Twist rates explained

[hildo]

I have been looking around a bit on twist rates in barrels, and it is not very clear to me.
A 1:48 twist, this means a 1 turn of the bullet (360 degree) on 48 inch barrel length. Is this correct?

Also noticed that on BP guns twist rates seeme to be slower. I saw on nitro rifles people talk on 1:7 twist rate, with .223 bullets, which seemes extremely quick.
Why the twist difference between BP and nitro guns?
Also have read that basically how heavier the bullet, the faster the twist rate should be. This messes up the statement above with the 1:7 twist rate on that small .223 bullet.
This seemes unlogical since when a large bullet is spinning it will keep rotating longer than a light bullet.
Then there seemes to be a difference between a round ball and an elongated bullet and howabout patches? Underspinning and over spinning seemes to give a bad accuracy. How fast a bullet spins obviously also has something to do with what speed it travels down the barrel too. How can one determin the right spin rate with a certain bullet at a certain speed?

The more I read the more unclear it becomes. Think I'm missing the basic point somewhere on how it works.

Can someone help me to get some clarity on these mysterious twist rates?

 

[Steve499]

I'm not qualified to explain all the questions you have, Hildo, but the way I have understood twist rates is that the longer (therefore heavier) a bullet is for any given caliber, the faster it needs to spin to stabilize. The slow twist rates commonly found on muzzleloaders is probably due to a patched round ball's tendency to not take the rifling if it's required to spin too fast or if it is propelled too fast, and a round ball tends to fly fairly true even if not spinning. Also, the thin skirted, pure lead minie bullets won't engage in too fast a twist, they strip on through if they aren't loaded very lightly.

The bullet's stability during it's flight can degrade after a certain velocity loss has occurred with small increments of increase or decrease to the bullet's length deciding whether it will be likely to yaw after travelling, say, 800 or 1000 yards.

There's all I suspect about it. Someone else can tell you the way it REALLY is, maybe.

Steve

 

[esmith]

Im not too sure on it either but from the research i have done there is a clear optimum round for certain twist rates. If an extrememly light bullet is traveling through a fast twist rate then its overstabilizes and can basically fall apart during flight due to high gyroscopic forces. Certain factors of the round define optimum twist rate (length of bullet, grain weight, etc.) I believe a professor at MIT made a formula that can be used to find the best twist rate for a certain round.

 

[Jamie C.]

Howdy, Hildo.
I'm gonna take a swing at this, and others can correct or clean up what I get wrong, or don't explain clearly enough.

You're right that a 1:48 twist means that the projectile makes one full turn in 48 inches. A 1:7 twist would obviously mean a full turn in 7 inches.

The reason for various twist rates is that the rates are matched to a particular bullet weight, shape, caliber... and velocity.

As I've mentioned elsewhere, how fast a bullet travels establishes how fast it spins, given a particular barrel twist. One traveling at 1000 fps is not only moving forward more slowly than one with a velocity of 3000 fps, it's also spinning more slowly, if both were fired from the same barrel.

Also, large heavy bullets seem to require less spin than small light ones. And I believe the shape of the projectile may have something to do with it too.

Searching through a catalog of modern-made muzzle-loading rifles, you'll see twist rates vary not only with caliber, but with the intended projectile as well. Usually a .50 caliber rifle will have 1:60 for round ball, and 1:48 or 1:32 for modern bullets and sabots.

Anyway, that's about as much and as well as I can explain twist rates.

Hope it helps.

J.C.

 

[alucard0822]

Twist rates are basically nothing more than solutions to physics problems. The greater the mass, the better spin momentum will be maintained after the bullet leaves the barrel, the larger the diameter (caliber) the greater the centrifugal force exerted on the bullet for the same RPM, the longer the bullet, the more force required to stabalize. Problems from too slow a twist are obvious and normally key hole shaped, too fas of a twist can result in separating components from speeds in excess of the "normal"(for 223) 250,000 rpm a bullet turns at, but accuracy can also suffer from overstabilization, and the target may have elongated holes similar to under stabilized bullets, the bullet travels in a degrading radius arc, where say for instance the bullet leaves the barrel pointed 15 degrees above the direct line to the target, if the bullet has so much gyroscopic force that it remains at this angle of attack the slipstream and stability in general are affected, the heavier rear carries more mass and is deflected less moving through the air (similar to why they yaw and tumble when they go through flesh) and as the radius of the arc decreases the bullet is angled more and more away from the path of travel and goes through the target at an angle. Basically a manufacturer has to use a twist that will stabilize a bullet over a wide range of distances without over doing it, and a certain range of velocities resulting in different rpm, that is why some bullets seem to "like" a certain velocity for a given twist rate.

 

[Omnivore]

Yes: A 1:48 twist means one revolution in 48 inches. Barrel length and twist rate are independent figures.

Basically, the longer the bullet, the faster the twist required to stabilize it, and it is possible to over-stabilize a bullet. Hence, optimally, the twist will be tailored to a specific bullet weight, or to put it another way, you will select the ideal bullet for your rifle’s twist. Its usually no big deal to slightly over-stabilize a bullet, so a round ball in a .50 barrel with a 48” twist for example, though over stabilized, will work fine. Using a long, heavy bullet with a 60” twist will probably not work too well, resulting in bullets hitting at odd angles (keyholing) and poor accuracy.

