Twist Rate

[BuckRub]

I hear different ones shooting a certain bullet from 100 yards to 400 yards then going to a bullet that is 60 grains heavier ( just an illustration) from400 to maybe 500 yards then say they go another 60 grains heavier to go from 600 to 1000 yards. ( the example is for a .223 )
I don't know but usually with my guns I find a certain bullet/powder combo that my gun loves and can shoot almost a ragged hole and use it for ranges from 1 to 600 yards, just have to aim higher or mil-dot to get to my destination. I feel you can actually sometimes jump in weight say 10-20 grains for a .223 and your twist rate can't handle that weight. Or what I'm trying to say is it can't shoot them accurately. If your gun can shoot multiple weights all accurately, care to explain ?
Shoot, I even tried loading a 22-250 with 55 bullets because I load 55 for 2 other .223s I got. All I could get that 22-250 to do was 3/4 inches. I bought some 52 grain bullets and starting playing with my powder and volia ! I just don't see guns shooting that big of a difference accurately.

[Bart B.]

BackRub, here's some insights on twist rates versus bullets and some things that will cause problems comparing them to each other...

Bullets need be at least a few ten-thousandths larger than groove diameter for best accuracy. If their diameter's smaller than the barrel's groove diameter, no twist rate and muzzle velocity will make them shoot really accurate. Across a bunch of bullets of the same weight and quality but with a small range of diameters, those smaller than groove diameter won't cut the mustard. Often, this is the cause of poor accuracy with otne make/model of a given weight bullet compared to others that shoot very well.

For any caliber, with bullets, well made and balanced, there's about a 30 to 40 percent spread in twist rate that they'll all shoot very accurate from a good barrel in a well built rifle. Best proof of this is 155-gr. 30 caliber bullets shot from 30 inch barrels leaving around 3000 fps from 1:10 through 1:14 twist barrels and shooting sub MOA at 1000 yards.

Bullets need to be spun at least fast enough to keep them stable but not so fast that their long axis doesn't stay parallel to the trajectory. Depending on the bullet weight, shape and balance, there's an rpm range that they'll shoot very accurate with. You have to use the right muzzle velocity and twist rate to get that. RPMs = muzzle velocity X 720 / twist rate. 3000 x 720 / 12 = 180,000 rpm.

For the example above, those 30 caliber 155's need be spun between 154,000 and 216,000 rpm. In warmer weather when the air's thinner, slower rpm rates will work well. When it's cold, they need be spun faster. Benchresters add 1/10th grain or more powder in cold weather compared to when it's hot; they shoot their 22 and 24 caliber bullets spun just enough to stabilize them in the atmosphere depending on how dense it is for ranges up through 300 yards. But these stoolshooters look for a few hundredths MOA change in accuracy at short range.

[tangolima]

For any caliber, with bullets, well made and balanced, there's about a 30 to 40 percent spread in muzzle velocity that they'll all shoot very accurate from a good barrel in a well built rifle.

30% to 40% spread in muzzle velocity is a lot. It is like 3000 +/- 900 fps. Did you mean 3 to 4%? Thanks.

-TL

[precision_shooter]

I think it's more like 20-30%, but definately greater than 3-4%.

In Bart's example, the 155gr 30cal bullet will work well being spun from 154,000-216,000 rpm which is variation of about 28% in rpm which equates to muzzle velocity. 3,000fps down to 2,100 or so...

[Bart B.]

Thanks, tangolima, for this alert.

I was referring to 155's all shot out at 3000 fps from 1:10 through 1:14 twist barrels. They've got to leave that fast to stay supersonic through a thousand yards.

But I goofed; typed in "muzzle velocity" instead of "twist rate." I corrected my post. Shame, shame on me. Please don't take me out behind the wood shed.

[57K]

It's specifically for their bullets, but if you go to the Berger website they have a calculator that gives the optimum twist rate for specific weights/calibers. Should at least give you a general idea.

[Unclenick]

Go to the JBM ballistics site and there is a stability estimating calculator that lets you enter bullet caliber, weight, and length, and velocity, and barrel twist, and atmospheric conditions. The answer comes out as a gyroscopic stability factor number. In general Sierra says the best match shooting will come from gyroscopic stability factors of 1.4-1.7 in match shooting, and 1.3 to 3.0 for hunting accuracy. These are based on making a compromise between the fact spinning faster makes the bullet more stable but also increases its wobble due to any tiny mass distribution asymmetries it may have. As a practical matter, you'll find many modern match bullets are made so well that they still don't wobble much even spinning with a stability factor of over 2, but if you were picking a twist rate you would not intentionally opt to spin them that fast.

