.308 twist rate?

[tt350z]

Re: .308 twist rate?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Turk</div><div class="ubbcode-body">http://www.6mmbr.com/barrelfaq.html#24640
I assume if one sees it as a function of time you are correct; but the bullet spins an x amount of revolutions for a given distance if I am not wrong? </div></div>

You can pick time or distance, it doesn't matter.

For the example below, let's just assume that the velocity doesn't change for the duration of it's flight for simplicity sakes.

A bullet at 2640 FPS (31,680 RPM) with a flight time of 2 seconds will have completed 1,056 revolutions in that 2 second span.

RPM (31,680)/second (60 in a minute) X (flight time(2) = 1,056

Same bullet at 2640 FPS spinning at 31,680 RPM will rotate 1,056 revolutions at the mile marker (5,280 feet)

31,680 / 60 = 528 revolutions per second
Since there is a 2 second flight @ 2640 to reach 5280, just multiply by 2 and you get the same answer, 1,056

 

[Turk]

Re: .308 twist rate?

1 revolution in the same distance but in less time w/ higher mv?

 

[tt350z]

Re: .308 twist rate?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Turk</div><div class="ubbcode-body">1 revolution in the same distance but in less time w/ higher mv? </div></div>

I get what your asking now

A slower MV will have the same amount of revolution when compared to a higher MV at any given distance.

But the bullets RPM is what determines how stable a bullet is throughout it's flight.

Use a motorcycle for example.

If you ride at 1 MPH for 1 miles the tire will rotate the same as a bike traveling at 60 MPH for 1 mile, barring any wheel spin.

But which one will be more stable? The bike going at 60 MPH because the wheels are traveling at a much higher RPM increasing it's gyroscopic force, which keeps it stable.

 

[Turk]

Re: .308 twist rate?

I really appreciate your scientific input! I was just trying to state that barrel twist rates are given as 1-10,1-11 and...
and mv doesn't change that? Agreed that speed has influence on stability; but I was trying to see where the trade off was w/ a faster twist barrel that produces slower mv and one w/ a slower twist and higher mv especially in shorter barrel in a 308. Again I appreciate your level of education and scientific input!

 

[tt350z]

Re: .308 twist rate?

You bet.

High school algebra from 6 years ago saves the day!

I should take some college classes...

ETA:

Just noticed something odd.

That's what I get for posting thing that require thinking late at night.

I was going from feet per second but totally forgot about needing to multiply by 60 to get RPM.

And also I needed to divide 12(inches per foot) by X(number of twists per inch).

So the actual formula is:

MV X (12/twist rate) X 60 = RPM

 

[biffj]

Re: .308 twist rate?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tt350z</div><div class="ubbcode-body">You bet.

High school algebra from 6 years ago saves the day!

I should take some college classes...

ETA:

Just noticed something odd.

That's what I get for posting thing that require thinking late at night.

I was going from feet per second but totally forgot about needing to multiply by 60 to get RPM.

And also I needed to divide 12(inches per foot) by X(number of twists per inch).

So the actual formula is:

MV X (12/twist rate) X 60 = RPM

</div></div>



Lets see how this works out because I come up with 159,000rpm for a 12 twist at 2650 fps and 150,000 at 2500 fps. . .

for a 10 twist the 2650mv works out to 190,800rpm and 2500 works out to 180,000 so your formula now works fine.

We have to remember that the gyroscopic action that actually stabilizes the bullet becomes more powerful with diameter so as the diameter decreases the spin rate needs to go up for the same stability. What works for a .50 cal won't stabilize a .30 cal. Also as we drop the velocity the spin rate drops and so does stability. This is the reason that we need a faster twist for subsonic shooting and slower for the really fast stuff.

Frank

 

[XTR]

Re: .308 twist rate?

Is the twist rate really that important in a .308 or is this a contagion from the 5.56 crowd that is giving people something else to obsess over? I certainly don't think that .308s are as sensitive, people have been accurately shooting with 1:12s for year with bullets up to 175.

 

[Rob01]

Re: .308 twist rate?

This is a perfect place for one of Lowlight's classics:

"You guys could over complicate and ruin a blow job left to your own devices."

 

[josephberry]

Re: .308 twist rate?

analysis paralysis