Twist rates

ivanthehunter

Tighter rates of twist!
Does a tighter rate of twist(ROT) cause increased barrel ware on the rifling groves?
Does the rifling start off slowly or is it at full kilter right off the blocks or at the top of the throat rim.

Rovi

ivanthehunter wrote: »

Tighter rates of twist!
Does a tighter rate of twist(ROT) cause increased barrel ware on the rifling groves?

In theory, yes, but it's apparently only really a factor when the cartridges are loaded very hot and the bullet is launched at very high velocities. Throat erosion from the very high pressures and heat involved in these sort of firearms tends to wear out barrels long before the rifling gets worn out.

ivanthehunter wrote: »

Does the rifling start off slowly or is it at full kilter right off the blocks or at the top of the throat rim.

I don't know of anyone making anything other than standard rifling these days; 'gain twist' rifling (where the pitch increases along the barrel) has been tried by some specialist manufacturers, but appears to have been dropped as it showed no appreciable improvement in accuracy and was more expensive to make.
It's apparently popular in large naval guns, old blackpowder rifles firing spherical lead ball, and paintball guns.

ivanthehunter

Will a tighter rate of twist(ROT) reduce the muzzle velocity for any given round.

I understand key-holing and what causes it but what is the opposite effect called? By this I'm asking what if a 40gr round is blasted down a barrel with a tighter ROT more suited to a 100gr round , say 1:8.
Is the stability effected or might the velocity be reduced due to excessive friction with the rifling groves?

meathstevie

I think you're starting to drift into very small tolerance highly tuned custom made stuff territory now Ivan. As far as I can see when you stick to quality of the shelf stuff in regards to rifle as well as ammo you won't encounter too many problems. Having said that, once you start talking high end target shooting I suppose it's exactly this sort of issues thta starts making the difference.

Sparks

once you start talking high end target shooting I suppose it's exactly this sort of issues thta starts making the difference.

Depends on the kind of target shooting you're talking about...

Jonty

ivanthehunter wrote: »

By this I'm asking what if a 40gr round is blasted down a barrel with a tighter ROT more suited to a 100gr round , say 1:8.
Is the stability effected or might the velocity be reduced due to excessive friction with the rifling groves?

Some cheaper bullets can fragment upon exiting the barrel. Its best to check this without a mod on!! I fired 40gr hornadys in a 1 in 8 barrel no probs.

jwshooter

Jonty wrote: »

Some cheaper bullets can fragment upon exiting the barrel. Its best to check this without a mod on!! I fired 40gr hornadys in a 1 in 8 barrel no probs.

is that factory
ammo .i have not heard it

ivanthehunter

Does the bullets length have any effect on stability. That is, will a longer bullet have more stability because it holds a better alignment to the barrel?

It would seem to me (a civilian lay person) that the 40 grain rounds disintegrated(in the 1;8 ROT) because their bodies are obviously shorter and as such the high ROT is so intense and violent as to cause the centrifugal force to exaggerate any and all errors in the distribution of the bullets mass to the point were the bullet starts to oscillate with in the barrel to some degree. This oscillation causes the shoulder to bed in to the bore and hence the kick back forces the bullet seat(rear) into the barrel wall which intensifies the the next and opposite kick by the shoulder into the barrel inner wall..This continues until the bullet disinterred.Thats all IMO BTW...

Vegeta

ivanthehunter wrote: »

Does the bullets length have any effect on stability.

Longer bullets need faster twist rates. As longer bullets are generally heavier it is common to think heavier bullets need faster twist rates when it is length that is the important factor.

ivanthehunter

What about the better barrel alignment. You seem to be set in your ways with statements like" it is common to think", I am new to this sport and as of yet i haven't formed any bad habits or preconceptions that normally effect the rest of the learned world after they have been forced to believe the findings of others as hard irrefutable fact.. Are you addicted to the dogmas of your own beliefs?:D
I have the advantage of thinking outside the box without even trying on this one:p

Vegeta

ivanthehunter wrote: »

What about the better barrel alignment. You seem to be set in your ways with statements like" it is common to think", I am new to this sport and as of yet i haven't formed any bad habits or preconceptions that normally effect the rest of the learned world after they have been forced to believe the findings of others as hard irrefutable fact.. Are you addicted to the dogmas of your own beliefs?:D
I have the advantage of thinking outside the box without even trying on this one:p

Well ivan you are the one who asked about rate of twist and bullet weight. So already you have linked bullet weight, rate of twist and stability. Think outside the box if you want but again you are the one who made that connection.

You asked if bullet length is an important factor in stability. It is.