You need bullet length for the calculation, and the JBM site also has a list of bullet lengths.

[precision_shooter]

According to that calculator, I wouldnt even drop below 2 unless I step up to a 208gr AMAX in my .308...

[tangolima]

But I goofed; typed in "muzzle velocity" instead of "twist rate." I corrected my post. Shame, shame on me. Please don't take me out behind the wood shed.

Not at all. Thank you for your information.

Actually you got me thinking. How much variation in muzzle velocity should it be if we want to shoot, say, 1 MOA group? Quick calculation shows about 10%. So if my load shows standard deviation of 3% it should be adequate. 3% is like 75 fps out of 2500 fps, which is not that hard to do. It is assuming range of 100 yd. It would be progressively more stringent as the range increases.

Sort of off the topic, but I thought folks would be interested.

-TL

[Bart B.]

TL, if all the bullets are perfect and leave the barrel at the same place in its whip angle and direction, you can find out how much vertical shot stringing happens for a given spread in muzzle velocity. For example, with the .308 Winchester cartridge, a 50 fps spread in muzzle velocity causes about 1/10ths inch (1/10th MOA) vertical shot stringing at 100 yards. Therefore, to get a 1 inch vertical shot stringing caused only by muzzle velocity (and therfore amount of bullet drop at target range) so the groups' are 1 MOA, the spread has to be about 10 times as much; 500 fps.

This assumes bullets leave at the exact same elevation angle relative to the line of sight for each and every shot fired. But that doesn't happen. Slower ones leave later than faster ones. If they all leave while the muzzle axis angle's moving up, that will compensate for the slower ones making them leave at a higher angle compensating for their otherwise greater drop and they'll strike the same spot as faster ones.

[tangolima]

Thanks, Bart.

50 fps for 1/10 MOA. That's one sided, i.e. 50 fps slower makes 1/10 MOA lower. 1 MOA in normal sense means +/- 0.5MOA. So 1 MOA group means +/- 250 fps spread in muzzle velocity. Exactly the same as what I have thought.

Knowing this I won't be busting my butt trying to chase a load of 0.2% standard deviation, if I am just trying to shoot 1 MOA group out 100 yd.

The little equation I have derived indicates that the percentage spread in muzzle velocity is inverse proportional to range. For 1 MOA group

100yd 10%
500yd 2%
1000yd 1%
etc.

-TL

[Bart B.]

A 50 fps spread with .308's means about 20 inch spread at 1000 yards by most software.

When positive compensation happens, it may mean zero. SMLE rifles shooting .303 ammo loaded with cordite and near 100 fps velocity spread shot much more accurate at long range than mid range. The Brits figured that out over a century ago

[BuckRub]

Ok Bart, I've shot through chronograph once only in my life. I really shoot paper and determine what is acceptable or not. But I've heard others state that certain loads with low SD would be most accurate and others say that you could get an extremely accurate load and it may be far from low SDs on a chronograph. Do you really believe a chronograph could say if you have a low SD then it is accurate?

[Been there]

Twist rate is determed by bullet length, longer bullets require a faster twist rate to achieve stability ie:
.222 1 in 14 twist for bullets 50 -55gr
.223 1 in 12 twist for bullets 55-69gr
1 in 8 twist for bullets longer than 69gr
.243 1 in 10 twist for bullets 80 -100 gr
6.5x55 1 in 8 twist for bullets 120 - 150gr
1 in7 1/2 twist for longer than 150gr
This is a general guide & assume bullets of normal construction.

[BuckRub]

I always assumed twist rate was to stabilize a certain weight (grain) bullet. So do you think if its by length then that bullets made of different material and both same length they could stabilize from same barrel even though one is 150 grains and the other is 250 ?

