Rifling Twist Rate

[necchi]

That is the interesting aspect of the twist rate that many don't put together.
I know it took me awhile to wrap my brain around the idea
That the entire goal of twist rate and velocity is to get the projectile spinning an optimum of revelutions per minute by and of itself.

Then take 14000 rpm's and divide by 60 for rev's per second, too get 233.333 revelutions per second and stuff that up against 1400fps and the ball spins in flight 6 times in a foot of travel,,
,but you just got that from a barrel that has a twist of one revelution in 66"s and the barrel is only 32"s long so the rifleing doesn't even make a half twist in the full length of the bore,
:shocked2: Ouch! :youcrazy: that stuff makes my head hurt,, :redface:

 

[George]

Must be that new math I keep hearing about.

Spence

 

[woodse guy]

Homemade scatter gun. :haha:

 

[Stumpkiller]

Then take 14000 rpm's and divide by 60 for rev's per second, too get 233.333 revelutions per second and stuff that up against 1400fps and the ball spins in flight 6 times in a foot of travel,,,but you just got that from a barrel that has a twist of one revelution in 66"s and the barrel is only 32"s long so the rifleing doesn't even make a half twist in the full length of the bore,
:shocked2: Ouch! :youcrazy: that stuff makes my head hurt,, :redface:

1/6th of one revolution in one foot of travel.


14,000rev/minute * 1 minute/60 sec * 72"/ revolution * 1 ft/12" = 1,400 ft/sec.

The 'ol math works. :hatsoff:

 

[necchi]

14,000 rpm
60 seconds = 233.333 rev per second

1400 feet per second
233.333rps = 6.0000857 revelutions per foot

:hmm: Where did I mess up?

 

[Stumpkiller]

You've got seconds in the denominator of both fractions. Have to invert one. 1400 fps * 1/233 rev per sec will cancel out the seconds and yield 6 ft per revolution, or 72" = 1 revolution.

 

[necchi]

1400 x .16666,,, = 233.3,,,
Got it!
math,

I need a new abicus,

 

[Dphar]

"Approximately"
Robby

Rotational drag is a fraction, a small one, of the drag effecting velocity and trajectory.

Dan

 

[msuspartan]

I need a new brain :surrender: ....ouch.... glad I shoot and the bullet hits the target and people tell me how to work up a load.

TOO much math, guess that is why I am a Draftsman..... :idunno:

The .22 story if funny in a wierd way tho'. :haha:

Cheers, DonK

 

[Robby]

That is why I said approximately.
Robby

 

[paulvallandigham]

Those thin jackets on those light weight bullets for a .22 hornet simply can't hold the bullet together at much higher velocities, and faster RPM. I read a study that when you go much above 4,000 fps, even the best jacketed bullets begin to blow up in mid-air, and they switched to using solid copper bullets for those higher velocity tests. I also read of a study where they made the bullets out of bronze- didn't do much for barrel life, but they held together for 6,000 fps!

Soft lead bullets actually stay together longer, altho they strip out on the rifling when fired too fast, even when patched.

The Slug Gun shooters, for instance, like to keep those big lead bullets at under the Speed of Sound at the muzzle for best accuracy. And they use the nose design on their bullets that is similar to what the Army and other researchers worked out for the .22 Long Rifle bullet back in the 19th century. At subsonic velocities, that round nose design produces the best long range accuracy.

Personally, I suspect that the round nose shape began its design life from looking at how accurate Lead Round Balls could be out to 100 yds. But, that is just my own opinion, with no actual facts to support it. I just think the idea, " If it ain't broke, don't fix it", has been around for a lot longer than anyone imagines. :hmm:

 

[Billnpatti]

It all has to do with basic physics. The greater the aspect ratio, the ratio of the bullet diameter to its length, the faster it must spin in order to be stabilized in flight. If an elongated projectile is fired from a smoth bore, it immediately wants to start tumbling end over end. When this happens, it looses all of its accuracy and becomes less accurate than a round ball fired from a smoothbore rifle. It must be spun faster and faster in order to overcome this tendency to tumble.

As one of the other responders said, the spinning of any projectile increases its accuracy by averaging all tendencies to deviate from the intended path so that it ends up flying in a straight line. Even a round ball needs to rotate to fly straight. However, since it has an aspect ratio of 1, any tendency to tumble is quite minimal and even if it does tumble, being round it has no effect on its desired line of flight.

Generally speaking, a twist rate of 1:66 or a bit greater is best for a patched round ball. A twist rate of 1:48 will work with both patched round balls and elongated bullets. It is a compromise since it is a bit fast for a patched round ball and a bit slow for a bullet but it does seem to work pretty well for both. The best twist rate for an elongated bullet will be dependent on its aspect ratio so the longer bullets will require a faster twist rate than the shorter bullets. There is a formula for getting you in the ballpark when designing a twist rate for a specific bullet. I recommend a book on basic firearm ballistics for a better explanation.

 

[Danbo]

Aspect ratios aren't really part of the equation in analysis of gyroscopic stability. Badminton shuttlecocks don't spin yet they are dynamically stable. Conical bullets are dynamically unstable.
Round balls are dynamically stable but aerodynamically unstable.
The meat of the issue has to do with centers of drag and gravity dislocation (pitching moments) which are functions of form, bullet material(s), velocity, caliber.
In discussion of twist rates for round balls, the previous info is correct...it is an aerodynamic issue, not one of gyroscopic stability. For conical bullets stability is gained when sufficient angular momentum is imparted by the twist rate and velocity to overcome pitching or overturning moments caused by the center of aerodynamic drag being forward of the center of gravity. Aerodynamic drag for a given form is a variable factor, so twist rates must be sufficient to stabilize the bullet at velocities where highest drag if found. Generally speaking, for black powder guns that is the Speed of Sound, or Mach 1.

 

[necchi]

Good point Danbo,
,but if it's true in theory, why won't a smoothbore shoot a ball as accurately as a rifled bore?
(given the round ball projectile only)

 

[Danbo]

Round balls are aerodynamically unstable (read: knuckle ball). Smooth bores demonstrate that fact. Given slow spin from a long rifle, imperfections of form and balance are presented uniformly about the axis of flight/rotation. Random rotations are eliminated as well by a rifled bore. I doubt that is a factor within the barrels of smoothbores, but once free and otherwise not stabilized, they are free to slowly rotate on any axis.

Not wanting to sound snooty, but it's not theory. Concise understanding of this came a long time ago. Greenhill was the first to make it generally understood. Recent aeroballisticians such as Vaughn and McCoy worked out some frightfully fine details with the help of hardware and staff at Sandia National Labs and Aberdeen. With that said, I'm the messenger, not the rocket scientist.

 

[necchi]

Given slow spin from a long rifle, imperfections of form and balance are presented uniformly about the axis of flight/rotation. Random rotations are eliminated as well by a rifled bore.

And a fine messenger at that. Well said.
:v

 

[nkbj]

When stability is marginal with a conical and the twist you have to work with, then boosting the RPM's with more powder can sometimes help. Sometimes I can't help but suspect that the bullet is just getting shorter. :haha: