Barrel Harmonics Part 2: Carbon Fiber Sleeve Installation and Testing
Part 2 of our Barrel Harmonics series will discuss carbon fiber sleeves and will answer whatever question you typed into Google (or a search engine that isn’t evil) that led you here. In the name of science, I have installed a carbon fiber sleeve on my Air Arms S510 and have documented the process. If you haven’t read P art 1, I would encourage you to check that out as a primer.
If you have read Part 1 of this series – let’s begin, shall we?
Long story short, my .25 Air Arms S510 shot beautifully out of the box but I wanted more power. I hot-rodded the thing and ended up with a killer tune. H&N Barracudas shot very well, but after competing with the ‘Cudas, I wanted to shoot the JSB 34gr Heavies since they are substantially better in the wind. Just one problem. This tune shot the JSB Heavies rather poorly. For reasons I will explain below, I believe that this was a harmonics issue rather than the barrel disliking the pellet or another more mundane excuse. Shot string was extremely tight (1% extreme spread) and head sizing did not measurably improve groups.
Why a Carbon Fiber Sleeve
Here is a target with three different 5-shot groups shot consecutively from my S510. 50 yards with the JSB Heavies singing along at about 840 FPS. Almost every target looked like the ones below.
Definitely not acceptable precision for a competition rig! BUT, there is much to be learned from that target. If you look closely, it wants to group. There is a trend developing: tight clusters with a flier (or 2) consistently off to the left. On top of that, shooting groups below the regulator pressure (at about 800 FPS or so), or increasing the hammer spring preload (to yield a velocity of about 860 FPS), resolved this issue entirely. Tightening or loosening the shroud (or removing it entirely) had little effect on group size.
This begs the question, “Nic, why not just shoot at those other speeds?”. Well, since you asked, I wanted 50 ft-lbs. 800 FPS is below that threshold and 860 FPS lowered the shot count too much for my liking. I demand 820-840 FPS! This sort of behavior indicates to me that we are outside of the harmonic sweet spot. In Part 1, I included the following diagram…
The “sweet spots” are at the crest and the trough. During the shot cycle, the barrel is constantly vibrating because of the forces exerted upon it. Therefore, if you try to tune your rifle so that the pellet exits the barrel while it is at its “normal position”, the barrel is literally moving past that point as the pellet is exiting the barrel. This means that slight changes in velocity can lead to noticeable point of impact changes. From shot to shot the pellet may exit while the barrel is in a different position relative to its starting place.
At the crest and/or trough there is a brief moment that the barrel stops moving. By definition that is the utmost extremity of its motion and it needs to stop before it can rebound and head back the other way. Since the barrel is stationary (or at least decelerating) at those points, that is when we want the projectile to exit. A stationary muzzle will always yield more consistent results than a moving one.
That is why I believe those fliers are caused by harmonic issues. The “core” group is good but because of extremely subtle changes in velocity and other variables, the muzzle is in a slightly different position than it was during the previous shot(s). A carbon fiber sleeve will hopefully “improve” the harmonics but will at the very least change them enough that it may improve the result…or not.
The point of installing a barrel sleeve is to stiffen the barrel and slow down the vibrations so that the sweet spots are longer. This increases our odds of having the pellet exit near the sweet spot. We want our airgun barrel to behave more like a bull barrel.
Here’s what this looks like. Red is a standard barrel, blue is sleeved/bull barrel. Again, this is not actual data.
The only way to find out if sleeving a barrel actually accomplishes this is to start gluing stuff to my rifle…what could go wrong?
Preparation and Methodology
Air Arms does not make an OEM carbon fiber sleeve so I took all the measurements and ordered a custom one. There are quite a few reputable suppliers who will make custom tubes. This one cost me about $45.
The tube fit the barrel perfectly. The barrel could slide freely into the sleeve with a little push. The sleeve will be epoxied into place. The only way to actually glean the possible benefits of a barrel sleeve is to epoxy or press fit the barrel into the sleeve. I do not believe that simply sliding the two into the gun is sufficient since the barrel/liner can still vibrate inside of the sleeve.
Ideally, the barrel sleeve should be recessed into the receiver. Simply butting the sleeve up against the receiver allows for hinging at that point. Recessing the sleeve into the breech effectively simulates a larger diameter barrel (i.e. stiffer, ceteris paribus)…which is what we are going for.