[Unclenick]

BuckRub,

It's actually five things. For a given twist and bullet shape, in order of importance they are:

1) Length (more makes the bullet less stable)
2) Weight (more makes the bullet more stable as long as length doesn't change from adding the weight; e.g using a more dense alloy)
3) Muzzle Velocity (more makes the bullet more stable by spinning it faster)
4) Air density (more makes the bullet less stable. This could be less or more important than velocity depending how much different we are talking about)

The reason you see charts showing weight is they are assuming both similar construction (usually cup and core) and similar shape (usually boat tail Spitzer with hollow point) and standard atmospheric conditions, etc.

What is actually going on is more complicated, but the estimator I linked to earlier comes close. You can vary length, weight, twist, velocity and factors that affect air density, and see which ones affect the result most.

Originally Posted by Precision Shooter

According to that calculator, I wouldnt even drop below 2 unless I step up to a 208gr AMAX in my .308...

That sounds about right. Keep in mind, the 10" twist is a military carryover from the .30-40 Krag, which fired a 230 grain RN bullet at about 1,900 fps during development, and for which a 220 grain RN bullet at about 2,000 fps became the first issue ammo. You've also discovered why the SAAMI standard twist for .308 Winchester is 12" rather than 10".


Tangolima,

You're forgetting something critical: As a bullet files further downrange it slows down, taking longer and longer to traverse each successive 100 yards, giving drift more time to carry the bullet further off course and gravity more time to further accelerate the bullet's velocity downward. Even in a vacuum where it didn't slow down, you would find it it's falling faster at each successive 100 yards due to that gravitational acceleration. As a result of those factors, while you are correct that with a common match bullet (175 grain SMK), 50 fps will only make 0.1 moa drop difference at 100 yards, as Bart says, by the time it gets to 1000 yards the difference is a little over 2 moa, or about twice what your formula predicts.

A common rule of thumb for long range shooting is to try to get MV SD to 10 fps or lower. Figure the extreme spread will be roughly 3 times that in most instances. But at 100 yards, as long as the muzzle is at the right range of phase of deflection for your velocity shifts, you might see no accuracy error because the drop differences are so small, or it could even be more accurate than some tighter SD load because its median barrel time (that deflection phase again) is more optimal.

The other thing is that wide standard deviations can indicate, though, is poor ignition consistency which can open groups up by causing short ignition delays. Usually these are on the order of milliseconds and too short for the shooter to tell, but they have the same effect as increased lock time, allowing small hold disturbances more time to act on the muzzle position.

Board member Hummer90 has copper indentation gages to check firing pin energy with. He says he's been surprised how poor some newer guns are and that some people he's spoken with at gun companies don't understand the idea. You could do worse than to get a new Wolff striker spring installed to see if that improves your SD. The other common cause is failure to seat primers adequately.

[BuckRub]

Thanks Unclenick and you too Bart. You guys seem to really know alot of info.

[Bart B.]

There's more than just muzzle velocity that controls vertical shot stringing and accuracy in that axis. While the more spread there is, simple reasoning says the more stringing in the vertical plane there is. Slower bullets drop more at a given range than faster ones. Here's a few other things that will change vertical shot stringing even with each bullet leaving at the same velocity.

Ballistic coefficient (BC) numbers are determined by the drag a bullet has. For each bullet shape, diameter and weight, it'll have a drag value that slows it down in flight. Sierra Bullets' has proved there's a different drag value for different velocity bands; some of their bullets have 5 different BC's for each of 5 velocity bands. As long as all of the bullet's spin axis stays parallel to its trajectory path, each one will have the same drag value(s). But all bullets have a tiny bit of unbalance; some an insignificant amount and others in the same lot enough to nutate just like a wobbly football the quarterback doesn't throw perfectly. Such wobbly bullets have more drag; there's more surface area going through the area so the slow down a bit more than those perfect ones. They drop further down range. The best match bullets may have less than a 1% spread but hunting bullets will have more. That's enough to cause additional vertical shot stringing. Bullets need be spun at the right speed to keep them spinning such that they have minimum wobbling for best accuracy. Sierra Bullets' loading manuals have had test data in them showing the different drag values for the same bullet spun a different rpms from different twist barrels.

The more unbalanced bullets are, the more the centrifugal forces there are that makes them jump off the muzzle axis when they exit the barrel. They'll take a path at all angles relative to the bore axis. How much depends on the forces implied; faster twists and more unbalance means a greater angle. And all this adds up to larger groups, less accuracy. The more gentle bullets are pushed into the rifling, the less deformed they are. Slamming bullets too hard into the rifling typically reduces accuracy by deforming the bullets a tiny amount. A small amount of jump to the rifling is OK and Berger's bullets seem to do best this way. Sierra's typically shoot most accurate when gently pushed into the lands when the round's chambered.

