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7thLightID

Help? Twist Stability Calculators

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Hi,

I did search for this beforehand, got nothing.

Am Honorable Veteran, and have been shooting since about 7. Done lots of shooting but have never bought a custom barrel b4.

In this proces, there is the specifying of the twist rate. To do this, I was trying to use various Twist/Stability calcs found online. Problem is, the calcs want "Bullet Length". Presuming they didnt mean "Cartridge" length, I set out to find the caliber's bullet length. A BIG kudos goes out to jbmballistics.com who is generating the only list, built by shooters/reloaders (I even tried Speer, Hornady etc.), where I could find this data.

Now, when putting in this data, I end up with some extremely high inch twist that I know is off. If I put in "Cartridge" length, I hit closer to realistic/common. Wth??

My questions are:

1. WHY would the added length of the brass be part of a calculation where the brass doesn't go down the barrel where the twists "reside"/affect trajectory of the real projectile? I can't see why including the case length would provide a better answer than that of what would actually be contacting the rifling when using these T/S calcs.

2. How does one use these accurately? The other factors (fps, caliber etc.) are all common sense. But what is it I am doing/not doing to get these oddball #s? Am I destined to bust out my school calc and do some "Miller" by hand? Ughh...

ty b4hand.

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Please link what you're talking about - where are they asking for this info?  That'll help, and I'll go look at it and see what they're doing...

Also, when were you at Ord?  I was there through '91, got shipped to Korea for a year and had a HAAP back to Ord.  By the time I was leaving Korea, my orders were changed to "7th ID(L), Forward, Ft Lewis, WA..."

Hit the Intro Section when you get the chance...

Edited by 98Z5V

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Also, what do plan on shooting/ using for projectile weights, and or specific projectile. 

 

And please, you are not texting, type things out. Nothing worse than reading illegible nonsense. Be eloquent and loquacious.

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39 minutes ago, edgecrusher said:

 Be eloquent and loquacious.

...must be VERY loquacious, when regarding such a subject as this...  

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98Z5V: Hell yeah Light Fighter! There in 85-88, went trough ROP twice because we got attached when the unit was 1st getting certified via DOA/DOD and had to go again about year later when unit was full man and the newbs had to pass ROP to stay. Sucked. "Hungry Lizard"/Hunter Ligget, Pioho Airstrip doing air assault and Palisades for rappelling. LOVED IT! Miss it now even today. They tore it all down judging by Google Earth. We were first down by Stilwell Hall and the sewage plant in the ol' WW2 barracks then moved up onto the "Hill". Sounds stupid, but I kinda miss that smell when we'd do our morning runs on the beach. Kinda wanna go back.  Miss my friends/comrades. A bond like no other. All you other Vets know exactly what I mean too. Yeah, a big part of me that I carry every day and proud of my time "In".

Anyway, sorry for being so "loquacious" and waxing nostalgic. ;)

Okay the websites there are many, but most are similar and will give the 2 that are different in approach:

1. http://mcgowenbarrel.com/twist2/

2. http://www.bergerbullets.com/twist-rate-calculator/

Here's the JBM one too on lengths:

http://www.jbmballistics.com/ballistics/lengths/lengths.shtml

According to the JBM header, this guy is doing this on his own and I think he deserves some Kudos for his/hers/they're efforts.

My Build:

45 Raptor. 16-18" bbl haven't decided yet, LR308 based.  

 

"And please, you are not texting, type things out. Nothing worse than reading illegible nonsense. Be eloquent and loquacious."...LOL Sorry, I try to only ask a question once. :)

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1 hour ago, 7thLightID said:

98Z5V: Hell yeah Light Fighter! There in 85-88, went trough ROP twice because we got attached when the unit was 1st getting certified via DOA/DOD and had to go again about year later when unit was full man and the newbs had to pass ROP to stay. Sucked. "Hungry Lizard"/Hunter Ligget, Pioho Airstrip doing air assault and Palisades for rappelling. LOVED IT! Miss it now even today. They tore it all down judging by Google Earth. We were first down by Stilwell Hall and the sewage plant in the ol' WW2 barracks then moved up onto the "Hill". Sounds stupid, but I kinda miss that smell when we'd do our morning runs on the beach. Kinda wanna go back.  Miss my friends/comrades. A bond like no other. All you other Vets know exactly what I mean too. Yeah, a big part of me that I carry every day and proud of my time "In".

EXCELLENT Loquacious response!!!   :thumbup::laffs:  I hear ya, exactly, and feel the same way.  That was the best unit I was ever in - it was no-nonsense.  I miss it, too, and only went back once, sometime between '99 and '01.  It was sad to see.  Went out to the old ROP site (CLC - "Combat Leader's Course" - my a$s...) - that sucked to see the condition of the Planning Bays and everything.  My barracks was right over the hill from you, then.  I was right there behind the diner and clothing sales, in the WWII buildings. 

I'm very, very familiar with the JBM Ballistics calculators, and use them alot.  I use "Trajectory Simplified" more than anything, and I've submitted a few of the Hornady bullet lengths for their library.

That McGowan calculator would be excellent, for a one-trick pony - build a specific twist rate for a known load that's never gonna change - I ran my 18" Grendel common load through there, and came up with a 1:9.7" twist.  Nope, I wouldn't want that on my Grendel - it's a 1:8" twist and does a great job with that.  I load up different projectiles and loads for different things, with different weighted projectiles, and the 1:8" serves me well.

932782367_twistrate18Grendelload.thumb.png.de819c9843c735bcd060ef64ba0d28bb.png

I'll get into that other one this weekend, and check it out...

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You bet. I look forward to it. 

It seems from what I can gather, is that the longer the projectile, the tighter the rifle twist. Meaning, the less number of inches per full twist and/or steeper angle of twist per inch. But it has to be the right amount. Too tight, it will over-gyrate the projectile causing it to "nose over" and begin to tumble. Too little of a twist rate and the bullet will drop faster, be less accurate and become far less consistent on it's point of impact overall. With this is velocity. The faster the projectile, in percentage of it's working velocities, the less (slightly) amount of twist becomes the ideal.

Are these correct or only parts?

This still leaves one wondering why these calcs use overall length in the calculations instead of actual projectile length. Again, the brass is not part of a bullets performance. A bullet starts its performance at the same place where the rifling/twist rate starts. The end of the chamber. So, how are these calcs correct in using"cartridge" length? I'm hoping someone can explain this.

Is everybody wrong on their choice of barrel twist? They can't be. The proof is found at the range. 

This leads me to determine that there is something I haven't grasped yet. What is it?

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2 minutes ago, 7thLightID said:

You bet. I look forward to it. 

It seems from what I can gather, is that the longer the projectile, the tighter the rifle twist. Meaning, the less number of inches per full twist and/or steeper angle of twist per inch. But it has to be the right amount. Too tight, it will over-gyrate the projectile causing it to "nose over" and begin to tumble. Too little of a twist rate and the bullet will drop faster, be less accurate and become far less consistent on it's point of impact overall. With this is velocity. The faster the projectile, in percentage of it's working velocities, the less (slightly) amount of twist becomes the ideal.

Are these correct or only parts?

This still leaves one wondering why these calcs use overall length in the calculations instead of actual projectile length. Again, the brass is not part of a bullets performance. A bullet starts its performance at the same place where the rifling/twist rate starts. The end of the chamber. So, how are these calcs correct in using"cartridge" length? I'm hoping someone can explain this.

Is everybody wrong on their choice of barrel twist? They can't be. The proof is found at the range. 

This leads me to determine that there is something I haven't grasped yet. What is it?

That is correct, and a way to simplify it, mentally, is this:  You are confined/restricted to a diameter.  .308 Win is 0.308" in diameter.  6.5 Grendel is 0.264" in diameter.  That can't change.  When you go to lighter or heavier projectiles, what can change? The length, or the material construction.  Most often, it's just the length. An exception to this is the Barnes TSX-family of bullets, as solid copper construction.  So,for the most part, you go to a heavier projectile for longer distances, which by it's nature makes it a longer bullet - and you would want a tighter twist rate to stabilize that heavier projectile. 

The AR15 is the most common example.  1:12" twists, 1:9", 1:8", 1:7", etc...   A 1:12" twist will be great for a light, 35gr varmint projectile.  That twist rate will suck for a 75gr Hornady HPBT.   And the opposite: don't try to run that 35gr varmint grenade through a 1:7" twist barrel...

In the McGowan calculator above, there's nothing about COL in there - there's no Cartridge Overall Length.  That's not a factor in the McGowan data, and isn't used.  I briefly looked at the other one, and didn't see it in there, either, but I'll look again...

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Okay, I just got into the Berger stability calculator - there's nothing about COL in it.  That "length" field they're wanting is the true projectile length - just the bullet.  I've been playing with a Ketrel weather meter/ballistic calculator for a few months now, and that's an important factor that you have to build into your cartridge profile in the meter, in order for the Applied Ballistics software to spit out accurate drop data for distance. 

I plugged in my same Hornady 123gr ELD-M projectile for the 6.5 Grendel loads.  First chart is G1 drag factor, second one is G7 drag factor.  It didn't change by changing the drag factor - which it shouldn't, if those drag factors are correct.

1768122580_18GrenbergerstabilityG1.thumb.png.01d29ff0a3a718bd07cfa102dd583298.png

896852883_18GrenbergerstabilityG7.thumb.png.a60aa3a7e5d87658ae8f24cc878fa7c4.png

Edited by 98Z5V

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Damn. Okay, I must be doing something wrong.

I'll go back and re-enter the numbers.  

I solidified some thinkings and picked up some new info.

Thanks 98 :)

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