressively longer drills to stop the drills wandering offline (gun drilling). The barrel is then honed and reamed until the bore size is correct and true.

Depending on the type of rifling, any exterior profiling work is then carried out. It’s best if this is carried out first as any work on the outside “stress relives” the inside, but this is not always possible.

For the process of putting in the rifling there are dozens of different patterns to use. The type chosen will very much depend on the type of bullet and propellant.

The most common type of rifling is the “Enfield” land and groove. In this, raised “lands” grip the projectile to impart a spin, the depth and twist of the groves and relies on the speed and strength of the projectile.

Overdo it and there is the chance that the bullet will strip some of its material into the rifling, which is bad news for accuracy. It is even possible to increase the rate of twist towards the muzzle to impart a greater rate of twist on the bullet before it exits.

This method of “progressive twist” rifling was common over 100 years ago on black powder rifles that fired short soft-lead projectiles.


Bore Finishing

The bore of the tube is honed to caliber, then polished to a mirror finish before the rifling is applied. Many military barrels are internally chromed to leave an immensely hard wearing surface which cannot rust – important in a military application.

The barrel at this stage is referred to as a blank.

To fit into the breech block, the barrel is now machined, often with a thread or taper, together with any work required at the muzzle end. For example grooves for a foresight, or threading for a silencer.

Any exterior modifications to a barrel must be made with extreme care, as any machining to the outside of a barrel results in stress relieving of the rifling itself, i.e. it changes tolerances.

This is especially true of threading for a moderator, which is carried out at the muzzle – the most crucial point – and can wreck accuracy. This can be avoided with button rifling, but at a price, as the machining can be unique for that type of rifle, and this work must be farmed-out to the barrel maker.


Rifling

Most barrels have a right-hand twist. I used to think this is due to most barrel makers in the old days being right-handed. It would have been more natural to rotate the cutter to the left as it is drawn through the tube.

But I now understand that bullets spun using a right-hand twist perform better in the northern hemisphere due to the “Coriolis effect” (the rotation of the earth). This is the same effect that causes water to seem to spin clockwise in the Northern hemisphere and counter-clockwise in the Southern.

I say “seems” as I have read elsewhere that this is a load of nonsense…
Whatever the shape of the rifling it will probably be made using one of three methods:
1.  Traditional hand cut
2. “Button” rifling
3.  Cold Hammering (Hammer Forging)


Hand Cut Rifling

With hand cutting, a tool with a single cutter is drawn through the tube cutting a single groove at a time. A ratchet usually controls the spacing of the grove. This is the oldest and most skilled form of riling and is usually carried out by a master craftsman.


Button Rifling

Market leaders Lothar Walther commonly use this process on airgun barrels. They make barrels for Daystate airguns, BRK and many others.

Daystate Guide To Airgun Barrels

Button rifling is a mass-production method, usually carried out on highly specialized and expensive CNC machinery. Button rifling machines are enormously expensive but once set up can mass-produce barrels quite quickly and to exacting tolerances.

In operation, a precisely made “button” which has the shape of the rifling on it, is pressed through a carefully tempered tube, forcing the shape of the button into the internal surface. No actual cutting takes place in this process.

In fact the local area of the compressed inner surface is hardened much as on a forged barrel, but the effect is much more localized.

An advantage to this type of rifling is that external fittings work, such as slots for sights, etc. can be machined before the rifling is put in place.


Hammer Forging

This was the original mass-production method for producing rifle barrels.

A “mandrill” with the shape of the rifling is placed inside the full length of the rod. The rod is then hammered onto the mandrill inside to leave the impression of the rifling. The mandrill is then withdrawn.

This process compresses the steel, hardening it. This is ideal for high wear applications such as full-bore firearms. It’s usually more expensive and slower than button rifling, but the result is a microscopically rough internal finish that is harder to polish.

It is particularly difficult to do this on smaller caliber barrels such as the target air rifle caliber of .177. Fortunately barrel wear is not an issue when used with soft lead pellets, so virtually all target air rifle manufacturers use button made barrels.


Thanks, Tony, for some great information! Next time he will cover more details of barrel manufacturing.