For most airgun designs, you would need to machine the receiver to accomplish this but not with the Air Arms S5xx series. There is a 10mm slot that the barrel fits into, but there is a 19mm long cavity that is ~15mm in diameter. This is the perfect host for a barrel sleeve.
Unfortunately, the carbon fiber tube was just a teensy bit too large in diameter and would not fit into the breech. Luckily, I have a machinist friend who sanded it down with his lathe and ensured everything is concentric.
Notice how little material was removed here. This gives a nice tight fit.
For epoxy, I used Devcon 31345 2-ton which fills gaps better than JB Weld and has a work time of about 30 minutes.
The final step was to prep the barrel. It should go without saying that I needed to pull it out of the gun before going about any part of this installation process.
I sanded the exterior of the barrel with 100 grit sandpaper to rough it up a bit and give the epoxy something to grab.
Installing the Sleeve
Installation is relatively straight forward. All you need is the epoxy, the sleeve, acetone, and a tongue depressor.
I did not take pictures while I was doin’ the gluin’. Frankly, I was terrified that the epoxy would start to cure before I was finished…sorry.
Here is the installation process:
Step 1: Clean the outside of the barrel with Acetone. Removing all oils and dirt is essential.
Step 2: Spread epoxy onto the first half or two thirds of the barrel. Less is more. As you push the sleeve onto the barrel, this will gob up. Spread excess epoxy onto the rest of the barrel as you go.
Step 3: Begin pushing the barrel onto the sleeve. Rotate the sleeve while pushing to ensure that the epoxy fills all the gaps and is evenly distributed between the sleeve and the barrel. Stop every two inches or so to spread out excess epoxy.
Step 4: Once the sleeve is fully in place give it another rotation or two. Clean all excess epoxy off using acetone and a rag. Don’t miss any epoxy…this could prevent the barrel from fitting back into the action or liner back into the barrel.
Step 5: Check fit and let it be! The epoxy needs time to set and cure.
Installation went smoothly. After inspecting the barrel, it appears that the fit is basically perfect.
I let the epoxy cure for 2 days. The barrel was sitting in a vice with minimal compression. Just enough to keep the barrel from falling out.
Here is the sleeved barrel installed back into the action. It’s pretty sexy if I do say so myself.
I made one small “executive decision” here. I decided to leave a small amount of the carbon fiber sleeve hanging over onto the 10mm muzzle end. By doing so, I can introduce a small amount of compression to the shroud guide, baffle assembly, and the barrel by completely tightening down the shroud. This allows me to further “fine tune” the harmonics by tweaking the shroud.
I have no idea whether or not that will be beneficial. It’s just a hunch but it’s easy enough to sand down if it sucks.
Nothing Ventured, Nothing Gain
Immediately, I had very interesting results. Here are four five-shot groups fired back-to-back at 50 yards. The only thing I changed was how tightly the shroud was screwed onto the receiver. Group 1 was tightened all the way and each successive group was loosened by 1/8th turn.
Groups one and three were similar to the pre-sleeve groups. Groups two and four however, were much better. Group two is outstanding – about .3” CTC at 50 yards WITH the JSB 34gr. This target is fascinating. You can actually see the various harmonic nodes manifest.
Here are a few more 50 yard groups shot a week later. These groups were shot while confirming my dope for EBR, otherwise, they would all be on the same target. All of the 50 yard groups are excellent. ~MOA or sub-MOA. The innermost thick black ring is ½” OD.
I am extremely happy with how this project turned out. It seems that in this specific case, installing a carbon fiber sleeve was the correct path to take. However, your mileage may vary. I have a hunch that the success of this project had a lot to do with the design of the Air Arms S510 breech, which allowed the sleeve to be recessed properly. A carbon fiber sleeve will likely not improve your situation if your tune is not in order or if you are trying to shoot ammunition that does not jive with the barrel.
I think it should also be said that higher power rifles stand to benefit much, much more than lower powered ones. The more power you are asking of your rifle, the more vibration you are introducing and thus the more room for benefit exists. There are exceptions to that, of course, but for most airgunners I think that holds true.
Based on sample size of one though…I wouldn’t hesitate to install one of these on another high power build.