 

[hildo]

Things are getting are slowly getting a little clearer

So, a big heavy bullet needs much less rotating speed than a small one to stabilize.
Does the length of the bullet make a difference in the appropriate twist rate or is it the weight only?
Maybe the actual caliber makes a difference too or is it just weight and/or size?
How can one determin the right twist rate for a certain bullet?
Are there twist rate charts for BP guns which include the typical heavy BP bullets? Or maybe somebody knows a formula, which seemes to excist as stated earlier by stealz10, so one can calculate the perfect bullet for the twist in a pistol or rifle that you already own?

Is there a twist rate difference between nitro & BP guns?
I have the idea that twists for nitro guns are faster?
If yes, does this have something to do with the deeper rifling commonly found in BP weapons or is the deeper rifling only to acommodate the BP residue?

 

[Jim Watson]

"Does the length of the bullet make a difference in the appropriate twist rate or is it the weight only?"

The other way 'round. The length of the bullet is the governing factor. In modern rifles, a boattail spitzer requires a faster twist for stability than a flatbase roundnose of the same weight because it is longer.

The reason big old guns get by with a slower twist is that the length-to-diameter ratio contributes a lot. If you go by the Greenhill formula, everything is based on bullet diameters, not inches. The Greenhill formula is given many places on the Net and you can get some useful approximations from it. They are only approximations, real bullet stability takes a LOT of computation, but since it was derived in the 19th century, it will be closer for an old gun than a new one.

Also, it was usual to not give a black powder firearm any faster twist than was absolutely necessary, to keep from accumulating fouling. The .30-30 which has always been smokeless, has a 12" twist rate. The .32 Winchester Special shoots the same weight and only slightly shorter bullets from a 16" twist barrel because it was intended to be sold with smokeless powder but reloaded with black because the early smokeless powders were thought too dangerous for handloading.

 

[arcticap]

Hildo, understanding the twist rate isn't so complicated .

Let's look at the round ball twist rate:

These 2 barrel makers use their own 2 formulas for finding their optimum twist rate for a round ball for any given caliber.

Cox's formula is: pi x bore diameter divided by .029
Ed Rayl's formula is: pi x bore diameter divided by .02618
pi = 3.14159

So, let's consider the .50 caliber round ball.
Cox's formula says that the optimum twist is 54.165
Rayl's formula says that the optimum twist is 59.998

Now many .50 round ball guns have a slower 1 in 66" twist. This means that the .50 round ball will usually need just a little faster velocity and rpm's to have the best accuracy. And this holds to be very true, most slow twist round ball barrels do require more powder for best accuracy.

When the twist is faster, the velocity of the ball usually needs to be slowed down a bit to obtain better accuracy. But sometimes the accuracy is just not quite as good as with a slower twist.

One of the most popular .50 caliber barrel twist rates is the 1 in 48" twist, which is a medium twist. The medium twist is designed to shoot both round balls and conicals. But this medium twist doesn't like long, heavy conicals, it likes medium size conicals, or shorter.
Medium twist likes medium conicals because the twist rate isn't fast enough to stabilize long, heavy bullets.
A medium weight .50 caliber conical will probably be around 300 grains maximum, maybe even a little less. Some folks shoot 350 grain conicals though and get great accuracy, but it depends on the bullet and the velocity, etc...
A .45 caliber bullet used in combination with a .50 caliber sabot is going to be longer for it's weight than a .50 caliber bullet of the same weight. So then a 250 grain .45 will approximately equal a 300 grain .50 in length (I'm guessing).

One of the Greenhill formulas allows for ascertaining the maximum optimum bullet length for shooting a known caliber bullet out of a barrel of a known twist, but there are limitations to the formula, one being that it wasn't designed for lead bullets. But none the less, it states that a .50 caliber bullet shot out of a 1 in 48" twist barrel shouldn't be longer than .78125 of an inch to be properly stabilized. Now, for practical purposes, that's about 1 inch. And it also doesn't take bullet shape into account. But you get the point, if a bullet is too long for the twist rate, it just won't stabilize as well, at least not over a longer distance.

http://stevespages.com/page8e.htm

Most modern rifles are smaller calibers shooting longer for caliber bullets, so the twist rates are usually always faster. Just like the twist rate for a smaller caliber round ball will be faster than for a larger caliber round ball. It's all based on bullet length, but bullet weight is usually a good indicator of bullet length.

Fast twists can stabilize longer bullets.
Medium twists can stabilize medium length (& weight) bullets and round balls.
Slow twists can stabilize round balls and the shortest length (& weight) bullets (sometimes).

 

[alucard0822]

I use min twist rate=([velocity/17]Xdia^2)/length

this is about the best greenhill formula adapted to modern jacketed bullets of various velocities, you can swich twist rate and lengths positions to solve for one or the other

by manipulating numbers you can see how dimentions affect twist, mass is somewhat implied as a similar calculation to the old pi*R^2 area formula

this is the formula for muzzle RPM=(twist rateXFPSX720) where twist rate of say 1/7 is expressed as division. so for an M-16 5.56x45(3200fps approx.) with a 1 in 7 twist RPM=329,142RPM

so for my best shooting 200yd 308 target loads of a sierra 175gr hpbt(1.230") over varget at 2700fps in my 1/10 twist savage the "minimum ideal" twist rate would be 12.25, so 1/10 gives a little better stability, and the bullet leaves the barrel at 190,800RPM