Author Topic: Rifling Twist (Read 7815 times))

rsterne · « **on:** September 30, 2012, 04:00:25 PM »

For the most part, this is a complicated, and poorly understood subject.... It has always fascinated me, and the lack of good information on subsonic ballistics has been a constant source of frustration to me.... A while ago, I discovered a Twist Calculator in the Border Barrels (Scotland) website which works not only for supersonic flight, but also for subsonic and the transition in between.... It has numerous inputs, and I would suggest you check it out.... http://www.border-barrels.com/barrel_twist.htm

First of all, let me state that I am NOT an expert in this field.... I have, however, done a lot of reading on the subject, to try and get a grasp of what is going on.... The original "standard" method of calculating the optimum rifling twist was the Greenhill Formula which dates back to 1879.... There have been many derived since, and one of the better ones is the Miller Rule, published in 2005.... However, even with that modern formula (probably because he had no interest in it), all velocities below the speed of sound used a velocity of 1120 fps.... The Border Barrels calculator, however, presents the optimum twist in graphic form from 500 fps up, and according to their website:

Quote

This program is basically a slick version of Robert McCoy's "McGyro" DOS program, written in the late 1980's when he worked for the U.S. Army Ballistic Research Laboratory at the Aberdeen Proving Grounds. It was later improved by William Davis Jr. of Tioga Engineering and the claim was an accuracy of 5% for super and subsonic velocities, and 10% for trans-sonic velocities.

The basic output of the calculator is a graph for a Stability Factor of 1.5, but you can also input the twist you are interested in, and it then presents a second graph giving the Stability Factor for that twist.... A Stability Factor less than 1.0 is unstable, and greater than 4.0 is considered "overstabilized" which simply means that any imperfection (ie imbalance) in the bullet can cause "precession" or wobbling, which tends to open up the groups.... The military chooses Stability Factors of 1.5 to 2.5, and benchrest shooters often opt for only 1.3, but 1.5 is generally considered a good design point....

Let's ignore velocity and bullet shape for a moment.... There are two basic concepts you have to understand about rifling twist....

First, you have to realize that all the twist equations originally worked in "calibers".... The length of the bullet, and the resulting optimum twist, were relative to the diameter of the bullet.... For a given shape of bullet (ie length to diameter ratio), the required twist was the same IN CALIBERS.... If a 15" twist was the best in a .25 cal (with a .50" long bullet), then you need a 30" twist for a .50 cal (with a 1.00" long bullet)....

Second, if you increase the length of the bullet (in a given caliber), you need to spin it faster in order for it to be stable.... Imagine how fast you would have to spin a pencil to make it fly straight (without vanes or feathers) and you get the idea....

Once you understand those two ideas, you are well on your way to understanding twist.... In the following posts, I am going to give some examples, using the Border Barrels Twist Calculator to produce the graphs....

Bob

BALLISTIC1 · « **Reply #1 on:** September 30, 2012, 04:19:40 PM »

Very interesting Bob! It is so much much more interesting and easier to understand when someone else does all of the 'hard' work for you... I know that AR15/M16 use a couple of different twist barrels and more applicable and of interest to me is the SniperSubSonic .22 for 10/22.. I was in the local gunstore a couple of weeks ago and heard someone making comments about not having the proper twist to use the SSS round... I asked if they thought that they could shoot their rifle well enough to tell the difference.. They didn't have much of a reply, more a dirty look!!!! Greg

rsterne · « **Reply #2 on:** September 30, 2012, 04:22:33 PM »

OK, so let's first look at the idea that twist is related to caliber.... Let's start by defining our "standard bullet".... I'm going to use a cylinder with a hemispherical nose, like this....

Let's start with a .25 cal that is .50" long....

Now let's look at a .50 cal that is 1.00" long....

Note that for all velocities, the optimum twist for the .50 cal bullet is twice as long as for the .25 cal bullet.... That means, however, that the twist rate is the same IN CALIBERS.... At just under 1000 fps, the optimum twist in .25 cal is 20", while in .50 cal, it is 40".... In both cases, that works out to 80 CALIBERS....

Bob

rsterne · « **Reply #3 on:** September 30, 2012, 04:24:23 PM »

Greg, it is entirely possible that the SSS bullet (60 gr?) is too long to stabilize in the 1 in 16" twist common in .22 LR barrels and may very well tumble....

Bob

rsterne · « **Reply #4 on:** September 30, 2012, 04:39:04 PM »

Now let's look at the idea that you need a faster twist as the length of the bullet increases (within a given caliber).... Again, we will use our "standard" Round Nose bullet....

.25 cal that is .25" long (1 diameter)....

.25 cal that is .375" long (1.5 diameters)....

.25 cal that is .50" long (2 diameters)....

.25 cal that is .625" long (2.5 diameters)....

.25 cal that is .75" long (3 diameters)....

