THE ACCURATERELOADING.COM FORUMS


Moderators: Mark
Go
New
Find
Notify
Tools
Reply
  
twist rates vs velocity
 Login/Join
 
<thomas purdom>
posted
Hi all: I posted this on another site and would like to see any answers you all might come with. I have a Ruger Mark II in 7x57mm with a 1x9.5 twist barrel. I posted before on if the 1x9.5 would best stabilize the 139 grain Hornady SST, or heavier bullets, such as the Hornady 162 grain boattail. The answer was it should stabilize both well. I use H414 and with 48.2 grains of it push the 162 grain at 2,750 feet range. Consistently goes into a .7 inch five shot group, and that includes one bullet which could be called a flyer. Now, with any given powder in this rifle and with 139 grain SSTs hitting 2,900 fps, the accuracy falls off. Sometimes I can get sub-moa groups, but then the next is not and opens to 1.5 inches. My question is, with the 1x9.5 twist, is the top velocity one should push to at 2,800 fps to get consistent sub-moa groups. I know the rifle is capable of consistent smaller groups because of the 162 grain, and the fact that on occasion it will shoot a sub-moa with the 139 grainers, but not all the time. I have not loaded the 139 grain SSTs to the 2,800 fps range yet cause the weather here in New Mexico won't allow me to get out on the dirt roads I need to get on in order to test the cartridges. So, if anyone out there knows if the max velocity with any given bullet for a 1x9.5 twist is 2,800 fps, please let me know. Thanks and sorry about the long-winded question ... Tom Purdom
 
Reply With Quote
<William E. Tibbe>
posted
Sorry that you are a late comer. This subject has been addressed in minute detail extensively on this forum.

When time permits I will be happy to get back to you on this and provide extensive information.

------------------
Bill Tibbe

 
Reply With Quote
<William E. Tibbe>
posted
THE ACCURATERELOADING FORUMS

The Accuratereloading Forums
Reloading
More about barrels, twist rates and accuracy.

profile | register | preferences | faq | search

UBBFriend: Email This Page to Someone! next newest topic | next oldest topic
Author Topic: More about barrels, twist rates and accuracy.
William E. Tibbe
unregistered posted 01-12-2001 08:59
--------------------------------------------------------------------------------
Some of the more obscure aspects of a good shooting rifle seem to be hidden from view. And very little is really said about them.
There are many generic statements about an individuals satisfaction with a particular rifle or a particular bullet. But usually those statements are just generic with explicit details missing.

So - lets put things under the microscope and see what is revealed:

Barrels:

There are old barrels and new, chrome-moly and stainless.

The rifling is produced by four methods; cut, button, hammer forged and the new etching process.

The "bore" is generally considered to be the inside diameter of the lands. The lands are the ridges. Thus a 30 caliber barrel, as an example, measures .300" across the lands and .308" in the grooves. Since there are two ends to the diameter, that makes the height of the rifling one half of the difference between .300" and .308" or .008"/2 = .004". In actuality tHe height of the lands may vary, from barrel to barrel, in the range of 0.002" TO 0.006".

The diameter of the bullet should be just the same as the diameter of the grooves or slightly smaller. We don't want the bullet to be choked and forced through the grooves.

The lands will engrave the bullet .004" deep on either side.

Rifling:

Many barrels are 6 groove. Some are 2 groove, 3 groove, 4 groove, 7 groove, 8 groove and some may be "microgrooved".

Twist Rate:

Here are Shilens recommended twist rates.

Caliber Twist

.172---9" For bullets heavier than 30 gr.
-------10" For bullets up to 30 gr.

.204/20---12" For all bullets.

.222 RF---14"* Twist for pistol barrels
-----------16" Standard twist for rifle barrels
-----------17"* Special twist for rifle barrels

.224 CF---8" For bullets heavier than 70 gr.
-----------9" For bullets up to 70 gr.
----------12" For bullets up to 63 gr.
----------14" For bullets up to 55 gr.
----------15"* For bullets up to 55 gr. driven 4,100 fps or more
----------16"* For bullets up to 55 gr. driven 4,300 fps or more

6mm/.243--8" Special for VLD bullets over 100 gr.
----------10" For bullets up to 120 gr. and VLD under 100 gr.
----------12" For bullets up to 85 gr.
----------13"* For bullets up to 75 gr.
----------14"* For bullets up to 70 gr.
----------15"* Special for bullets up to 70 gr.

.257------9" For bullets heavier than 100 gr.
----------10" For bullets up to 100 gr.
----------12" For bullets up to 90 gr.
----------13"*For bullets up to 80 gr.
----------14"*For bullets up to 70 gr.

6.5mm/.264-8" For bullets heavier than 130 gr.
------------9" For bullets up to 130 gr.

.270------10" For all bullets

7mm/.284---9" For bullets heavier than 140 gr.
-----------11" For bullets up to 140 gr.

.307------13"* Special size and twist

.308-------8" For bullets heavier than 220gr.
-----------10" For bullets up to 220 gr.
-----------12" For bullets up to 170 gr.
-----------14"* For bullets up to 168 gr.
-----------15"* For bullets up to 150 gr.

7.65mm/.311-10" For all bullets

8mm/.323--10" For all bullets

.338------10" For all bullets

9mm/.355--14" For low-velocity wadcutters
16" For all other bullets

.38/.357--14" For low-velocity wadcutters
-----------18" For all other bullets

.358------14" For all bullets

.375------12" For all bullets

10mm/.400-16" For all bullets

.411------14" For all bullets

.416------14" For all bullets

.44-------20" For all bullets
-----------16" For low velocity wadcutters

.451------16" For all bullets

.458------14" For all bullets

*Stainless steel only

Black powder barrels (1.250" x 30")

.32 14" .320" groove

.38 20" .379" groove

.40 20" .403" groove

.457 20", 14" .457" groove

Lets take the .30-06 for an example. Bullet weights can range from 110 grains to 220 grains ( heavier double the weight of the lighter ).

The twist rate is the distance a bullet must travel to make one full revolution. Therefore a twist rate of 12" would mean that a bullet would turn 360 degrees, or one full revolution, each foot of travel or each 12".

The forward motion of the bullet is measured in feet per second and/or meters per second. The bullet is travelling, or translating, or has a translational velocity ( forward speed ) It is, at the same time, rotating and has a rotational velocity.

When the bullet is fired there is a sudden rise in pressure, to a peak, then a sloping pressure curve drop until the bullet exits the barrel where the pressue returns to atmospheric. If the barrel is 24 inches long, with a 12 to 1 twist, it turns two times in the barrel. The gas velocity is generally estimated at about 5,600 to 7,000 feet per second. Using a "hot" load lets assume that we have a bullet velocity of 3,000 feet per second. The lapsed time for the pressure peak to occur is somewhere in the range of a couple of milliseconds. The bullet is already up to maximum pressure in the first few inches of the barrel, from the chamber. It's speed, or velocity, continues to accelerate along the entire length of the barrel until it exits the muzzle. Lets assume further that our target is 100 yards away ( 300 feet ) if the bullet maintains its constant turning rate at 12 to 1 and travels 300 feet then it should turn 300 times. If our target is an animal with a 1 foot thick chest cavity, the bullet would turn one complete revolution while traversing the chest if unimpeded. But it won't because it decelerated very rapldly when encountering resistance. Therefore the bullet isn't windmilling like a food processors whirling blade, but rather it is essentially being "pushed" through with virtually no rotation. You might say the bullet is "stabbing" the animal or corkscrewing slightly or twisting its way through. Therefore, the "petals" on a Barnes bullet, for example, are not "chewing" their way through like a rotary lawnmower blade. They are merely ploughing and pushing obstructions out of the way.

Rotation:

The rotational speed, and rotational distance travelled, is measured in revolutions per minute, or rpm, or rotations per second, or even rotations per millisecond and circular inches or feet ( or centimeters and meters ). The distance travelled, of a pencil point mark on the side of the bullet, can be found by calculating the circumference of the bullet. The circumference of .308" is pi x D or .308" x 3.1416 = 0.9676128" ( close to an inch ). A bullet travelling 3,000 feet per second would rotate one turn for each foot travelled and rotate 3,000 times during that second if its flight time was actually of one second duration, which it isn't. Since its rotational distance travelled is 0.96 inches per revolution it will be 0.96" x 3,000 = 2,902 circular inches.

To make the figures sound more impressive we could quote rpm - "revolutions per minute". Thus we would multiply 3,000 revs per second by 60 seconds in a minute and say 180,000 rpm. * ( an ultra high speed hand grinder possibly could attain 20,000 rpm's and a "Roto-tool" 30,000 rpm's.)

Here again are the twist recommendations for a .308" caliber.

.308-------8" For bullets heavier than 220gr.
-----------10" For bullets up to 220 gr.
-----------12" For bullets up to 170 gr.
-----------14"* For bullets up to 168 gr.
-----------15"* For bullets up to 150 gr.

The spread is 8" for bullets heavier than 220 grains and 15" for 110 grains up to 150 grains. Quite a spread. The rpm's, it can be seen from the reloading manual, will depend on the powder load which in turn dictated the velocity. Since the maximum pressure is always a controlling factor, it follows that as the bullets grain weight increases, there has to be a corresponding drop in velocity.

A 110 grain Jacketed HP bullet using 60 grains of #748 would give 3387 fps at 49,000 CUP. It would call for a twist of 15 to 1 in a stainless steel barrel.

Whereas a 220 grain Jacketed RN using 57.5 grains of H-4831 would give 2583 fps at 50,000 CUP. using a chrome moly barrel. It would call for a 10 to 1 twist.

A bullet heavier than 220 grains would use an 8 to 1 twist.

It can be seen that a lighter, shorter bullet does not need to rotate as fast ( 15 to 1 ) as a heavier bullet ( 8 to 1 ). The heavier bullet needs to be spun faster to "spin stabilize" it. If a heavy, long bullet is spun too slowly it will precess and become erratic. The more it yaws, the worse the group. Conversely a light bullet may be spun too fast or overspun.

The preceding may account for some of the so called "phenomenon" that some shooters are reporting. They may have a twist that just isn't appropriate for a given bullet taking into account its weight and length.

A spread of 8 to 1 to 15 to 1 is too great for one single barrel to cover. It cannot apparently give peak performance to any, and/or all bullets. Therefore, a choice has to be made by the shooter.

Do some homework. Calculate your bullets weights versus rpm's.

We will welcome any comments from anyone who has observed variations, improvements, or deterioration in accuracy due to twists vis-a-vis bullet weights.

Bill Tibbe

[This message has been edited by William E. Tibbe (edited 01-13-2001).]

[This message has been edited by William E. Tibbe (edited 01-13-2001).]

IP: Logged

Bruce Gordon
Super Member
Posts: 116
From: Tulsa, Oklahoma USA
Registered: Dec 2000
posted 01-12-2001 13:15
--------------------------------------------------------------------------------
I have a question that you might be able to clarify concerning the information posted above.
Is the bullet weight the determining factor in rifling twist, or is the length the determining factor, or a bit of both?
The reason I ask is that on another post I asked what weight of bullet was correct for a particular rifle that I have and was told that bullets made from a less dense medium, such as pure copper bullets, require a different optimum twist than the same weight of conventional jacketed lead core bullet, which would also be different from a pure lead bullet.
If that is the case, would bullet shape be important as well? For instance, a flat nosed wadcutter bullet is considerably shorter than a hollow point spire point boat tail bullet design and might need a different twist rate to get optimum accuracy.
Another question I have is on how to correctly extrapolate the twist listings. For instance on the .308" diameter bullet it lists 12" twist for bullets up to 170 grain and 10" for bullets up to 220 grain. Are these twists for a particular velocity or is there a particular velocity they are optimized for. In other words, can I use the chart for a 2000 fps bullet and for a 4000 fps bullet and still trust that the twist rate is the optimum one? If not, how much adjustment do I need to make?
[This message has been edited by Bruce Gordon (edited 01-12-2001).]

IP: Logged

Bill Leeper
Super Member
Posts: 227
From: Elko, B.C. Canada
Registered: Jun 2000
posted 01-13-2001 01:52
--------------------------------------------------------------------------------
The bullet length is indeed the determining factor. Obviously, because it affects length, shape is also a determining factor. a larger diameter bullet of a given length does not have to rotate as fast. Although it may not be exactly true it is nearly true that the surface speed of the bullet different diameter will be the same to achieve equal gyroscopic stability.
Velocity does have an effect but it takes a lot of velocity to equal a little twist. Regards, Bill.
IP: Logged

William E. Tibbe
unregistered posted 01-13-2001 06:30
--------------------------------------------------------------------------------
Bruce:
Six excellent questions.

As a prelude to answering, there are two "numbers" that are provided by many bullet companies, about their bullets:

S.D. = Sectional Density

B.C. = Ballistic Coefficient

However, there is a third number that is not so easily obtainable and infrequently seen or mentioned.

S.F. = Stability Factor

The optimum range for the gyroscopic stability factor for accuracy is 1.5 to 2.5. If the stability factor for any given bullet is known, its approximate stability for any other twist can be calculated by multiplying the known stability factor by the square of the ratio of the twists.

If the stability factor is unknown, it can be determined by the Greenhill formula. Sir George Greenhill discovered the formula before WW 1. The formula is:

Length of bullet "in calibers", multiplied by the twist rate in calibers is 150.

For example a 30-06 bullet weighing 173 grains is 4.21 "calibers" long.

4.21 x t = 150

t = 150/4.21--------- t = 35.62

Where:

t is twist rate
l is length of bullet in calibers

To convert 35.62 calibers into inches, multiply by the bullet diameter, .308", yielding a twist of 10.97" or about 11"

The Greenhill formula correlates very well with modern bullets where the measured values are available.

The formula results in a gyroscopic stability factor in the range of 1.5 to 2.0 for typical boattail bullets and 2.0 or more for pointed flat-base bullets or short boattails travelling at high velocities.

To simplify the calculations here is a twist rate calculator:
http://www.z-hat.com/twistrate.htm

Two values must be entered: #1. caliber and #2. Length. After entering the caliber and first length, then change lengths ( for example 1/2"; 3/4"; 1"; 1 1/4"; 2" etc.... or in decimal 0.5"; 0.75"; 1"; etc. ) and see how the twist rates change with each ordered recalculation.

In answer to your questions:

#1. The Greenhill formula asks for the length. Naturally that is linked to weight. Since lead and copper have two different specific gravities those differences, for a given weight, mean different lengths. The stability factor is determined in the factory using a complicated, and expensive set of equipment. But the Greenhill formula, using length, gives a very good correlation.

#2. Length is, again, the prevailing element regardless of the bullets composition. However, there are multitudes of other "things" to consider relative to accuracy. Drag functions, sectional densities and ballistic coefficients to name a few.

#3. & #4. There are optimum velocities. And there are maximum - minimum velocities. The maximum would be when the bullet flies apart and doesn't reach the target. The minimum would be a very light load wherein the powder would lay along the low side of the case and the flash, or flame, from the primer would flood in over the top into the void. A dangerous situation could arise because there was insufficient gas pressure to expel the bullet, which could lodge in the barrel, thereby causing a "trapped" pressure spike.

Using one specific bullet with a lead core and copper cladding, with specific known metallurgial characteristics, that bullet would fly apart at 307,000 rpm's. Here are the twist rated and velocities to cause such a failure.

Twist = 7----Max vel----2980 fps
Twist = 8----Max vel----3416 fps
Twist = 9----Max vel----3843 fps
Twist = 10----Max vel----4270 fps
Twist = 11----Max vel----4697 fps
Twist = 12----Max vel----5124 fps
Twist = 13----Max vel----5551 fps
Twist = 14----Max vel----5978 fps
Twist = 15----Max vel----6406 fps
Twist = 16----Max vel----6832 fps

#5. If you use the chart to select, it should be at, or near, optimum for whatever weight and twist you select. The range of coverage should be wide enough to reach in both directions for some distance. The Greenhill formula does not have an element for velocity, but correlates well with actual tests. The " twist rate calculator", however, does have a velocity criteia selection. You must enter the velocity either above or below 2800 fps. Note however, that if you stray out of the .308" caliber there are notes particularly in the .224 CF - 15 and 16 twist *( stainless steel barrels ) at velocities over 4,000 fps.

#6. There is no element, or instrument, to make adjustments in the Greenhill formula. But the twist rate calculator does have a velocity provision. The best that can be done is select a twist for a given caliber, given bullet weight, from the chart, compare it to the Greenhill calculators results, and bench test it. In this regard you can refer to the Accuratereloading load data for the .30-06. It will be noted that at the bottom of the listings the bullet lost its stability and keyholed ( flew sideways ). That was excessive velocity for that powder and for that velocity.

It may be further noted that some bullets do fly true remarkably well at low velocities. These are the "Whisper" series. They are shot at sub-sonic velocity and are very consistent at very long ranges. The bullets are long and pointed VLD ( very low drag ).

It can be stated, therefore, that the twist rate would be for one specific bullet and when the shapes vary there may also be some varition in the performance.

Bill Tibbe


[This message has been edited by William E. Tibbe (edited 01-13-2001).]

IP: Logged

Powderman
New Member
Posts: 1
From: Enumclaw, WA USA
Registered: Feb 2001
posted 01-13-2001 11:48
--------------------------------------------------------------------------------
One question--what is etched rifling? Sounds intriguing.
------------------
Happiness is a 200 yard bughole.

IP: Logged

Kevin Gullette
New Member
Posts: 3
From: Wichita Falls, Texas, USA
Registered: Dec 2000
posted 01-14-2001 01:16
--------------------------------------------------------------------------------
Gentlemen,
A much more up-to-date source of ballistic information is available at the JBM Ballistics web site.
I feel it is vastly superior to useage of the Greenhill formula, etc.

Have 17, Will Travel
Wire Gullette
Wichita Falls

IP: Logged

William E. Tibbe
unregistered posted 01-14-2001 06:39
--------------------------------------------------------------------------------
Powderman:
Etched rifling is just being introduced. It is the most sophisticated and modern of all rifling procedures.

It is a chemically etched, cation rifling. Metal is removed by applying a strong acid to the groove surfaces. When the desired depth is achieved, the process is halted by passivating the acid.

The finished barrel has an excellent finish and a uniform bore.

Bill Tibbe

IP: Logged

Powderman
New Member
Posts: 1
From: Enumclaw, WA USA
Registered: Feb 2001
posted 01-14-2001 15:56
--------------------------------------------------------------------------------
Extremely interesting. I wonder what this will do with practical accuracy, as absolutely no barrel stress is imparted with this method. Also, the small ridges left behind by chambering reamers will be missing, too. Sounds great--the barrels are finish lapped, aren't they?
------------------
Happiness is a 200 yard bughole.

IP: Logged

All times are Dubai Time
next newest topic | next oldest topic


Administrative Options: Close Topic | Archive/Move | Delete Topic
Hop to:
Select a Forum

List of Forums:


Category: Hunting
--------------------
African Big Game Hunting
Big Game Hunting
Varmint Hunting
The AccurateReloading Book

Category: Guns, Politics, Gunsmithing & Reloading
--------------------
Gunsmithing
Reloading
Big Bores
Small Calibers
Cast Bullets
Gun Ownership & Plitics

Category: Target Shooting
--------------------
Rifle Target Shooting
Pistol Shooting
Rim Fire Rifles & Pistols

Category: Other Topics
--------------------
Humor
Shotgunning
Air Rifle
Contest
Moderators Only
Miscellaneous Topics
Black Powder
Classified

Contact Us | accuratereloading.com


Ultimate Bulletin Board 5.47d


------------------
Bill Tibbe

 
Reply With Quote
<Don G>
posted
Thomas,

I get accused of telling people how to build a watch when they ask what time it is. Bill seems to suffer the same malady!

The practical answer is that for any given bullet there is an optimal rate of rotation that stabilizes it under all atmospheric conditions and supersonic velocities.

If you rotate it faster than that there is a slight increase in group size due to overstabilization for bullets that have slight imperfections. Most hunting bullets are more imperfect than Sierras, but still, moderate over-spinning alone does not usually cause measurable increase in group size. (As I recollect, Vaughn in RIFLE ACCURACY FACTS comes up with numbers like .05 MOA using intentionally imperfect bullets.)

Your doubling of group size with only a slight increase in velocity (and thus RPM) is more likely a function of barrel vibrations ("barrel whip") or bedding or both rather than RPM.

I've found that it's usually best to let the rifle tell me what it likes rather than the other way 'round. It's cheaper and easier that way, too!

That mule deer won't know the difference between 2750 and 2900 fps.

Don

 
Reply With Quote
<thomas purdom>
posted
Bill and Don: Thank you so much for your posts. I really do appreciate it. When the muck clears off the roads here (in New Mexico the mud makes road travel impossible, even for 4-wheel drive units) I'm going to drop down some in the 139 grainers just to see what happens in the 2,800 fps range. The 7mm Mauser rifle is my catchall weapon. I have used it for elk, mule deer, whitetail deer, javalina, turkey (in south Texas with a 182 yard neck shot and steadystix) and wild pigs. I love to handload (have a Lyman orange crusher kit) for it and my .223 in Howa 1500 heavy barrel. Keep up the good work gentlemen. Tom Purdom
 
Reply With Quote
<Don G>
posted
Tom,
Once you find a load that a hunting rifle likes, it is quite likely that the same rifle will like other loads with the same "barrel time". This is the time between primer ignition and the bullet leaving the barrel. The reasons for this have largely to do with barrel vibrations.

Your talk of fliers makes me wonder about the bedding in your rifle.

Don

 
Reply With Quote
one of us
posted Hide Post
Okay, I've started from zero knowledge but I've read these posts and I'm learning fast.

My .22-250 heavy barrel has a 14" twist.

I have in my hand two different 55 grain bullets one of which, according to Greenhill, is unlikely to be stable with 14" twist. Yet the other bullet creates a very short cartridge loa with a relatively large jump to lands, despite which it is remarkably accurate for a factory load.

So, do I stick with bullet length, or do I stick with the 55 grain limit as mentioned somewhere up this thread?

Re the longer bullet (V-Max), I'm just in the middle of working up optimum loads at 100 metres before working it back to 200 and 300 metres - and possibly further.

Pass me a damp towel please?:-)

------------------

 
Posts: 360 | Location: Sunny, but increasingly oppressed by urbanites England | Registered: 13 February 2001Reply With Quote
<Don G>
posted
Pete,

The 1:14 may stabilize the V-Max, they are worth a try. Work up to max velocity before you make your decision to quit on them. The bullet may keyhole at low velocity and run fine at higher velocities. The rate of spin stays almost constant after the bullet leaves the barrel, so if it is stable at 100 it will be stable until it drops subsonic.

The longer bullets have a reputation of being more susceptible to wind than the short ones. The BC may be worth it if you are trying for long ranges.

Don

 
Reply With Quote
<Warren Jensen>
posted
The Greenhill Formula is a simplified method for determining mathematically the amount of spin necessary to stabilize a bullet. It was worked out in 1879 by Sir Alfred George Greenhill who was a Professor of Mathematics at Woolwich and teaching the Advanced British Artillery Officers Class. It was considered satisfactory for bullets having a density of .392 lbs/cubic inch or greater. (Lead has a density of .409 lbs/ cubic inch, and copper has a density of from .318-.325 lbs/cubic inch, depending on the alloy) The formula is Twist required (in calibers) = 150 divided by the length of the bullet (in calibers). It makes no allowance for nose shape, considering round noses and all spitzers and spire points as the same. It does not work for bullets having a density below .392 lb/ cubic inch. All copper or brass solids and most heavy jacketed bullets have average densities below .392 lbs./cubic inch. Notice I said average, as the formula makes no allowance for bullets of variable construction, linearly. The formula was a shortcut and was useful at the time, as most bullets were roundnoses and were lightly jacketed, if jacketed at all. Because the math is simple, the Greenhill Formula has remained in use to this day. Just a few years ago I had an engineer at a major ammunition manufacturing firm quote me the Greenhill Formula as a method for calculating the spin required to stabilize a long, 10 caliber spitzer, 7mm 175 gr. , variable density, hunting bullet. Needless to say, he was not even close. The Greenhill Formula is accurate when used in the context for which it was intended, but it is not an accurate formula for calculating stability of all bullets.

Bullet length, nose shape, material density, velocity, and twist all are factors with different levels of importance. Many shortcuts or rules have been developed for "standard bullets". For calculating stability for bullets with non-standard shapes, densities, or other factors you have to use the whole equation to be accurate.

The actual formula is much more complicated. It is Gyroscopic Stability (GS) = the spin rate (in radians per second, squared) times the polar moment of inertia, squared, divided by the pitching moment coefficient derivative per sine of the angle of attack times the transverse moment of inertia times the air density times the velocity squared. (My keyboard does not have all the correct symbols and that is why I wrote it out). For the bullet to be stable, GS > 1.0. This is actually a short version as the pitching moment coefficient component is a complicated calculation that derives the center of gravity and the center of reverse air pressure. The equation is basically calculating the linear difference between the center of gravity and the center of reverse air pressure on the nose of the bullet. The greater the difference, the greater the spin required to keep the bullet pointed nose forward.


------------------
Warren Jensen

Warren@lostriverballistic.com
lostriverballistic.com

[This message has been edited by Warren Jensen (edited 04-11-2001).]

 
Reply With Quote
one of us
posted Hide Post
Don, Thanks. I later realised I should have added that Hornady's own data specifically confirms that 60 grain V-Max and hollow point bullets will not stabilise in 14" twist and slower. The absence of any similar remark suggests the 55 grainer should be okay.

Warren, thanks also. I'm not sure but you seem to be suggesting that Greenhill may be inappropriate (and inaccurate?) for this calibre? I understand it's all complex stuff but please put me right.

------------------

 
Posts: 360 | Location: Sunny, but increasingly oppressed by urbanites England | Registered: 13 February 2001Reply With Quote
<Warren Jensen>
posted
Pete,

I am not suggesting that at all. As the discussion had turned technical I was just pointing out the the Greenhill Formula was and is a mathemetical shortcut and within which parameters it may not be reliable. The longer the ogive, or the more pointed a bullet becomes, the greater the chance that the Greenhill Formula will result in an inaccurate answer.

There are other factors besides whether the bullet is properly spin stabilized that should be considered. For example, a bullet when engraved, rotates around it's center of form. When it escapes the confines of the barrel it will then rotate around it's center of mass. If this difference is large then the bullet will undergo a period of significant yaw and wobble, even if it is correctly stabilized. This can mean at short range you may be getting keyhole imprints that are not due to insufficient rotational velocity. A properly designed bullet will have yaw dampening characteristics such that any initial yaw, as described above, will be dampened with time. This pattern could be misinterpreted as instability when it is something else entirely. It is important when trying to understand the nature of a bullet's flight that you also understand any limitations to the tools that you may be using. The Greenhill Formula has definite limitations. I am not trying to overcomplicate the discussion, but some of the things you have been discussing do not lend themselves to simple answers.

The question, as I recall, was is it bullet length or bullet weight that is the limiter for stability in a given caliber? The answer is "neither". The linear difference between the center of reverse air pressure and the center of gravity is the determiner as to how much twist is required to keep the bullet point nose forward.

------------------
Warren Jensen

Warren@lostriverballistic.com
lostriverballistic.com

 
Reply With Quote
<Geoffrey Kolbe>
posted
Generally, the rate of twist required to stabilise any given bullet will increase as the square root of the bullet velocity.

So, for example, a 150 grain .308 bullet might require a 14" twist to stabilise it at 2500 ft/sec. At 3000 ft/sec the same bullet would only need a 15.3" twist to achieve the same stability factor.

Yours aye,

Geoffrey

 
Reply With Quote
one of us
posted Hide Post
Quick follow-up on my previous postings. Today I tried the 60 grain bullets partway through a session with the chronograph.

As I was forming one hole- and touching hole groups with the 55 gr V-Max and then couldn't hit a sheet of A4 paper with the 60 grain bullets I shan't be bothering with them again! Time to unload some rounds!

------------------

 
Posts: 360 | Location: Sunny, but increasingly oppressed by urbanites England | Registered: 13 February 2001Reply With Quote
  Powered by Social Strata  
 


Copyright December 1997-2023 Accuratereloading.com


Visit our on-line store for AR Memorabilia