.220 Swift - Non Lead - 1:14 twist

scope-eye

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That is not quite correct, the reason they have recomended twist rates for certain bullets is because that bullet in particular needs a certain RPM to stabilize. OK here we go a bullet traveling 3000fps with a 12 twist will exit the barrel at 180,000 rpm. A bullet traveling 3500fps with a 14 twist will also exit the barrel at 180,000 rpm. In my case I have a 220 swift AI with a 16 twist and I can send that same bullet 4000fps and they will also exit at 180,000 rpm.
The formula is Muzzle Velocity X 720 Divided by Twist Rate. Hope this helps

Dean

PS: That 220 Swift AI has just shy of 50gr of powder capacity and can send those 50gr Barnes 4500fps.
 
Last edited:

Mikecr

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Scope-eye, you're totally wrong. An RPM calc is not a stability calc.
There is no 'time' in stability requirements, but displacement per revolution(each).
And no credible bullet maker would ever declare stability requirements in RPMs. Have you ever noticed that?

Example: a typical 55gr 22cal bullet under standard ICAO conditions(SL at 59degF):
in 14tw
Sg = 1.06 at 2kfps, kRPM 103
Sg = 1.11 at 3kfps, kRPM 154
Sg = 1.14 at 4kfps, kRPM 206
Sg = 1.24 at 10kfps, kRPM 514 (still marginal)

in 12tw
Sg = 1.44 at 2kfps, kRPM 120
Sg = 1.52 at 3kfps, kRPM 180
Sg = 1.56 at 4kfps, kRPM 240
Sg = 1.69 at 10kfps, kRPM 600

You can see from this that there is no direct correlation between RPM and stability, and that no realistic amount of RPM's could provide full stability in 14tw with that bullet.
Well, unless you consider 27,000fps / 1,337,000 RPMs realistic....

What DOES make a difference is the relative displacement:
Same bullet, in 14tw, 2kfps, but 100degF and 7800' elevation, Sg = 1.50
 

scope-eye

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Go to bottom othe page to the last paragraph.
Or even better from they article

"That said, we note that bullet-makers provide a recommended twist rate for their bullets. This is the “safe bet” to achieve stabilization with that bullet, and it may also indicate the twist rate at which the bullet shoots best. Though the RPM number alone does not assure gyroscopic stability, an RPM-based calculation can be very useful. We’ve seen real world examples where a bullet that needs an 8-twist barrel at 2800 FPS MV, would stabilize in a 9-twist barrel at 3200 FPS MV. Consider these examples"

Dean

MV = 2800 FPS
8-Twist RPM = 2800 x (12/8) x 60 = 252,000 RPM

MV = 3200 FPS
9-Twist RPM = 3200 x (12/9) x 60 = 256,000 RPM
 

TxPhred

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Nov 4, 2012
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Location
Indiana
Title says it all. I have to shoot Non-Lead in Kali.
Does anyone have a copper bullet that will stabilize in a 1:14 twist barrel?
I'm down to my last 80 of 52 gr. HPBT lead and will have to re-load non-lead. I would like to duplicate my current load of 41gr 4350 with a copper.
I have had very good luck with the Barnes TSX 45 gr bullet in my 220. very fast and accurate for me.
 

ButterBean

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Scope-eye, you're totally wrong. An RPM calc is not a stability calc.
There is no 'time' in stability requirements, but displacement per revolution(each).
And no credible bullet maker would ever declare stability requirements in RPMs. Have you ever noticed that?

Example: a typical 55gr 22cal bullet under standard ICAO conditions(SL at 59degF):
in 14tw
Sg = 1.06 at 2kfps, kRPM 103
Sg = 1.11 at 3kfps, kRPM 154
Sg = 1.14 at 4kfps, kRPM 206
Sg = 1.24 at 10kfps, kRPM 514 (still marginal)

in 12tw
Sg = 1.44 at 2kfps, kRPM 120
Sg = 1.52 at 3kfps, kRPM 180
Sg = 1.56 at 4kfps, kRPM 240
Sg = 1.69 at 10kfps, kRPM 600

You can see from this that there is no direct correlation between RPM and stability, and that no realistic amount of RPM's could provide full stability in 14tw with that bullet.
Well, unless you consider 27,000fps / 1,337,000 RPMs realistic....

What DOES make a difference is the relative displacement:
Same bullet, in 14tw, 2kfps, but 100degF and 7800' elevation, Sg = 1.50
I'll have to agree with Scope Eye MV can overcome marginally stable bullets as I have done it myself, All you have here is some numbers that you calculated which serve as a guideline and nothing more
 

Calvin45

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I'll have to agree with Scope Eye MV can overcome marginally stable bullets as I have done it myself, All you have here is some numbers that you calculated which serve as a guideline and nothing more
I’ve emailed Sierra some time ago about their newer 200 grain smk (not the old one, there’s 2 different 30 cal 200 smks, new one is wicked slick looking). They recommend a 1:9 twist. I asked about it in my 300 win with a 10 twist. They replied the 1:9 is recommended for .308 win velocity stability, that at 300 win speeds I’d have no problems. It seems the ballistics experts there believe speed helps stabilize.
 

ButterBean

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I’ve emailed Sierra some time ago about their newer 200 grain smk (not the old one, there’s 2 different 30 cal 200 smks, new one is wicked slick looking). They recommend a 1:9 twist. I asked about it in my 300 win with a 10 twist. They replied the 1:9 is recommended for .308 win velocity stability, that at 300 win speeds I’d have no problems. It seems the ballistics experts there believe speed helps stabilize.
Say it aint so ;)
 

Mikecr

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Then allow me to speak more on improving stability with velocity increases.
With my earlier example:
in 14tw
Sg = 1.06 at 2kfps
Sg = 1.11 at 3kfps

Sg of 1.06 is barely stable, and if your air density bumps one day you could dip to <1.00 (where bullets will tumble).
So maybe you're advised that this will still work at higher velocities, and sure enough 3kfps MV does raise Sg back to marginally stable. The bullets impact point forward now, and that certainly is a big affect.
But you should know that it's a poor plan to be on that edge. That marginal stability holds detrimental affects to BC, and accuracy.
And regardless of velocity, that bullet/twist combo would never reach better than marginal stability under SL std air densities.

Ideally your Sg would be around 1.5, and with this, velocity will not make one bit of difference about anything stability-wise.
It's a goal you should be solving your plan for.

Notions about matching RPMs to work around actual stability requirements will lead you to failed tests.
Go ahead and explore it with credible calcs. You will see this. No need to believe me at all.
I am just trying to help you see this.
 

ButterBean

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Messages
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Location
West Terre Haute Indiana
Then allow me to speak more on improving stability with velocity increases.
With my earlier example:
in 14tw
Sg = 1.06 at 2kfps
Sg = 1.11 at 3kfps

Sg of 1.06 is barely stable, and if your air density bumps one day you could dip to <1.00 (where bullets will tumble).
So maybe you're advised that this will still work at higher velocities, and sure enough 3kfps MV does raise Sg back to marginally stable. The bullets impact point forward now, and that certainly is a big affect.
But you should know that it's a poor plan to be on that edge. That marginal stability holds detrimental affects to BC, and accuracy.
And regardless of velocity, that bullet/twist combo would never reach better than marginal stability under SL std air densities.

Ideally your Sg would be around 1.5, and with this, velocity will not make one bit of difference about anything stability-wise.
It's a goal you should be solving your plan for.

Notions about matching RPMs to work around actual stability requirements will lead you to failed tests.
Go ahead and explore it with credible calcs. You will see this. No need to believe me at all.
I am just trying to help you see this.
I see perfectly fine and once again Numbers is all they are and while the math is correct its still just a reference point,
 

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