The muzzle axis of all free floating barrels whips the most in its vertical axis and typically a few hundred cycles per second. And it's very repeatable from shot to shot; the frequency doesn't change regardless of the load fired. But the amplitude does a little bit just like a guitar string plucked gently or hard. If all the bullets leave while the barrel's on its upswing, slower ones leave later than faster ones; this compensates for their greater drop and they tend to hit the target where the faster ones do. However, if the bullets leave on the the muzzle axis down swing, the opposite occurs and vertical shot stringing increases.

With all the bullets being perfectly balanced and shot at exactly the same velocity as the leave, us humans add our variables to the equasion. If we could just hold the rifle exactly the same way for each shot, we could shoot 1/8 MOA groups all day long at 600 yards in good conditions. Meanwhile, our own variables cause the muzzle axis to point at different places, hopefully in a very small area on target. Shooting medium size, 30 caliber cartridges holding onto a 9-pound rifle as it rests atop something on a bench as we sit beside it pulling its 1 pound or heavier trigger, darned few of us can keep all our shots inside 1/2 MOA at 100 yards. But that same rifle fired in free recoil untouched by humans from a test cradle will shoot its ammo into near 1/8 MOA. Benchresters shoot their rigs in free recoil; only touching the 1- to 2-ounce trigger with a gentle finger tip.

And those subtle winds down range we cannot see play havoc with precision accuracy.

My best example of a given caliber bullet's need for different twists to get best accuracy is comparing two 30 caliber bullets. A 240-gr. leaving at 2200 fps from a .308 Win. barrel needs a 1:8 twist, a .30BR shooting 112-gr. bullets at 3000 fps needs a 1:18 twist barrel.

If the same caliber, I think bullets made of different material and both same length they could not stabilize from same barrel even though one is 150 grains and the other is 250. It's the same as shooting a given bullet out then another of the same but its lead core missing. Both the same shape and size, but greatly different in mass.

[old roper]

Here is interesting article on small group
http://bulletin.accurateshooter.com/...-gun-and-ammo/

Here is F-Class also with barrel tuner
http://bulletin.accurateshooter.com/...sm-f-open-rig/

As you see it's little more than just bullet and barrel twist

[BuckRub]

Alot of good info in this post. Thanks

[tangolima]

You're forgetting something critical: As a bullet files further downrange it slows down, taking longer and longer to traverse each successive 100 yards, giving drift more time to carry the bullet further off course and gravity more time to further accelerate the bullet's velocity downward.

Unclenick,

Thanks for your comment. You are right about the slowing down of the bullet. I realized that but did not include that in my equation for simplicity. So it is assuming vacuum. I just want to make some sense of the mechanism, knowing it is over optimistic. It was for 100 yd mostly, and I guess it is not too bad. I am quite convinced that it is no point chasing perfect standard deviation for such short range. The other factors would dominate the group size.

-TL

[Unclenick]

Not necessarily. If the high SD is due to erratic ignition causing ignition delays, letting small disturbances introduce inconsistencies in muzzle position at the moment of bullet exit, it may open even your 100 yard groups up. It just won't necessarily be vertically. But it's also possible to have, say, 20 fps SD that's due to powder moving around, but that involves no ignition delays. Faster powders, in particular, are easier to light up and might do this, in which case you could still have a maximum accuracy load that's not the lowest SD load you can cook up.

I thought it might be fun to look at bullet drop over 1000 yards. If you sit and count the drop distances based on 1 g over the Time Of Flight (TOF) numbers given, you will find these look a bit off. That's because of the bullet points into the trajectory (traces) so its drag has a component of effect in the direction of gravity both climbing and falling. Hence the bullet seems to drop too much for 1G on the upward arc of the trajectory, but not enough on the falling arc.

