Rifling Rate of Twist

[Winter Hawk]

In response to a question on another thread, and in order not to take that thread over, I figured it would be best to start a whole new topic.  The question had to do with accuracy of the patched round ball versus the rate of twist of the rifling.

The rate of twist of a rifle is directly related to the bullet length compared to its diameter, as well as the shape of the bullet, its velocity and the material it is made from.  Basically, the longer the bullet with respect to its diameter, the faster it has to spin in order to stabilize it so it doesn't start to tumble.  A round ball's "length" equals its diameter so it doesn't need a great deal of spin imparted by the rifling, thus a slow twist rate is used.  Since velocity is also part of the equation, to maintain accuracy with a faster twist we can decrease the bullet's speed as with the 1:48" barrel.

With an elongated bullet we need to increase the twist to stabilize it.  That is why the Lyman Great Plains Hunter has a (IIRC) twist of 1:32" since it is designed to use heavier, longer bullets and (shudder) sabots.

A very good treatise of the subject is found here:
http://firearmshistory.blogspot.com/2012/11/how-to-calculate-twist-rate-i.html 

It doesn't talk about traditional M/L firearms, but for an explanation of what happens and how to determine the rate of twist you want it is very good.

If I am doing a no-no by posting this, please delete it and let me know!

~WH~

[William]

Thanks for making this link available, I'm going to read it with great interest.

One thing though; you state that you can "decrease the bullets speed by using the 1:48" ROT.  How does that work?

[flintboomer]

Thanks for making this link available, I'm going to read it with great interest.

One thing though; you state that you can "decrease the bullets speed by using the 1:48" ROT.  How does that work?

I'm pretty sure he meant reducing the charge for lower velocity in a faster twist barrel might help. (It does!)

[Hanshi]

:48" ROT. 

I'm pretty sure he meant reducing the charge for lower velocity in a faster twist barrel might help. (It does!)
[/quote]



Yes it does in the case of shallow rifling. 

[Red Badger]

Nothing wrong with the link Winter Hawk,  we allow other sites information to be posted as long as you tell us where it came from, so posting the link is the best way to do that! If you copy and past the article and forget to say where we got it - that is a problem.  (Although If I see that I usually try and find the original and post a link to it for you ....)

Red Badger

[Tim Ault]

I could not and still can't wrap my mine around how a round ball (sphere) and be stable or unstable in flight a bullet yes understand that well and how twist effects it . But a sphere can not yaw or tip in flight on its own axis no matter how fast or slow its rotating..... it's round !!!     Sigh !   I stopped trying to figure it out and just use the amount of powder in that barrel the yields the two best accuracy nodes in my Dixie 50 cal with  1:56 twist that's 40 and 70 gr of 3F

[DandJofAZ]

See, you don't have to understand it....just have to do what works.... Ain't black powder fun...

Doug 

[Winter Hawk]

Sorry, I've been away for a while.  I understand Tim Ault's problem; the round ball question is one I don't understand fully either.  I understand that there are some old rifles with straight rifling, so no twist at all.  I dunno.  But rifling does make a difference over a smoothie so it works (by magic?).

And yes, I did mean that we reduce the powder charge to get better accuracy from a faster twist barrel.

Now I have been told that there is a correlation between round ball shooting and baseball pitchers, something about how the way the ball is twisting in flight changes what happens by the time it arrives at the batter's box but that eludes me also... 

It's magic, must be! 

-WH-

[pilgrim]

      years ago when I got deeply involved in Paintball,  I used a rifled barrel.   At the time it was a New innovation.  It improved the accuracy as well as the range.  But  if there was any dirt that adhered to the paintball,  the ball would curve,  just like when a golfer slices the ball  or when a pitcher throws a curve ball.     Now imagine what will happen to the round ball if  is not loaded with the sprue straight up or straight down.    That makes rolling the balls perfectly round a good idea. 

[Winter Hawk]

That makes rolling the balls perfectly round a good idea.

Or buying swaged balls from Hornady.

-WH-

[Tim Ault]

Interesting theories , after reading I revisited this in my mind especially the comparison to a baseball . And although a patched round ball is round indeed like a baseball or golf ball it's not perfectly smooth after it's been rammed down the barrel and kicked back out under 8000 or so psi of pressure it's gonna be not so perfectly round and marks for the patch indented in it deeper where the lands are also . So just like a baseball with the threads on it   Wonder if the same holds true with say a solid brass or copper ball that would maintain a perfectly spherical shape better than lead ? Would it shoot to the same accuracy level no matter what the velocity ?   Gotta get more coffee in me before I start into this deeper

[Hanshi]

Though not a very good analogy, consider how the gyroscopic inertia will keep the wheels of a bike stable and upright.  That same gyroscopic inertia bestowed upon the ball by the rifling also keeps it stable.  At least that's the way I envision it.

In a smoothbore it depends on how much, or either no rattle, the ball experiences in the bore whether or not a measure of stability is granted to it.  Velocity can also affect stability.

[pilgrim]

      Does a smoothbore get loaded with a patch and ball,  if so then there should be NO ball rattle.

[beowulf]

      Does a smoothbore get loaded with a patch and ball,  if so then there should be NO ball rattle.

there are two accpeted ways to load a smoothbore .  one is with a patch and ball works fine if you swab the barrel every few shots , as fouling makes it difficult to ram the ball down , the other way ( and both have been used since day one ) is to push a wad down the barrrel , drop the ball down , and use a card or folded piece of paper over it ! both give pretty good accuracy , I`ve used both methods ,  while it is a PITA I do prefer patched ball , because it`s a tad quicker !

[Hanshi]

My best smoothbore accuracy is with prb; and I don't load it anywhere near as tight as I do in my rifles.  I also get deer-worthy accuracy at 50 yards using a bare ball load, as described by beowulf.  The closer the ball is to bore size, usually the better when shooting with no patch.