Whiscombe rifles and barrel harmonics – Part 4Long-range testing in .177

by B.B. Pelletier

Part 1
Part 2
Part 3

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.

Baseline test
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.

Sweet spot!
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!

18 thoughts on “Whiscombe rifles and barrel harmonics – Part 4Long-range testing in .177

  1. B.B.
    I recently perchased a Beeman R9 (20 cal.)for hunting and couldn’t be happier. It is a little “buzzy” though- as you stated in your review, but if you can hold it right it is extremely accurate. My question is in the instuctions it says to not leave it cocked for an extended period of time- to cock it just before firing. This is problematic when you’ve just spent an hour stalking a rabbit or squirrel and then your going to risk spooking it by cocking the gun? Is a couple of hours to long to leave it cocked? Second, this gun is a bit heavy, which is great when shooting but tough to carry in the woods all day- what are my options when it comes to slinging it. Barrel bands would seem to affect accuracy and weaken the barrel hinge eventually- besides being ugly. What about putting a sling swivel stud near one of the forward stock screws, on the side of the gun? Thanks for any info.- great site.


  2. Tom Gaylord did a test in which he evaluated four different mainsprings left cocked for an entire month in an R1 – a Weihrauch factory spring, a Venom, a Maccari and a Beeman Laser. Thre worst spring still had 93 percent of its original power after 735 hours of being fully compressed.

    Your sling Idea sounds okay, as long as the weight of the rifle isn’t too great for the forearm. Those wooden “fingers” on either side are thin, so you don’t want to overload them.

    What about a hunter’s carrying strap that’s just tied around the barrel? Looks bad, but it works. If you carry the rifle with the scope upright, there will be less strain on the hinge pivot.

    B.B.



  3. Don,

    A weight must be attached to the barrel so it can be positioned along the barrel’s axis. As it is moved, the nodesx move and you tune the harmonics.

    You test it by shooting groups at long distance.

    B.B.


  4. I have several gamo air rifles. I really like my wood stocked hunter but my recon is accurate on some days, and shoots bad on others. At 8 yards it can put 3 shots overlapping but sometimes struggles to stay with in a nickel. Any suggestions. Also the recon’s trigger is pretty bad. I’ve heard gamo triggers are adjustable, how do i make it so there is actually two stages instead of one loooooong single stage. The hunter’s is ok (its also plenty accurate: I can hit a shotgun shell at 30 yards). Thanks

    hb


  5. HB

    I have a gamo cfx that probably has the same trigger as your recon. Theres a small screw behind the trigger. Take a screw driver and turn it one way to the max and shoot some shots. Then turn the screw all the way to the other way. I did it this way because doing it in small amounts is hard to notice. I dont know if this make the trigger lighter or just quicker to letoff but I noticed it was easier. If you havent already tried this I think it will help.

    Kyle



  6. You know, This blog simply turned me green with envy on 2 levels…
    1. The rifle. Now I want one (with all the barrels) but I imagine it is not exactly a “Budget” weapon
    2. 1/2″ groups at 50 yards.. Are you sure your not shooting 22lr?? :P

    Really interesting post though. nice work.




  7. ah, so that’s how it works. not a gimmick after all! and when you think about the vibration from the springs, this system must be that much more useful. so then, i must ask, why do some of the elite (i.e. law enforcement, olympic biathlon/10m, etc) still shoot free floating barrels? isnt it just attaching a threaded weight to the end of the muzzle? with a quarter inch improvement, it’d be worth the trouble!


  8. free floating a barrel means having the very end of the barrel (chamber) touching the action. The rest of the barrel is not touching anything ei stock. That way the barrel doesn’t bounce of the bedding so it’s more consistent. It doesn’t make a difference in pcp barrels because there is no recoil. The anshutz 2002 ca i shoot has a barrel band and it will always shoot a pellet through the same hole. I’ll be shooting that gun in Akron Ohio this summer in nra precision air rifle.

    hb

    not going to Ft. Benning but possibly to bowling green


  9. Thanks for the price check… that is on the higher end of price, still it seems to be on the bleeding edge of arigunning, and it really is tempting to cancel christmas and get one. It would be hard to justify hitting that can every time as opposed to most times for $4,000. The wife may just kill me first.


  10. hi again
    i was wondering if a higher twist rate (say 8 to 1) would have any stabalization effects over a slower twist rate (say 15 to 1) in an airgun or powderburner. sorry it just been bothering me for a few days
    thanks


  11. hb,

    yep, i read the same thing. but if the BOSS system is proven, then one would think it would be simple to emulate, to machine two chunks of steel, thread them, and weld it on to a slip-on piece. system decreases accuracy, slide it back off. it works, leave it on.
    as shown in this posting, this reduces barrel movement as much as possible. the free float alone just stops it from vibrating against the stock. and powderburners create plenty of vibrations to deal with, i imagine, with the rapid explosion of powder and the force of the bullet being engraved (increase in barrel diameter) and launched out of the barrel (decrease in barrel diameter). its a good idea, this BOSS system. better yet since it actually works, unlike some other things (air pistol compensators, for example.)



  12. Twist rate,

    Yes, the twist rate means everything to stabilization in a firearm. Diabolo pellets are much less affected, but solid pellets are bullets and rely on the twist rate, alone for stabilization.

    The science of twist rates has been deleoped over the past 200 years to the point that it is possible to calculate the correct rate for a given bullet caliber, weight and length.

    B.B.


  13. It is even possible to “evaporate”
    (centrifugal disintigration) a bullet which has happened in .22 cal. Think about it. 1:9 twist rate, 4300 fps = 38,700 rpm = brass/lead dust at end of barrel. E


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