80 grain SMK at MV 2650 fps
Range	   TOF	         ΔTOF	      Total Drop    Drop Over Previous 100 yds
   0 yd	 0.0000 s	0.0000 s	  0.0 in	  0.0 in
 100 yd	 0.1176 s	0.1176 s	  2.6 in	  2.6 in
 200 yd	 0.2449 s	0.1273 s	 10.9 in	  8.3 in
 300 yd	 0.3833 s	0.1384 s	 26.0 in	 15.1 in
 400 yd	 0.5345 s	0.1513 s	 49.0 in	 23.0 in
 500 yd	 0.7007 s	0.1661 s	 81.8 in	 32.8 in
 600 yd	 0.8839 s	0.1833 s	126.2 in	 44.4 in
 700 yd	 1.0872 s	0.2033 s	185.0 in	 58.8 in
 800 yd	 1.3141 s	0.2269 s	261.5 in	 76.5 in
 900 yd	 1.5682 s	0.2541 s	360.2 in	 98.7 in
1000 yd	 1.8505 s	0.2822 s	486.7 in	126.5 in

[tangolima]

Yeah, I noticed that too. But out of both laziness and simplicity, I normally assume the vertical velocity of the bullet is small and would generate insignificant amount of vertical drag. It is generally true for short time of flight. But for long flight of flight, vertical drag will become visible.

Interesting point. Thank you.

-TL

[Bart B.]

As bullets from .308 Win cases drop less than 2 inches vertically for every 36 inches travelled horizontally at 1000 yards, I think vertical drag's not an issue at all. Besides, BC's are only calculated for the bullet's long axis being parallel to the trajectory as far as I know. Sierra uses time of flight between two screens.

[Unclenick]

Bart,

This is one of those instances in which minding your vector P's and Q's gives you an unexpectedly significant result.

You are correct that the actual bullet velocity doesn't err much from the linear velocity because the angles are so small. For th 175 grain MatchKing in my example, to hit the target at 1000 yards the angle of departure is about 0.775°, and the angle of fall (at which the bullet strikes the target) is about 1.49°. Even at 1000 yards, the difference between actual bullet velocity along the trajectory and bullet velocity over the range, is, at its most extreme, only about 1/3 of a foot per second. It's not much error. And with Sierra measuring over a shorter 300 yard course by launching at different velocities, the error is smaller, still, so the measuring method at Sierra doesn't introduce a significant error with respect to calculating bullet transit time over a long range.

But what we are interested in here is bullet drop, and gravity is a smaller accelerating force than the drag force from air pressure on the nose of the bullet. Therefore, it takes only a small percentage of the drag force, as a vertical vector component, to neutralize a significant percentage of the gravitational force.

So here's what we've got: The total drop is the difference between where a bullet actually hits on the vertical axis and where it would hit at the same range, based on angle of departure, if there were no gravity over the same bullet transit time. From the table in my previous post, you can see the total drop calculated by the 3 DOF QuickTARGET Unlimited software is 486.7 inches during 1.8505 seconds of travel. But if you go to calculate drop due to gravitational acceleration over that same period of time, you get 661 inches.



What accounts for the difference in the two numbers is the vertical vector component of drag as the bullet falls, which opposes gravity. The bullet is decelerating at -325 ft/s². It strikes the target falling at an angle of 1.49°. So how much does that affect gravity working to pull the bullet down? Well, if -325 ft/s² (the minus sign here shows opposite to the direction of bullet travel) is in a straight line over the range, that becomes the drag deceleration vector on the horizontal or X-axis, pointing back at the firing point. So, what is the vertical component? Well, the Tangent of the angle is the ratio of the opposite over the adjacent side. Tan(-1.49°)=-0.026. -0.026×-325 ft/s² = +8.45 ft/s². A standard gravity is -32.17 ft/s² (minus sign denoting downward direction). -32.17 ft/s² + 8.45 ft/s² = -23.72 ft/s². So, instead of falling with an acceleration of -32.17 ft/s², the bullet is falling with an acceleration of -23.72 ft/s² toward the ground at the time it impacts.

Yes, drag adds to gravity when the bullet is climbing and subtracts from it when the bullet is falling. These tend to neutralize each other, but not very exactly. Drag is greater at the firing velocity, but the firing angle (angle of departure) is smaller than the angle of fall. Plus, the bullet reaches apogee at about 585 yards, which is beyond the half way point, so the angle of fall is steeper than the angle of departure, and, because it's going slower, the bullet actually winds up spending a little more time falling than it does rising. The end result is the 174.3 inch difference between total drop in a vacuum and actual total drop.

For those not following, this illustration depicts the terms used here: