The Daystate Guide To Barrels And Barrel-Making – Part Two

This is the second in a series of three posts about barrels and barrel-making by Daystate’s Tony Belas.

You can read part one here.


 

Rifling Patterns

Many different patterns can be used, from a conventional “land and groove” pattern, to what is a polygonal shape, which is twisted. There are advantages to both but none rules supreme…

Sometimes the lands can be very shallow – especially for airguns – and sometimes they are deep and there is a lot of them. There are even barrels out there with two groves in which a special bullet is inserted. Maybe a subject for another day?

Below, you can see the pattern of lands and grooves in this slug that was jammed in a rifle barrel.

Muzzle Energy And Consistency For Slugs


Twist Rate

Twists are measured in turns per Inch. The “standard twist” for airguns is 1 turn in 17.7 Inches expressed as 1:17.7 or rounded-up to 1:18.

A very high-powered airgun might have a slower twist of 1:30 that is one revolution in 30 Inches. Over on firearms, a military weapon like the M16 is about 1:9 while a .50 cal is 1:15

In long range firearms, the twist rate has many effects on the ballistics, and ballistic predictions. On airguns it only effects accuracy. As far as ballistic calculation programs go, I feel that twist rate is a bit of a waste of time.

However, the software ballistics programs need twist rate information to work as they are written for firearm shooters. So if you are asked, “What’s the twist rate of my airgun barrel?” – just say 1:18…

Borescoping Air Rifle Barrels


Chokes And Tapers

Like a shotgun, rifle barrels can be choked. But whereas on a shotgun the choke is there to tighten the shot pattern, a rifle choke tightens up the projectile’s fit in the barrel to maximize the accuracy. This happens just before the projectile exits the barrel.

Chokes are very common on air rifles. Deformation of the soft lead pellet as it travels on its way up the barrel can be compensated for by a choke.

By tapering the barrel over the last few Inches, a similar result to a choke is achieved but with a superior, more gradual effect. However, as you can imagine putting a gradual taper into a small caliber bore is very difficult (and therefore expensive) to do.


Crowning

For it to be accurate, the projectile must exit the barrel square to the bore. So one of the final processes in barrels and barrel-making is to machine the muzzle of the barrel square.

This is referred-to as “crowning”.

Crowning is mainly done in a lathe where care must be taken that the bore is central.


Shotguns

A shotgun barrel usually starts with a tube though (in the case of Breda – an associated company of Daystate), it can also be gun drilled from a solid rod.

A shotgun barrel is drilled, honed and polished much as a rifle barrel. But depending on its eventual use, the barrel may need a fixed choke to tighten up the pattern of shot, this is machined-in during this initial process.

After that the barrels are externally reduced to the required external size, a process referred to as “striking”.

With shotgun barrels it is possible to fit an ‘Multi Choke’. Here the muzzle of the barrel is internally threaded to take a screw in choke, which is available in a range of sizes. Some manufacturers offer the option of a choke tube that screws on the end of the barrel, but internal multi chokes tend to give a slightly better pattern.


Proofing

To ensure the safety of the shooter, under the United Kingdom 1888 Gun Barrel Proof Act, all firearms which discharge explosive substance must be proofed at either the London or Birmingham proof houses.

Often in other countries, such as the USA, proofing is done in house by the manufacturer, but firearms imported into the UK must be re-proofed before they can be sold there.


Cosmetic Finishing

Depending on its use, the barrel will have one further process. Military weapons undergo a form of chemical rustproofing before painting, civilian sporting rifles, are just painted or powder-coated.

Many sporting weapons and most air rifles are still “blued” or “blackened” – an age-old chemical process. This requires that metal parts are polished to a mirror finish to get the best results, however drawn steel is quite porous, so the barrel needs center-less grinding before polishing if the barrel is not to end up with a matte finish.


Testing And Straightening

One more point about barrels and barrel-making. The final stage is for the barrel to undergo inspection and measurement to check that the rifling lands are evenly cut. Also that there are no bulges or cracks.

The trained and highly-skilled eye – as far as I can tell – is still exclusively to carry out straightening.

For the ultimate test to see if the barrel is any good it must be shot*.

That’s why all Daystate and BRK air rifles are test-fired for accuracy before shipping from the factory.

Daystate Guide To Barrels And Barrel-Making

Daystate Guide To Barrels And Barrel-Making

This can result in the barrel being made, finished, choked, machined, center-less ground, polished, blued, assembled into a test rig or the actual rifle and finally, when it is found to be inaccurate – scrapped!

*Many years ago, during the run-up to Desert Storm, I was lucky enough to be invited to watch test firing of Challenger Tanks at Lulworth Cove tank range in Dorset, UK. Even with a $100,000 120mm tank rifled barrel, the final test before it is shipped out for active service is to shoot at a considerable distance. A technician explained at the time: “Despite all our measurements, it’s the only way we can be sure that it’s accurate”.


Next time Tony will cover more details of barrel manufacturing.