What makes a good barrel?
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Rifling methods
- Barrel steel
- The rifling method
- Availability of steel
- Changes to technology
- Inexpensive barrels
- Business practices — cheap
- Business practices — quality
- Stress relieving
Today’s report is in answer to a reader question. Riki from India asked the following.
“BB,what makes a good barrel? I mean in all smallbore air rifles the twist rate is the same, then why is the lothar walther barrel so much more coveted than a chinese barrel ? What does lothar walther do different from the others that their barrels are so accurate?”
I told him the answer would take an entire report, and did he really want to know that much? He said yes and several other readers chimed in, as well. So here goes.
You need to know up front that B.B. Pelletier is no barrel expert. I am writing this partly from what I have read about barrels over the past 50 years and partly from researching them for this report.
Let’s get this out of the way first. Rifling methods do make a difference to the accuracy potential of a barrel, but I have already written a lot about this subject in reports titled How are barrels rifled?, The invention of rifling and The importance of rifling. I recommend that you read those reports, and I will proceed as though you understand what is in them.
This is a big one! The type of steel used for barrels has to match: 1. the rifling method; 2. the speed at which the rifling is done and 3. The desired result. I will explain.
The rifling method
Some steel irons out smoothly and is well-suited to broach or button rifling. Other steel tears when it is broached and leaves a surface that’s full of microscopic divots and bumps. You would think it’s a no-brainer, then, to select the best steel for the job, but there are a few catches. First, if the proper steel costs significantly more than another steel that is acceptable but also has a few known problems, some manufacturers will choose the cheaper product and think it’s good enough. And if the cheaper steel will also support higher speeds in the rifling process, it becomes even cheaper because of reducing the overall manufacturing time and it’s even more attractive to those watching the bottom line.
The speed of manufacture
Chinese barrel makers run their rifling as fast as the tooling will allow, to lower the per-item cost. Premium barrel makers take longer and also they reject barrels that don’t meet a minimum quality standard.
Availability of steel
In a closed economy (one that is managed by a central authority beyond the manufacturer’s control) the best steel may not be available. The central authority decides what quantity and quality of resources go to which manufacturer. In that case, the central authority, and not the manufacturer, is the one who says it’s good enough. I am referring to how things work in a communist state.
Changes to technology
As time passes, technology changes how things are done. In the 1960s no precision shooter wanted a stainless steel barrel because they could not keep up with carbon steel barrels in the accuracy department. They were fine for hunters and plinkers, but not for those who chased precision. But, over the next two decades, the alloys of stainless were changed and the methods of rifling changed with them. Today a stainless barrel can be the most accurate barrel of all.
Many barrel makers drill the barrel on the horizontal axis, but I have been told that Feinwerkbau drills on the vertical axis, thus lowering drill bit runout (a wider hole drilled because the bit bends as the barrel turns) from gravity. We would be hard-pressed to see any deflection of a long deep-drilling bit, but apparently FWB feels there is an advantage to their method that’s worth the expense.
Not all barrels are drilled, however. Many airgun barrels are made from seamless tubing — especially for pneumatics and CO2 guns. Since the tubing is formed by a drawing process anyway, the steel chosen is suited for button and broach rifling techniques. So it works. Before you go all ballistic about this choice, know that it does work very well.
However — and this is a big however — there are things that can be done to improve on barrels made this way. Seamless tubing does respond well to drawing a button or broach through the tube, and it is possible to use the barrel that way without a lot of additional work. But if a better barrel (more accurate) is desired, the tubing can be honed before rifling. Even though the inside of the tubing looks smooth to the eye, there are minor imperfections that honing will address. It’s an extra step that costs extra time and money, but one manufacturer recently discovered that it pays off in greater accuracy. And greater accuracy pays off in increased sales.
Business practices — the desired result — cheap
If a company wants the absolute cheapest barrels they can produce, they use rifling broaches that start out on the large size, so they can be sharpened more times and last longer. Get a gun that has a barrel rifled with a broach like that when it is new and the barrel will be slightly oversized. I have seen this a lot in Chinese spring guns like the B3 underlevers and TS 45 sidelevers. That, Riki, is where cheap and inaccurate barrels come from — along with everything else I’ve said. I don’t see it at all in the rifles made in China for the famous brands like Benjamin, Umarex and Ruger. Just when the Chinese are left to their own devices, will they pull a stunt like this.
Cheap barrel makers who use seamless tubing will not hone the bore before rifling. They also won’t choke the bore, though that step doesn’t have to cost much if it is designed into the rifling process.
Business practices — the desired result — quality
A barrel maker who concentrates on quality will always hone the bore before rifling. If they drill the barrel they have to both ream and hone it, but the better makers will even hone seamless tubing before rifling. They will also choke the bore and chamfer the breech for easier loading of pellets with large heads. This is how a company like Lothar Walther makes a better barrel.
Then there is the hand-lapping that I discussed last week. A top barrel maker will hand-lap their barrels, either as part of the manufacturing process for all the barrels they make or as a special-order option. Lapping is different from honing, though both are aimed at smoothing the surface of the barrel metal. But lapping occurs after the rifling is completed and smooths the cuts that were made to create the lands. Lapping uses much finer abrasive than honing, and it is usually done by hand while watching the bore carefully with a borescope. At least that is how a barrel maker does it.
Beyond lapping there is stress-relieving through a cryogenic process. When a firearm barrel heats up it will warp and twist from stresses in the metal. A cryogenic treatment done properly will relieve these stresses and realign the crystalline structure of the steel, giving the bore a smoother surface with less tendency to warp and twist. You probably figured out that airguns don’t heat up when they are fired, but as far as I know, nobody has experimented yet with the cryogenic process for smoothing the bore. I don’t know if it is worth it or not, but since an airgun barrel does not wear like a firearm barrel, you could have a barrel that is already in service with a known accuracy badseline cryogenically treated and see the results.
Riki, there you go. That’s why a cheap Chinese airgun isn’t as accurate as a premium airgunb. Of course there is a lot more to the story when the rest of the gun is considered, but the barrel is where most of the accuracy lives, so this report addresses the primary differences.
What’s surprising to me is not the fact that there are cheap barrels, but the fact that a barrel can be made economically and still be quite accurate.