Barrel rifling technique vs. accuracy.

[Alan]

Subject matter here, and on other airgun forums, proves there is a lot of interest in how a barrel is rifled. They're all covered here (https://en.wikipedia.org/wiki/Rifling), except for smooth-twist.

A lot of folks swear that smooth-twist barrels are the epitome of accuracy. I really doubt that is true. And, I look at them from different perspectives. First, it is easier and cheaper to rifle just part of the barrel than to rifle its whole length. If the technique was actually superior to full-length rifling, everyone would be doing it. The same goes for the other, shall we say irregular rifling (i.e.: oval vs. straight; hammered vs. cut; gain-twist, some swear are superior. 

It is obvious that a lousyily-made barrel won't be very accurate. That's a given! However, a well made one will only be accurate if the correct velocity, twist rate, and projectile are used. That's also a given! So the question remains, why all of the interest in smooth-twist rifled barrels?

 

[broekzwans]

I don't think the smooth-twist barrel in particular is more accurate, I think in general the rifle behind it makes the difference. FX rifles are pretty much tuned right out of the factory, a lot of other air rifle makers just make air rifles and test if they shoot and you have to do the finetuning yourself (if you care about real accuracy).

As you've said all well made barrels will be accurate as long as you use them right. But the consistency in FX rifles is what makes them tick (there are exceptions of course). Also the smooth part of the barrel has less resistance so less horse-power is needed to speed up the pellet. That they therefore use a lower twist rate at the end is also pretty logical too I think, applying the torque to get the pellet rotating right from the start is easier than when it's already flying 900 fps. If the twist rate would be too high it'll just fly straight through the rifling I guess and it'll probably be as unstable as can be . (I think the smooth twist is not usable for firearms for this reason, well maybe for rimfire (22LR) since their velocity and twist rates do not differ that much from most airguns). Apparently the twist it gets from the smooth twist is enough to stabilize the pellet for it's flight.

 

[Guest]

Because of the design of a pellet the center of gravity (CG) is ahead of the center of pressure (CP). That is the opposite of a bullet. There is therefore no lever arm working to upset the pellet and start it tumbling. Rather the lever arm, in the case of a pellet, tends to aggravate spiraling. A pellet is far easier to stabilize and, because the CP is behind the CG it is easier to over stabilize a pellet than a bullet. Assuming that a pellet is balanced with respect to the center of the bore (the CG and CP are coaxial with the bore) extreme over stabilization results in a pellet which is not flying nose on at long distances. When over stabilized an oscillation can be set up between the gyroscopic effect (spin) and the lateral pressures against the skirt (drag) when the pellet "tries" to stay pointed along the stabilization axis. If either the CG or the CP are not perfectly located on the center of the bore (the pellet is not balanced along it's longitudinal axis) there will be a spin rate to velocity ratio which results in the pellet spiraling. Slower twists avoid this problem by relying more upon drag stabilization.

Pellets are subject to the same gyroscopic drift gremlins as bullets. These problems are exacerbated by the lower sectional densities of pellets as well as the resulting lower BCs which lead to long flight times at shorter ranges. Smoother bullet surfaces are less affected by these gyroscopic effects because they create less drag and therefore less lift. Smooth twist barrels do less damage to the surface of the pellet as it is engraved by the "rifling". There is less "lift" created by the spinning pellet and therefore less gyroscopic drift. This is why these smooth twist barrels are winning bench rest competitions. Gyroscopic drift IS significant and variable for outdoor shooters even in air gunnery. This is especially true of bench rest shooting where a quarter of an inch might as well be a mile. Even a very light shift in wind direction or velocity can cause a miss when your target is smaller than the pellet you are shooting. It is that "drift" which can not be accounted for by simply zeroing your rifle.

 

[aa_limited]

 if you get a high quality slow twist barrel of regular lothar walter.. well then the st is not more accurate.
more of like being equal

 

[Alan]

Unless I missed it, the winner of the last 4 bench rest shoots (pellets) all used full-length rifling. Two of those were Lother's. That sort of speaks volumes.

 

[ncstan]

Extreme bench rest winner 2014 USFt,2015 FX boss,2016 FX impact.Usbr competition dominated by full twist LW barrels.Alan as you stated several factors twist,pellet,power factor determine accuracy in quality barrels.I own a FX Royale 500 that is a ragged hole at 25 yd.I also Own a WAR Flex 30 cal that just as accurate at 25 yd and better accuracy past 50yd.Quality is Quality whether it is full twist or smooth .

 

[hasenpfeffer]

"Alan"If the technique was actually superior to full-length rifling, everyone would be doing it.

Maybe they will, after the patents expire.

 

[Guest]

The FX process for the adding the twist to their tubing blanks may be patented but the smooth twist type barrel has been around for 100 years.