by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- The first airgun rifling
- Important points
- Choked barrels
- Rifling twist rate
- Other twist rates
- Smooth Twist
Today we will look at airgun rifling as it has evolved over the past century. For many years airguns were not rifled, but in 1905 the BSA company built what many consider to be the first modern air rifle and it was rifled — as the name implies.
The first airgun rifling
The first underlever air rifles made by BSA were taploaders. A pellet was loaded into the open tap, nose-first, then the tap was rotated closed and the gun was fired.The pellet expanded inside the tap and was driven into the rifled barrel at high velocity — well, high for airguns at the time. Maybe 500 f.p.s. for a .177 pellet and 325 f.p.s. for a .25-caliber pellet.
The rifling had to be aggressive enough to catch the pellet and not let it slip through the bore out of control, but it also couldn’t be so aggressive that the pellet was slowed by excessive friction. So the question arises — how did “they” know what to do?
Of course they didn’t know. Not until they began shooting pellets did the developers know for sure how the rifling would react with the lead pellet.
Pellets were also evolving at this time. First, the diabolo pellet had either just been invented or was about to be invented because the rifled barrel needed it so much. I believe the former to be the case.
Diabolo pellets contact the inside of the barrel at two points along their axis. Their heads are small enough to ride on top of the rifling sticking up, or if they are engraved, it’s only slightly. The skirts stick out farther and are engraved by the rifling. They are what makes the pellet spin in accordance with the twist rate of the rifling as they travel down the bore.
It turned out that shallow rifling and narrow lands were all that was needed to properly guide the diabolo pellet. If you recall from Part 1 of the rifling series, Ballard rifling was invented in the late 19th century and was considerably shallower than the rifling that preceded it. But in 1905 the first airgun barrels were rifled with lands only about half as high as traditional Ballard lands. Instead of 0.005-inches high, the airgun lands were around 0.0025-inches high. This proved to be enough to catch and hold the pellet at the velocities it was traveling.
Next you’re going to want me to tell you how many lands and grooves they used. I wish I could. But in researching this topic I was struck by the fact that the writers of the day got away with phrases like “finely rifled” and “polygroove rifling” to obfuscate the fact that they either did not know the number of lands and grooves or they didn’t care. I have personally encountered vintage barrels that had 12 lands and grooves and a few (much fewer, in fact) with just 6 of each. There may have been different numbers, but there isn’t enough information to go by.
The point is, the BSA designers understood very well that the airgun they were building didn’t have a lot of energy to spare. Anything they could do to lower the pellet’s friction with the barrel was a plus. Apparently lots of low lands was the solution to lower friction.
I’m about to launch into the largest part of this report, but before I do I would like to say a few things about choked barrels. We hear about them a lot and perhaps you have wondered if choking the barrel is all it’s cracked up to be.
First of all, what is a choked barrel? Rifle bores are meant to be the same diameter from one end to the other. The military used to gauge their barrels and they marked those that were the most uniform with a star at the muzzle. These are called star-gauged barrels.
But barrel makers like Harry Pope discovered that if there is a slight taper — a choke — near the muzzle of the barrel, that barrel will be more accurate. Even his barrels that were loaded from the muzzle responded to a choke because when the black powder went off it obturated (squashed out) the base of the bullet out into the rifling. So, when it arrived at the choke, the bullet got squeezed down even farther.
Airgun barrels are not typically choked, but sometimes spring gun barrels that have dovetails for the front sight get squeezed down inside a little and it feels like a choke what a pellet is pushed through. The only airgun barrels that are usually choked intentionally are those found on precharged guns. The makers say the choke helps size all the pellets the same before they leave the muzzle. Spring guns size their pellets by squashing the skirts out into the rifling when they fire, so a choke isn’t necessary.
Rifling twist rate
If the number of lands and grooves in a vintage airgun barrel is a mystery, the twist rate of the rifling is as open as a book. It was always one turn in 16 inches of travel. I would like to say something at this point about twist rate. The pellet doesn’t “spin up” to its rotational speed as it travels down the barrel. It’s spinning just as fast at one inch down the bore as it will be when it leaves the muzzle. The length of the barrel has very little to do with how fast the pellet spins. All it can do is increase the pellet’s velocity, which does make it spin faster. But it is the rate of rifling’s twist that determines how fast the pellet spins.
You may recall that I did a huge 13-part test to see what affect the twist rate has on a pellet’s velocity and accuracy. Dennis Quackenbush made special barrels for my Talon SS with twist rates both slower and faster than the standard. I gathered lots of data in the test, but the bottom line was the 1: 16″ twist rate was the best — not back in 1905, but in 2013. Read the series if you haven’t already, because it will help you will understand airgun barrels better.
Other twist rates
Have other twist rates been tried? Yes, they have. I have heard of a 1:14″ rate and a 1:18″ rate. Both of these, though, are so close to the 1:16″ rate that there really isn’t enough of a difference to sort things out. But there is another barrel with a twist rate that’s vastly different.
The Smooth Twist barrel is a proprietary rifling system employed by FX Airguns. The bore is smooth until the final few inches, then the pellet reaches the rifled portion and the rifling takes over. The rifling is pressed in from the outside of the barrel (hammer-forged?) If you read the internet comments about Smooth Twist you get opinions that range from it doesn’t work (not too many of those) to it’s the greatest way to rifle an airgun barrel.
Some of the claims for the benefits of the Smooth Twist barrel are:
Less cleaning required
Superior accuracy — particularly at long range.
They like any domed pellet
Some of the detractors will say:
They cost too much (this is a universal response to anything that’s not free)
They are very picky about which pellets they shoot (one popular claim is they were made to only shoot JSB pellets)
They only shoot well in a narrow velocity band
I have investigated the twist rate reports for the Smooth Twist barrels and found rates that range from 1:45″ to 1:96″. However, these same people who measured their Smooth Twist barrels are measuring other airgun barrels and finding twists of 1:16.798″ and so on! So I suspect they are not measuring the Smooth Twist barrel twist correctly.
It would appear that the Smooth Twist barrel does have a slower twist rate than a conventional barrel. And it also appears that these barrels do favor domed pellets over other shapes. And perhaps they perform best within a certain velocity range, but so do all conventionally rifled barrels.
The Smooth Twist barrel is a different kind of rifling for the 21st century. Who knows where it will go?
The one thing we can say for certain about airgun rifling is, “It ain’t over yet!”