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Slavia · Moderator

I was reading in another thread about "smooth twist" barrels:
http://www.airgunhome.com/agforum/viewtopic.php?t=7998&start=30

Wanting to learn more about them, I found this video from FX:
http://www.youtube.com/watch?v=xPF6dWAqfhY

I'm not sure I completely grasp this concept:
1. How does the smooth twist barrel "increase the ballistic coefficient"? (Is it choked so heavily that it extrudes the pellet with a higher sectional density?)
2. How does the smooth twist barrel make a pellet "less affected by external factors like wind speed"? (Compared to a pellet leaving any other barrel with the same angular velocity.)
3. What's special about the smooth twist spiral "forming the pellet into a projectile"? (I'm pretty sure I can do that to a rock without any gun at all.)
4. What is the definition of "precision ratio"? A ratio of what to what else?

Am I that far behind the times, or is this the Swedish equivalent of tethered wild boar hunting?

SMP · Silver Status Member

Lol...had to get in on the controversy, EH

Firstly in order to get properly conditioned for any conversation on "precision ratio" we must first go watch 8 hours infomercials

So We'll continue this conversation after that Mr. Green

radar · Silver Status Member

SMP · Silver Status Member

rsterne · Moderator

Well if it's the polygonal rifling that does all those good things.... you can also get that from LW.... they sell both conventional and polygonal rifled barrels....

Bob

WhitleyStu · Veteran Member

The following is my take on the ST barrel and just my 2 cents. Here is a video showing the inventor of the smooth twist barrel, Freedic Axelsson, making each ST barrel then testing it…
http://www.youtube.com/watch?v=V_HTQwrTG6s&feature=related
A smooth twist .22 has a twist rate of 1:156" or (1:13’). As you can see this is a very slow twist rate. It is this fact that lets the pellet move through the air with very little spindrift. In the video where they use the term “precision ratio” is probably a way to simply say less spindrift. “Increasing the bullet coefficient” might be FXs way to say the projectile can accelerate faster with little rifling in the majority of the barrel. I read up on the Smooth Twist before ordering my FX only to receive a rifle with a Lothar Walther barrel and now I am glad I did. Over on Benchrest Central the smooth twist is not highly thought of for an ultimate target barrel. The benchrest gents still opt for conventional rifling.

radar · Silver Status Member

Slavia · Moderator

The video says that Ben Taylor ("ben" of Theoben) came up with the idea. Axelsson is the founder of FX. From what I have been reading, The rifling in the last few inches of the barrel is not cut in the traditional way. Instead, a spiral clamp is applied to the outside of the barrel, causing smooth spiral "bumps" on the inside. No cut edges. Then the clamp marks on the outside are turned off and the lost metal is replaced with a screw-on tube.

I'll allow that this could enhance exterior balistics and retard lead fouling. It is also probably cheaper to manufacture than conventional rifling. The magnitude of those improvements over cut rifling will probably be seen as the design becomes more common.

My intent in starting this was not to put down the design, but rather was a reaction to the tone of FX's video. I detected the faint aroma of snake oil, and it made me wonder if I ordered immediately, could I get the Ginsu knives? But wait, that's not all...

rsterne · Moderator

If the twist rate is, indeed 1 in 156", then there is very little spin stabilization occurring.... We have been told for years that our diablo (waisted) pellets are drag stabilized to a large degree, so perhaps that is all that is necessary.... None of the twist calculators predict stability with such a slow twist rate, and since the Smooth Twist barrel works, then apparently the combination is working.... The very slow twist rate, and the (presumed) smoothness of the pellet as it exits the muzzle, could indeed influence the BC and the vertical dispersion in a crosswind due to the Magnus effect.... both of which are beneficial.... BTW, a pellet from a Smooth Twist barrel travelling at 867 fps will still be turning 4000 RPM....

You can bet that during the development process, smoothbores were tried, and then choked smoothbores.... and then they added/subtracted twist until they got stable flight.... Since I first heard about this idea I have always figured it was a way of producing barrels with adequate accuracy at lower cost than either drawn rifling or hammer forged.... Logic tells me that it's a lot cheaper to make a smoothbore and then squeeze the end that either of the earlier methods.... Before you jump on me for using the word "adequate" let me give you MY definition.... I would consider an airgun to have adequate accuracy if from the bench, in windless conditions, it can produce MOA groups.... While this may not be "match" accuracy, 99.99% of airgun shooters would never know the difference.... especially once you throw in the variabliity between pellets....

What I do find surprising is that other manufacturers have not jumped on the bandwagon.... Perhaps they are waiting until the Smooth Twist barrels start winning benchrest matches.... or perhaps there is a patent issue (although I can't see that stopping the Chinese).... Maybe the chance of producing a "poor" barrel is greater without extreme quality control, I don't know.... What I am very sure of.... is that if this is the way of the future it won't be long before everyone is doing it....

Bob

SMP · Silver Status Member

Slavia · Moderator

Take a look at about 6:30 - it looks like a tighter twist than 1:156":
http://www.youtube.com/watch?v=joguUbL35Ks

SMP · Silver Status Member

Ya it sure does

WhitleyStu · Veteran Member

rsterne · Moderator

I would agree that the photo certainly looks a lot faster than 1 in 156"....

Bob

mod90 · Veteran Member

I am compelled to ask a question. It seems as though the smooth twist barrel is essentially a spiral crimped smooth bore tube, with a really slow twist rate by comparison to most other barrels. So this is my question. Wouldn't it have been simpler to achieve the same or a more effective twist rate on the pellet by rifling the skirt of the pellet at a particular angle & depth prior to it entering the barrel, & leave the barrel smooth & choked tight?
Just a crazy idea i had.