You will note that as the length of the bullet increases, you have to spin it faster to keep it stable.... The same thing would happen in a larger caliber, but the twist rates would be slower (in inches)....

Bob

rsterne · « **Reply #5 on:** September 30, 2012, 05:03:20 PM »

By now you will have noticed that there is a "discontinuity" in the graphs right at the speed of sound.... As the bullet breaks the sound barrier, shock waves form around it, and they can have a dramatic affect on the stability.... You have probably heard of pellets "tumbling" when they drop below the speed of sound, and that can definitely occur if the twist rate is too slow for the pellet to be stable right at Mach 1.... Here is a graph of such a combination of bullet and twist rate....

The graph was generated using a twist rate of 18", and I adjusted the length of the bullet until the stability dropped below 1.0 at 1100 fps.... Although the bullet would be stable in supersonic flight, and at low speeds, it would not be stable at 1100 fps, and even at 1000 fps, the Stability Factor is only 1.1, which may be unstable depending on temperature and altitude (ie air density).... Note that if the pellet was significantly shorter.... and/or if the twist rate was faster.... this tendency to tumble right at Mach 1 could be eliminated....

Bob

rsterne · « **Reply #6 on:** September 30, 2012, 06:30:03 PM »

This post will try and explain what happens when you change the shape of the bullet.... I don't fully understand it myself, and the over-riding factor in determining the twist is the length of the bullet.... but here is what I found out for a few examples.... All these bullets are .25 cal, 0.50" long (ie 2 diameters)....

.25 cal cylinder (ie shaped like a tin can)....

.25 cal Round Nose, with a Meplat (flat) of 0.125" diameter....

.25 cal Round Nose (as in the earlier post)....

.25 cal Pointed (nose radius extends all the way to the base)....

Other than just as the bullet breaks the Speed of Sound (where the optimum twist for all is about 23.5"), the fatter the nose, the faster the bullet needs to spin to be stable.... If we look at the optimum twist (for a stability factor of 1.5) at just below the speed of sound, we find that for the cylinder it is 16", for the round nose with meplat it is ~17.3", for the round nose, it is 18", and for the pointed it is 18.5".... That means that the blunter the bullet (and the more forward the CG) the FASTER it has to spin to be stable.... I guess the center of drag is more forward with the blunter shape (makes sense, now that I think of it) and moves aft on the more streamlined shapes so they are more stable....Note that our Diablo shaped pellets, with a solid head and hollow skirt, have the CG further ahead, and the CD further aft, giving them greater stablilty to start with.... Unfortunately, the Border Barrels calculator doesn't allow us to "thin out" the waist, or make a hollow base, and see what that does to the stability.... About all we can do is assume that it increases it.... That means we can PROBABLY get away with a slower twist than a solid bullet of the same length....

Bob

robert w · « **Reply #7 on:** September 30, 2012, 07:31:01 PM »

a caliber that had lots of trouble was a 6.5 carcano ,not a 6.5 sweedish mauser . they tried gain twist as well as normal twist . i owned a carcano long barrel and it was fast but not good for over 100 yards . it had gain twist . the twist is slower at the start and gains twist as it gets closer to the end of the barrel . bullets would just tumble . now a 6.5 sweed was and is a very fast accurate caliber . and the bullet lenght matched the twist . so bob you have a very interesting subject dug up . i will look that site over as you never know enough about how and why a bullet flies

rsterne · « **Reply #8 on:** October 03, 2012, 11:23:38 PM »

NT.

Bob

PakProtector · « **Reply #9 on:** October 04, 2012, 11:04:39 AM »

hey-Hey!!!,
Now I have also been thinking about this, and I saw an anomoly; the FX paradox/smooth twist barrels evidently put about a 44"/rev spin on the pellets..and that is just a wee bit out of the stable region and yet I have seen these shoot quite well. What int he heck is going on?
cheers,
Douglas

Powder burner · « **Reply #10 on:** October 04, 2012, 11:31:11 AM »

QVTom · « **Reply #11 on:** October 04, 2012, 11:58:36 AM »

Quote

the FX paradox/smooth twist barrels evidently put about a 44"/rev spin on the pellets.

Douglas, I see this number a lot but don't think it is correct or verified. Unfortunately, FX doesn't feel it is necessary to reply to questions on their twist rate.

Tom

PakProtector · « **Reply #12 on:** October 04, 2012, 12:22:51 PM »

Quote from: QVTom on October 04, 2012, 11:58:36 AM

Quote
the FX paradox/smooth twist barrels evidently put about a 44"/rev spin on the pellets.

Douglas, I see this number a lot but don't think it is correct or verified. Unfortunately, FX doesn't feel it is necessary to reply to questions on their twist rate.

Tom

FX does not see fit to resond to lots of inquiries.

It should be fairly easy to tell if there is any possibility of the pellet engaging the rifling at the end. It is moving at speed V, it needs to accelerate to rotation rate W in so many microseconds...T=I*Alpha. Alpha will of course be huge. Given even a small I, I'll bet is will significantly exceed the shear strength of pellet lead. Since it isn't going at the groove/land rate...how much slower is it going? That ~44"/rev looks entirely possible from here.
cheers,
Douglas

Boundary conditions: 850 fps pellet speed at rifling entry. 4" rifling. .00035 seconds to traverse rifling. assumed revolution rate equal to 18"/rev rifling.

Pellet will have to accelerate to 34k rpm in .00035 seconds or 10.2 Million radians/second^2. The FX rifling twist is not an accelerating twist so the pellet *WILL* skid and grip available to exert torque will drop with increasing deformation.

rsterne · « **Reply #13 on:** October 04, 2012, 12:30:12 PM »

RE the FX twist rate: I saw the twist rate quoted as 1 turn in 13 FEET (156") but have no idea what the actual number is, nobody has been able to get an answer from FX.... If the twist rate is that low, then the primary factor in the stability must be the diablo shape of our standard pellets....

Bob

rsterne · « **Reply #14 on:** October 28, 2013, 01:15:13 PM »

To compliment the recent thread I started on Bullet Design and Drag.... http://www.gatewaytoairguns.org/GTA/index.php?topic=54796.0 .... I thought I would expand on this thread.... First up, adding a Boattail to our "standard" bullet.... Once again, a .25 cal bullet, .50" long, with a hemispherical nose, but this time with a boattail half the length of the bullet and half the caliber at the base....

Compare this to the graph without the boattail....

Now I realize that this is a very extreme design, but look at what happens to the required twist with the boattail.... At just supersonic, it goes from 23.5" to 15".... At just subsonic, it goes from 20" to about 8".... and at 500 fps it changes from 22" without the boattail to 16" with it.... Although the CG is further forward in the boattail design, the Drag (Center of Pressure) is further forward as well.... It's like trying to throw an arrow backwards, you better be spinning it VERY fast....

More to come....

Bob

rsterne · « **Reply #15 on:** October 28, 2013, 01:50:50 PM »

OK, so let's get a little more realistic on bullet design, things that are practical.... How about a 2-Ogive bullet with a 50% Meplat of the same proportions as above.... ie .25 cal x 0.50" long.... with the nose one caliber long (0.25") and a 0.125" Meplat.... This design lends itself to a Hollowpoint with no profile changes, but for now let's just look at a pure lead bullet with a Specific Gravity of 11.4....

We're going to concentrate on the subsonic portion of the stabilty curve, ie only the area to the LEFT of the discontinuity at Mach 1.... In the recent sister thread, we determined that pushing bullets past 0.8 - 0.9 Mach (900-1000 fps) is literally a drag anyway, plus it takes a ton of air for little reward anyway to go transonic....

This bullet requires a bit faster twist than our "standard" round nose design, 18.8" at 1000 fps instead of 20".... Now let's see what happens when we add a boattail that is 80% of the caliber in diameter (0.20") and 20% of the bullet length (0.10")....

Wow!... The required twist at 1000 fps changed from 18.8" to 14.5".... and that's not a very exaggerated boattail.... However, the subsonic drag decreased by about 45%.... For a discussion of that, look in the other thread.... I'll mention one other thing before leaving this post.... If you make any bullet design into a Hollowpoint, the required twist rate increases.... You are lightening the bullet, which reduces its SG, and hence it has less gyroscopic stability.... I have found that a lot of HP designs are 10-12% lighter than the equivalent solid.... That means their SG is about 10.0 instead of 11.4.... substituting that in the calculator for the boattail design above requires a twist of about 13.5" at 1000 fps (instead of 14.5"), and for the non-boattail version it increases from 18.8" to 17.5".... The actual increase in twist rate wouldn't be that much as you are removing lead near the centerline of the bullet where it has less rotational moment of inertia.... so somewhere in between those values would be correct for a hollowpoint....

Bob

QVTom · « **Reply #16 on:** October 28, 2013, 05:05:36 PM »

Interesting stuff! Having fun

Your last graph sheds some light on the suitability of the Pile Drive pellet. (loosely applying the 25 cal. data to 22 cal.) Stability goes south near 600 fps with a 1 in 16 twist. Does this explain why they are reported to perform poorly in HP PCPs like the Rainstorm?

Tom

rsterne · « **Reply #17 on:** October 28, 2013, 08:11:54 PM »

Just measuring the .22 cal Piledrivers off my screen from a photo, and then putting the numbers into Border Barrels Twist Calculator, it shows a 16" Twist as optimum at ~800 fps and below should be fine.... However, it also shows them going unstable from about 1000 fps to Mach 1.... With an 18" twist, they have a SF of less than 1.5 all the time, and go unstable at 900ish....

Bob

Powder burner · « **Reply #18 on:** December 19, 2013, 03:45:21 PM »

what is "NT"?

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Rifling Twist

Author Topic: Rifling Twist (Read 7815 times))