Sparks

ivanthehunter wrote: »

I have the advantage of thinking outside the box without even trying on this one:p

I am reminded of the story of the guy who went to a Nationals with his buttplate upside down because he was "thinking outside the box"....
...he didn't finish with a medal, oddly enough...

Red Renard

Sparks wrote: »

I am reminded of the story of the guy who went to a Nationals with his buttplate upside down because he was "thinking outside the box"....
...he didn't finish with a medal, oddly enough...

Is that when the rot set in. I would have to err on the side of caution than end up in a twist with my buttplate upside down.:D

ivanthehunter

Vegeta wrote: »

Well ivan you are the one who asked about rate of twist and bullet weight. So already you have linked bullet weight, rate of twist and stability. Think outside the box if you want but again you are the one who made that connection.

You asked if bullet length is an important factor in stability. It is.

Never mind the other poster here! I,ll respond to your polite post Vegeta.
Firstly you have claimed that it is common to believe that longer bullets require tighter rates of twist because you believe them to be heaver than shorter bullets. here lies your error, the fact you made bold assumptions

I can see you point, but you have not commented on the physical interaction of the longer round holding better alignment due to increased contact surface along the bore....
This extra stability must have some impetus into the amount of extra stability required to stabilise any round of a unit weight. This would therefore IMO mean that as weight/grain-size increases so does the requirement for a tighter rate of twist in order to control such a weight up till the point were the bullet characteristics start to change the dynamics of the interaction. But remember that there probably exists some where a study on the optimum shape for a bullet and I'll bet its bloody complicated and its some how related to charge and caliber and rate of twist..

What does twist achieve and how does it do it. This can be best understood with a look at a simple arrow and the two hen feathers and the cock feather. An observer can easily see that the alignment of the feather is set in such a fashion as to cause the rotation. The benefit of this rotation is to allow any aerodynamic errors to ironed out of the arrows trajectory. An arrow which didn't spin might have a tendency to bear right due to inconsistencies in the design, but through rotation this tendency to bear right is transformed in to a spiral trajectory which to all intense purposes ids deemed as being straight..

The bullet is no different and the rate of spin is designed to help the bullet sail through the air is albeit as straight a line a possible, surely a longer stream-lined bullet will be subject to aerodynamic forces which will deter it from veering of is trajectory

Sparks

An arrow which didn't spin might have a tendency to bear right due to inconsistencies in the design

What are you on about? Arrows don't have a tendency to bear right, they bear left, and it's not because of a design flaw, it's because they rest on the left hand side of the outside of the bow (unless you have a left-handed bow), which is why your bow sights are always set to compensate.

The bullet is no different

Bullets are very different. Much shorter, much denser, much more energy, much shorter flight times, far less stable, they approach mach one during flight and they have a very different distribution of mass.

a longer stream-lined bullet will be subject to aerodynamic forces which will deter it from veering of is trajectory

Yes in theory. In practise, the aerodynamic forces that stabilise it are hugely exceeded by those which destabilise it and that's why you need the spin (gyroscopic stabilisation trumps the destabilising influence of turbulence and drag, at least for a while).

Vegeta

ivanthehunter wrote: »

I can see you point, but you have not commented on the physical interaction of the longer round holding better alignment due to increased contact surface along the bore....
This extra stability must have some impetus into the amount of extra stability required to stabilise any round of a unit weight. This would therefore IMO mean that as weight/grain-size increases so does the requirement for a tighter rate of twist in order to control such a weight up till the point were the bullet characteristics start to change the dynamics of the interaction. But remember that there probably exists some where a study on the optimum shape for a bullet and I'll bet its bloody complicated and its some how related to charge and caliber and rate of twist..

Do you have any diagrams of bullet designs for differnt length bullets of the same caliber? I'd be interested in looking over some of them.

I currently have 75 grain BTHP and 50 grain HP flat based bullets for my .223. I haven't measured them but the amount of the bullet in contact with the rifling would be very close I'd imagine. The nose (which is not touching the rifling) and the boat tail (again not touching the rifling) of the 75 grain round add to the overall length and therefore weight but I don't know for a fact if it adds to the amount of copper touching the rifling versus the 50 grain round.

I also don't think (could be wrong and am very open to new ideas if someone has a differing opinion) the difference between the length of copper touching the rifling from say a 75 grain round compared to the 50 grain round is enough to introduce an instability. How could it? The instability we are talking about occurs perpendicular to the direction of motion and the round is a good fit for the rifling/bore or it is not. If it is a good fit it will not move, regardless of length. If the round is not a good fit and does not make a secure contact with the rifling then it is crap ammo.

long and thin needs to be rotated fast. That's how I see it

ivanthehunter

Vegeta you could be right,

take a look http://en.wikipedia.org/wiki/Rifling


Was the thread monster around with his hover again?;)