Shooters Forum banner

1 - 20 of 29 Posts

·
Registered
Joined
·
32 Posts
Discussion Starter · #1 ·
what does it mean? While looking at the barnes tipped tsx at the store i noticed it said something about the barrel twist. I dont remember the numbers or what faster or slower means but when I looked the bullets up on barnes website it doesnt mention anything about barrel twist. So now what does faster or slower barrel twist mean(with examples...7" faster than 9" or vise versa). How to pick a bullet based on barrel twist? I shoot a remmington model 7 7mm-08 with a 20" barrel with a 9 1/4" twist. So with that I am looking at reloading my own rounds and not sure how o pick what bullet to go with. I want an all around load that will be good for deer and elk(I alo hunt pig but not really worried about using a dfferent round for it). I have always used 140gr federal nozler partition factory rounds but have everything but bullets and possibly powder. I have plenty of brass and some LR CCI primers and IMR 4350, 4895 and 3031. that I can look to use. I have read alot of people using the hodgdon varget powder with good results. So main question is what do bullets and barrel twist mean when trying to match them for a hunting round?
 

·
Banned
Joined
·
674 Posts
http://kwk.us/twist.html

That link will give you a calculator to tell you what you need for a particular bullet in a caliber better than I can but basically a large slow moving bullet will require a fast twist to stabilize the bullet and a small fast bullet will require a slower twist to give you the same stabilization.
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #3 · (Edited)
thanks. I have done some more research I think I get it. (9 1/4" is basically a 1:9" twist which means 140gr and up and a faster or tighter twist than say a 1:11. meaning 1 rotation in 9" is faster than 1 rotation in 11". That calculator seems over my head right now with my newbieness. but I want to figure out how that will help me.
 

·
Banned
Joined
·
415 Posts
Ok, so your bullet diameter is .284" On a 140 gr. bullet, I believe we're looking at 2800 fps muzzle velocity. A 150 gr. should be around 2650 fps. The specific gravity is the density of the bullet. This is just kind of a relative guess, since as far as I know, none of the bullet manufacturures actually state that information. The TTSX is 100% copper, though it also has a plastic tip. I would guess an SG of 8.8. Now it's a matter of playing with lengths until you get the correct twist for your gun, then finding bullets near those lengths.

So using a 150 gr. (2650 fps) at .284" and an SG of 8.8, you would need a 1.42" bullet to stabilize. Of course, you have room to play (in fact, usually a fair bit of room), but this is considered to be the ideal. I should just mention that the SG is very important, as a copper bullet (having a lower SG, and therefore being less dense) must be longer to make up the same weight as a lead bullet.

I do use these calculators, and I suppose they give you an idea of things...but I'm really not satisfied with them.
 

·
Banned
Joined
·
415 Posts
cvc944 - You may be able to remain set in your ways yet! Many companies are working on new alloys (usually using tungsten as the old standby) to equal the mass of lead, yet remain "better" to the environment.

I wonder how long it will take before we find some reason for all of these copper and tungsten alloys to be banned for damage they're doing to our environment/ecosystems?
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #6 ·
Thanks guys. They did have the nozler partitions there I had just heard all the rave(not necissarily for being all copper) about how good the TSX bullets are. Makes sense they have to make a bullet longer due to lack of weight as its really the only direction to go. So with all this new fangled smartness I'm etting would it be wise to try to find a 150gn as a intermediate load for deer/elk or keep the 140gn as theres tons more options in the 140. I dont doubt a 140 will kill an elk cause Ive done it twice out of 3 hunts(last hunt didnt see anything we could shoot. Oh by the way I currently reload my own pistol stuff so reloading is not new but all this stuff for riffle is because I'm much more worried/interested in the best setup as its for hunting and the pistols are for plinking and HD.
 

·
Banned
Joined
·
415 Posts
Sure, I would think that would be a good bet. My uncle hunts moose and elk with 150 gr. Speer Trophy Bonded Bear Claw (or occasionally the Barnes TSX, also in 150 gr.) in his .270 Win, and he's never had to shoot one twice. This includes a 400 yard shot on a large bull moose. I believe bullet selection is VASTLY more important for hunting game than the caliber. Use the right bullet for the job, put it in the right spot, and everything will end just fine. And a 140 gr. in nearly any type of bullet (pointed soft points, TSX, TTSX, ballistic tips, etc). I should just mention that ballistic tips kill animals. However, at any reasonably close range (under 100 yards) they quite often do a LOT of damage to the meat as well. I personally only use them on small game now.

If you use a 140 gr. in the Nosler Partition, the Swift A-Frame or the Barnes TSX (TTSX, or even the MRX), anything you shoot will die. For heavier game, I agree with my uncle and use the Speer Trophy Bonded Bear Claw. Using that in (I believe the more common for your rifle would be) 160 gr. and any elk you shoot should be DRT.
 

·
The Hog Whisperer (Administrator)
Joined
·
37,103 Posts
what does it mean? While looking at the barnes tipped tsx at the store i noticed it said something about the barrel twist. I dont remember the numbers or what faster or slower means but when I looked the bullets up on barnes website it doesnt mention anything about barrel twist. So now what does faster or slower barrel twist mean(with examples...7" faster than 9" or vise versa). How to pick a bullet based on barrel twist? I shoot a remmington model 7 7mm-08 with a 20" barrel with a 9 1/4" twist. So with that I am looking at reloading my own rounds and not sure how o pick what bullet to go with. I want an all around load that will be good for deer and elk(I alo hunt pig but not really worried about using a dfferent round for it). I have always used 140gr federal nozler partition factory rounds but have everything but bullets and possibly powder. I have plenty of brass and some LR CCI primers and IMR 4350, 4895 and 3031. that I can look to use. I have read alot of people using the hodgdon varget powder with good results. So main question is what do bullets and barrel twist mean when trying to match them for a hunting round?
Don't get wrapped around the axle on twist rate. All off-the-shelf hunting rifles have a twist that is fast enough for the heaviest (longest) common bullet in that caliber, and will likely shoot the lighter (slower) ones just fine. It's possible to make a bullet come apart in flight but rare and usually only happens with varmit bullets that are driven as fast as possible.

It's just not a a problem with usual hunting rifles that the bullet spins faster than it needs to. In fact it's certainly better than the alternative.

Pick a bullet weight that others can recommend as a good choice for the caliber and game, and get started! What your rifle likes is what it likes, regardless of the twist rate.....
 

·
The Troll Whisperer (Moderator)
Joined
·
23,917 Posts
To expand a bit on MikeG's input - you can fool around with twist rates and bullet weights forever chasing the elusive "best" combination. As stated, most manufacturers produce barrels with twist rates for common, middle of the road bullet weights for that caliber/chambering. Your 140 gr selection is smack dab in the middle of the 1:9 twist rate for a .284 bore.

The best thing is to find the bullet/powder/primer/seating depth that your particular firearm likes. Since you have experience reloading for handgun as well as rifle, I have no doubt you can juggle these combinations to find the "sweet spot" for your rifle.

I've always found the best accuracy is obtained when loading just below the maximum listing for the combination of choice of components. A bullet traveling 100 fps faster will not kill any better than the slower one - placement is still the answer. You just get more recoil and muzzle flash! :p
 

·
Super Moderator
Joined
·
13,632 Posts
I'll move this thread to ballistics, then try to sort out a few mistaken notions for you.

You are correct that shorter twist is faster because it makes the bullet rotate faster for any given muzzle velocity.

The number of inches given for a full rotation of barrel twist is called the pitch of the twist, though you usually only see the word "pitch" used in ballistic formulas; most folks have shortened "rifling twist pitch" to just "twist", when they speak. Your 9¼" twist makes one complete 360° rotation in the bore for every nine and one quarter inches of barrel length. It does not directly tell you what bullets you can shoot.

All bullets have a minimum twist they need for a given muzzle velocity or they will tumble and keyhole in targets and fail to shoot accurately. They will also do well with a certain amount of extra spin rate up to a point. Thus, each bullet has a range of barrel twists it can work with. As Mike and Kdub said, commercial rifle barrels have twist rates chosen to work with most of the commercial jacket and core bullet weights available in the caliber, tending to be near minimum for the longest and heaviest and near maximum for the lightest and fastest. This is what gives Cvc944 the impression you don’t need to worry about twist rate choice with common jacket and core bullets, but the wrong twist will throw them off, too. An example:

A .222 Remington has a 14" twist barrel as standard. These will shoot most jacketed bullets about 55 grains or lighter with excellent accuracy. Try to shoot an 80 grain match bullet, though, and you are out of luck. It will hit the paper sideways if it doesn't miss it completely. Change to an 8" twist barrel, and the 80's will shoot fine. This is why, in the .223 Remington, rifles for light varmint bullets can have a 12” twist or even the older 14” twist, while match rifles for longer range, like an AR match rifle, will have an 8”, 7 ½”, or even a 7” twist barrel to shoot long heavy bullets. The bullet makers will warn you that you need at least a “x inch” twist for their heavier target bullets. Berger, for example, lists twist rates with his bullets.

What twist does, of course, is determine how fast the bullet spins for any particular muzzle velocity. That rate of spin determines the gyroscopic stability of the bullet. When a bullet flies, air pressure at the nose constantly tries to turn in. It has to spin like a gyroscope to resist being turned. If a bullet doesn't spin fast enough, it will be turned, causing it to tumble and move off its intended trajectory. If it doesn’t miss the target completely, it often strikes sideways or partly sideways in random locations. In paper targets that produces an oblong “keyhole”, instead of a round hole.

If a bullet is made to spin too fast, three things can go wrong: Any tiny imperfection in its weight distribution around the spin axis makes it wobble and the faster it spins, the bigger the wobble; accuracy is deteriorated by wobble. If it is a jacketed bullet and you try to drive it too fast for the twist in your barrel, the rotational acceleration can cause the jacket to slip free of the core and actually spin faster than the core. This is called core stripping and it results in reduced accuracy. A bonded core bullet should not do it nearly as easily as a common pressed core. In the worst case, a bullet can be made to spin so fast it flies apart on its way to the target.

The longer a bullet is, all else staying the same, the more twist it needs. Bullet length is the lever arm for the air pressure trying to turn the bullet. Length is the most critical single factor in choosing twist rate, because a small increase in length can cause a large increase in spin requirement. A solid base wadcutter, being blunt, needs less twist than a pointed bullet the same weight and same density. That is because the pointed shape is longer. For any particular shape, longer bullets are also heavier, so they get to lower muzzle velocities and spin RPM. Thus, the faster twist they require doesn’t normally force them to spin so fast they suffer the bad effects outlined in the last paragraph. In some overbore guns, they may, though.

The lighter a bullet is, length staying the same, the faster the barrel twist it needs. What makes a gyroscope hard to turn is its spinning mass. The lighter the mass, the faster it has to spin to be equally hard to turn. Thus, a less dense bullet, like a Barnes solid, needs more twist than a denser conventional bullet the same size and shape. Tungsten bullets, for example, are more dense than lead, have very high BC's, and need less spin to be stable. Depleted uranium bullets even less.

Being less dense, a Barnes solid is also longer than same-weight bullets of the conventional jacket and core construction. That fact has the most effect on its need for a faster twist. However, if you have a twist that will stabilize regular jacketed bullets that are about 6% longer than the Barnes bullet, it will then stabilize the Barnes bullet, too. Remember, we are dealing with a twist range here, and not a fixed number.

