What IS a good tuneup?
by B.B. Pelletier
As long as we're looking at tuneups, it might be good to discuss the goals of a good tuneup. So many times I've read the comments of beginning tuners, who haven't got an idea of what they are after except for greater velocity. I've been shooting spring-piston airguns for almost 30 years, and I think I can tell a good tune from a bad one. Perhaps reading this discussion will help some of our newer shooters set their sights on what is possible.
The spring-piston powerplant is the harshest type!
Compared to pneumatic or gas guns, spring guns vibrate, recoil and require lots of holding technique to shoot well. So, the chief goal of a spring gun tuneup should be to make the gun shoot smoother.
Anti-recoil is not the answer!
If you've never experienced a non-recoiling gun, you might think they're the solution to traditional spring gun problems. They do away with all recoil, which seems like a good thing until you actually shoot one and discover what is left behind. The vibration and the firing impulse is still there in most recoilless airguns. A Diana model 6 or 10 pistol may not kick, but you still feel the jolt when it fires. A Diana 75 rifle also has a jolt, as does an FWB 300. In fact, the 300 often has a bundle of vibration, besides. A Diana 54 is the same, with lots of jolt and some vibration when it fires. Even a handmade Whiscombe rifle, which is the gentlest of all recoilless airguns, still packs a good jolt on firing.
Therefore, the primary goal of a good spring gun tune is to eliminate the firing jolt and vibration. The recoil can remain, because, of the three forces, it turns out to be the least objectionable.
Tight tolerances remove vibration
The fit of the moving parts in a spring gun powerplant is what causes vibration. Also, things like a bent mainspring (see How long does a mainspring last? Part 1) or a bent spring guide can contribute to vibration. By making the clearance between moving parts the smallest space commensurate with good operation, you can remove much of the vibration.
Reduce the jolt by balancing the pellet to the powerplant
The other BIG trick is to use a pellet that moves at a time that prevents the piston from bouncing or slamming into the end of the compression cylinder. You can feel a good pellet by the LACK of firing impulse it has! Conversely, when a pellet makes a gun jolt, it's not a good idea to use it even if it's accurate. The ideal pellet will be both smooth-shooting AND accurate.
Power comes last
After you have smoothed the powerplant, you can look for more power. As long as more power doesn't make the gun harsher, it's okay. The goal for a spring gun tune should be smoothness, less recoil and power - in that order.
Accuracy never changes as these factors change. It's easier to shoot a smooth gun more accurately, so that might be an additional benefit.