Whiscombe rifles and barrel harmonics - Part 4
Long-range testing in .177
by B.B. Pelletier
We last looked at the Whiscombe on November 28, 2006. I said then that I wanted to use the rifle for a lot of other testing, and this is one of those tests. In fact, this is the test that started the whole series. Does harmonic barrel tuning affect accuracy?
What are harmonics?
Anything that vibrates does so at a certain frequency. A guitar string is often used to represent the harmonic wave, because it really does vibrate with a sine wave pattern.
When a guitar string vibrates, it does so in a sine wave like this. The places where the curved line crosses the straight line are called nodes. They are places where the least amount of movement takes place.
A guitar string changes pitch (the number of vibrations per second) when it is either lengthened or shortened. The space between the nodes grows shorter as the pitch becomes higher. If we were to increase the mass of the guitar string, that would also change its pitch.
A rifle barrel vibrates just like a guitar string when the gun fires, and you can change its pitch or harmonic frequency by adding or subtracting mass. Or, if there is a movable weight on the barrel, changing its location also changes the location of the nodes. If you can get the muzzle to be located at a node, there will be the least amount of dispersion of the bullets coming out. That's what the Harmonic Optimized Tuning System (HOTS) does. It's no different than Browning's BOSS, and they hold competitions for them.
The HOTS weight is screwed in or out and locked in position to change the location of the vibration nodes.
Starting with good pellets
I tested three pellets of known performance in an attempt to find the best one. The were JSB Exact domes at 10.2 grains, Crosman Premiers at 10.5 grains and Beeman Kodiaks at 10.6 grains. All three performed well, but the Kodiaks were slightly better than the other two, so they were the ones I selected for this test. That's not to say the other two pellets couldn't have been adjusted to be just as accurate, just that the Kodiaks were closer to start with.
Before the barrel was adjusted, the gun was shooting Kodiaks into groups ranging from just under .75" to just under an inch. It took about an hour of adjusting and shooting, adjusting and shooting, before I found the sweet spot. Fortunately, I've done this before so I know that I can make large adjustments until the groups start to shrink. Then, things have to be done in small steps.
This group of five Beeman Kodiaks at 50 yards measures 0.926 between centers. It's representative of how the rifle performed before the HOTS was adjusted.
I finally found the sweet spot and locked the adjustment in place. The point of impact walked around the target while I adjusted, because I purposely made no attempt to keep the group centered on the bullseye. It doesn't make sense to adjust the scope before you have adjusted the barrel vibration, because the shots will move with every adjustment.
With the HOTS properly tuned, my groups shrank to just over a half-inch between centers. That's pretty good shooting for 50 yards! It was at this time that I shot the Crosman Premiers, and, once again, just because the HOTS was adjusted for Kodiaks doesn't mean it's right for Premiers. The barrel has to be tuned for each pellet.
Big improvement! The HOTS really does allow you to "tune" the barrel vibration for better accuracy. This group measures 0.578" between centers.
Harmonics can be tuned
So, we've answered the question about harmonic tuning. It does make a difference that can be demonstrated with an adjustable barrel weight. Now, I have a different question to consider. Would there be a difference in accuracy if I speeded up the pellet? Since I planned to do that anyway for the Crosman Premier hollowpoints, it was easy enough to check on the Kodiaks at the same time.
Come back tomorrow when I show you what happened with that, plus a lot more!