Finally, atmospheric conditions affect twist requirements. With a given bullet and velocity, air that is more dense pushes harder against the bullet to try to turn it. It takes more spin to resist that. So, any condition that makes air more dense calls for faster barrel twist and vice-versa. Lower altitude, lower temperature, and lower humidity all call for faster barrel twist. Higher altitude, temperature, and humidity all call for slower twist. The military has sometimes used faster twists than commercial makers (10” instead of 12” for .30-06, for example) for same size and weight bullets, just to be sure they would still be stable in extreme climate conditions.

How much twist do you need? Ballisticians speak in terms of the gyroscopic stability factor, G.S., or more often just s. This is a number specific to your bullet, figured from its physical shape, weight, spin rate, and velocity. It is calculated such that when s=1.0 or higher, the bullet is stable in flight. Anytime s is less than one, the bullet is unstable and will tumble. How big s can be without causing too much wobble depends on the opinion of the ballisticians consulted, but a Sierra ballistician told me he sees best accuracy with their bullets comes when s=1.3 to 3.0, IIRC?

That information forms the basis for a recommended twist range. A twist that gives the longest and heaviest bullets you will shoot nothing less than s=1.3 and the shortest and lightest no more than s=3.0 should have you good to go. If you add in your extremes of weather conditions (coldest, driest, highest barometer reading for the long bullets, vice versa for the short bullet) it will necessarily narrow the range of choices.

If you want to pick an optimal twist rate for a set of atmospheric conditions, Harold Vaughn recommends s=1.4 as optimal. Don Miller recommends s=1.5 as optimal. Lower numbers don’t let a bullet recover from in-barrel tilt and bullet jump as quickly in flight, affecting short range accuracy adversely. Faster numbers get more wobble from imperfections in the mass symmetry of the bullets, affecting accuracy at all ranges. The Vaughn and Miller numbers are where they believe the best compromise lies. I believe them both, so I split the difference and use s=1.45.

The formula that Old Grump linked you to is the Greenhill formula, devised by George Greenhill in the late 1800’s for use with artillery shells. That it happens also to give working numbers for many supersonic bullets is a happy coincidence, but it is not really letting you optimize twist by providing an s value.

At the other extreme are Robert McCoy’s comprehensive analytical equations. They are beyond the scope of persons not versed in calculus and differential equations.

The best tool I’ve seen is Don Miller’s revision of the Greenhill formula to include the influences of air pressure and temperature and the effect of velocity continuously (and not just at a break point of 2800 fps, as with some Greenhill variants). His version gives you the value of s for your result. The only thing it does not do is compensate for the increased drag in the transonic range, but if you stay above 1400 fps or below 1050 fps at sea level, it’s not a concern.

You can use a free on-line version of the Miller calculator at the JBM calculator site. That site also has a growing list of bullet lengths to use. You can also go to the file repository I have and download an Excel version I created. I added a couple of features. It lets you enter the s you want and also the barrel twist you have or are looking at buying and it gives you both the twist you would need to get the desired s, and also the s you will get from the twist you entered. It also has an atmospheric barometric pressure estimator for altitude. I also put in a second worksheet page that calculates s from the angle of the diagonal stringing caused by wind on actual targets, so you can see what you’ve really got? If you don’t have Excel, I have tested it to be sure it runs in Calc, which is the spreadsheet program that comes in the free Open Office Suite.
 

·
Super Moderator
Joined
·
13,632 Posts
True enough! Sorry for any misinterpretation. The Internet is plagued by that hazard.
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #12 ·
WOW! some seriously smart talk going on now. Thanks for the good read and the added info. I think ill be doing some more research just because its pretty interesting. Sorry about the wrong location too! It's the newbie in me....
 

·
Banned
Joined
·
415 Posts
Also, just wanted to point out Nick, that the link Old Grump provided does NOT use the Greenhill equation, but rather uses it as a basis in his introduction to his twist rate calculator. The equation used on the link is Les Bowman's basic equation (1962, according to the site), with corrections provided by Ken Howell (1999).

I do agree, however, that the JBM calculator is a better one to use.
 

·
Banned
Joined
·
1,002 Posts
what does it mean? While looking at the barnes tipped tsx at the store i noticed it said something about the barrel twist. I dont remember the numbers or what faster or slower means but when I looked the bullets up on barnes website it doesnt mention anything about barrel twist. So now what does faster or slower barrel twist mean(with examples...7" faster than 9" or vise versa). How to pick a bullet based on barrel twist? I shoot a remmington model 7 7mm-08 with a 20" barrel with a 9 1/4" twist. So with that I am looking at reloading my own rounds and not sure how o pick what bullet to go with. I want an all around load that will be good for deer and elk(I alo hunt pig but not really worried about using a dfferent round for it). I have always used 140gr federal nozler partition factory rounds but have everything but bullets and possibly powder. I have plenty of brass and some LR CCI primers and IMR 4350, 4895 and 3031. that I can look to use. I have read alot of people using the hodgdon varget powder with good results. So main question is what do bullets and barrel twist mean when trying to match them for a hunting round?
I ran your barrel twist thru the Greenhil Formula. A .284" groove diam. with 9.25" twist shows to be good for bullets up to and including 1.30" long.
 

·
Super Moderator
Joined
·
13,632 Posts
Marsms,

Mea culpa! I was hurrying and glanced through and missed that. Thanks for the catch.

I took a more careful read of that page. Hatcher's Notebook (p. 556-557 of the 1961 edition) cites the British, Textbook of Small Arms discussion of Greenhill as the source of bullet density compensation same as Harris did, but he never says whether or not it was part of Greenhill's original equation or part of Greenhills follow-up discussion of it? I had believed it was the former, and several sources have it written that way. Hatcher further adds in a similar compensation for the density of the fluid medium the bullet flies in, but does not take credit for it, either, which implies it was in the same British book.

Greenhill worked on early artillery shells, not cast of pure lead (whose density is 11.34) nor of lead alloy. I always assumed the 10.9 density used by Greenhill was for a steel artillery shell filled with lead, but I am suspicious that he changed over to a projectile less expensive to study later. Looking through my books, I find Scottish ballistician Geoffrey Kolbe says Greenhill determined the equation for "prolate spheroid (egg shaped) projectiles made of lead", but it couldn't have been pure lead with a density of 10.9. Robert Rinker gives the source as Greenhill's, On the Rotation Required for the Stability of an Elongated Projectile, published in volumes 10 through 14 or the Royal Artillary Institution which must be or have been a journal or set of books? Hatcher gives the Krag bullet as an example of a projectile having about the 10.9 density. More modern jacketed bullets are usually credited with a density of more like 10.7, but that varies with the design, of course.

I'd always understood Greenhill's original equation to be:

T=C/L x √(S.G./10.9)

With
C=150
T and L being in calibers

But I could be wrong. Online I find that form cited in several places. Even the Wikipedia has it, but in the non-caliber form, with D² in the numerator of the first term to allow length units to be used for D and L and T.

If you use a jacketed bullet density of 10.7, then √(10.7/10.9) gets you 0.99, which is so close to 1.0 the S.G. compensation can still be be ignored. I assumed that was why it gets dropped from most citations of the Greenhill equation? But I'll have to dig out that Royal Atillery reference if it can be done. It's got my curiosity up now.

It appears that Bowen/Howell did add velocity consideration by calculating C rather than using a constant. The general admonition has been to change 150 to 180 at 2800 fps. Rinker mentions that "some think" there should be two change points for the constant at 2200 and 3200 fps, but doesn't provide simple numbers to substitute; just a suggestion to research typical twist rates used between 2200 and 3200 fps for the bullets of interest, and he mentions a rule to decrease the twist predicted by Greenhill by one turn per inch for each 350 fps above 3200 fps.


EPD99,

I don't know if you tried the Miller calculator? If you saw Old Grump's link, you'll note that C.E. Harris found the Greenhill calculation tends to overstabilize bullets some, giving a gyroscopic stability factor, s, of as much as 2. He thought 1.4 was minimum and 1.7 optimum. Vaughn theinks 1.4 is optimum, and Miller thinks 1.5 is. Geoffrey Kolbe agrees with Miller. The calculating method Harris used is not specified to compare.

If I run a 7 mm Barnes 175 grain TSX using Don Miller's adjustments and assuming about 2500 fps MV from a load of IMR4895 in a 24" tube, then, despite its 1.513" length, it gets s=1.33 from a 9 1/4" twist under standard temperature and pressure conditions. This means it would still be stable from your barrel but not enough for the very finest accuracy. That would take more like a 8 3/4" twist. So, you're not far off and could likely use that bullet for minute-of-large-game at ranges that aren't too great.
 

·
Banned
Joined
·
415 Posts
It appears that Bowen/Howell did add velocity consideration by calculating C rather than using a constant. The general admonition has been to change 150 to 180 at 2800 fps. Rinker mentions that "some think" there should be two change points for the constant at 2200 and 3200 fps, but doesn't provide simple numbers to substitute; just a suggestion to research typical twist rates used between 2200 and 3200 fps for the bullets of interest, and he mentions a rule to decrease the twist predicted by Greenhill by one turn per inch for each 350 fps above 3200 fps.
Nick, should you happen to dig that stuff up, let us know, will you? I suppose it's largely inconsequential to this discussion, but it certainly can't hurt to know! As for the velocity thing...I was always under the impression (though I can't actually recall reading this anywhere) that Howell had used both the 150 and 180, and sort "averaged" everything in between, so there wasn't an immediate jump? I DO recall reading, however, that he did use both points.
 

·
Banned
Joined
·
674 Posts
It's been my experience that most who play with ballistics swear at Greenhill than swear by him. The link was provided more for the write up because it gave a basic look at why twist is a factor than to provide an introduction into the mysteries of esoteric calculations. I can do the numbers but I have found paper numbers rarely work for me in the field. A little experimentation works better for this old dinosaur but understanding the concept guides my choices and explains some of my failures.
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #19 ·
Marsms,

EPD99,

I don't know if you tried the Miller calculator? If you saw Old Grump's link, you'll note that C.E. Harris found the Greenhill calculation tends to overstabilize bullets some, giving a gyroscopic stability factor, s, of as much as 2. He thought 1.4 was minimum and 1.7 optimum. Vaughn theinks 1.4 is optimum, and Miller thinks 1.5 is. Geoffrey Kolbe agrees with Miller. The calculating method Harris used is not specified to compare.

If I run a 7 mm Barnes 175 grain TSX using Don Miller's adjustments and assuming about 2500 fps MV from a load of IMR4895 in a 24" tube, then, despite its 1.513" length, it gets s=1.33 from a 9 1/4" twist under standard temperature and pressure conditions. This means it would still be stable from your barrel but not enough for the very finest accuracy. That would take more like a 8 3/4" twist. So, you're not far off and could likely use that bullet for minute-of-large-game at ranges that aren't too great.
What if I am using the 140gr out of a 20" barrel vs the 175 you mentioned. Does mass play a part in that or not and looking to be up closer to 2750-2800fps based on some other min-max loads printed. sorry but alot of this talk is still way over my head.
 

·
Banned
Joined
·
674 Posts
What if I am using the 140gr out of a 20" barrel vs the 175 you mentioned. Does mass play a part in that or not and looking to be up closer to 2750-2800fps based on some other min-max loads printed. sorry but alot of this talk is still way over my head.
Short answer is barrel length only costs you a few feet per second of velocity and has nothing to do with stabilizing the bullet. 20"- 22" is optimum barrel length another inch only gives you another 25 fps.

130 - 150 grain bullets are optimum for your rate of twist.
 
1 - 20 of 29 Posts
Top