Twist rate questions???

R

reynolds357

New member
I have always built with twist rates that calculate out to at least 1.5 on the heaviest/longest bullet I intend to shoot. My best shooting rifle ever is around 1.75. I have recently heard that 1.2 to 1.3 is more ideal. Do those bullets remain stable at 1000 yards? 1200 Yards? Do they de stabilize when the velocity drops low enough to bring the calculation down to say .9 or .8 etc?

I am wondering because I am considering trying some slower twist barrels.
 
I would say it depends.
If your shooting 300, 600, even out to 1,000 yards, but not more, then yes go with the stable twist.

Shooting "ULR" 1,500+ yards, then it's being discussed that what is supposed to be unstable, is actually working quite well when coming downs through the sound barriar.
 
reynolds357 asked:

..... Do they de stabilize when the velocity drops low enough to bring the calculation down to say .9 or .8 etc?
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Please explain your thinking and calculations about this question.
I've always thought that the spin (rpm) of the bullet didn't slow down near as much as the velocity of the bullet slows down?!
 
RaySendero said:
Please explain your thinking and calculations about this question.
I've always thought that the spin (rpm) of the bullet didn't slow down near as much as the velocity of the bullet slows down?!
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Using the calculators such as Bergers twist rate calculator. I dont know exactly how spin slows in relation to velocity(hopefully someone here does) but it has to slow. It is encountering friction on its spin.
 
Yeah, I would like someone that knows for sure, too.

Reason I asked; I kinda remember working thru a table several years ago showing a bullet velocity drop of 42% at 500 yards yet only a 4% drop in spin/RPMs at that distance.
I can't find the source, but it did impress on me the large magnitude of difference between velocity loss and the bullet RPM dropping.
 
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You can calculate until the cows come home and you'll still find that barrels come in standard for calibre twists. Unless you pay very big money for a custom twist.
"...the heaviest/longest bullet..." Heaviest.
"...Do they..." According to that Newton fellow's laws, objects in motion will stay in motion until force is applied to change that motion. Changing velocity doesn't affect the rotation of a bullet.
Read this. Note that the Berger calculator needs environmental variables and that it's just a guess like all calculators of the type. You should also remember that bullets are not stable when they come out of the barrel. That takes time and distance. In the case of a .30-06/.308 it's about 300 yards.
http://bulletin.accurateshooter.com...rstanding-bullet-stability-twist-rate-and-mv/
 
T. O'Heir said:
You can calculate until the cows come home and you'll still find that barrels come in standard for calibre twists. Unless you pay very big money for a custom twist.
"...the heaviest/longest bullet..." Heaviest.
"...Do they..." According to that Newton fellow's laws, objects in motion will stay in motion until force is applied to change that motion. Changing velocity doesn't affect the rotation of a bullet.
Read this. Note that the Berger calculator needs environmental variables and that it's just a guess like all calculators of the type. You should also remember that bullets are not stable when they come out of the barrel. That takes time and distance. In the case of a .30-06/.308 it's about 300 yards.
http://bulletin.accurateshooter.com...rstanding-bullet-stability-twist-rate-and-mv/
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Atmospheric air drag upon the rotation effects speed of spin. I am not planning on shooting in a vacuum chamber.
 
Bullet spin rate slows down about 10% over 1000 yards. Approximate formula developed by Geoffrey Kolbe provided by Unclenick:

Nm = the spin rate the bullet had at the muzzle
N = the spin rate of the bullet after your time of flight to the range of interest
t = time of flight in seconds from the muzzle to the current bullet position
d = bullet diameter in inches
e = natural logarithm base, 2.71828…

N = Nm × (-0.035 × t / d)^e

Good quality, well balanced bullets, even 30 caliber ones, are well stabilized by 100 yards else they would not shoot 1/10 to 2/10th inch or smaller 10 shot groups at 100. Berger and Sierra match bullets are examples when they tested at 100.

If you review benchrest aggregate records at 100, 200 and 300 yards, groups enlarge about 1/10 inch every 100 yards past the first 100.
 
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1.5 is ment to make a bullet stable to about 250 yards. 1.2 or 1.3 is ment to make a bullet stable as far as you want to shoot. do not have to long a point on your bullet, a round nose is better. if you have a boat tail your rifle must discharge it 3600 to 4000 ft. per second. a flat base bullet is over all more accurate and doesnt need to be pushed fast to shoot. most over the counter barrels on factory made rifles shoot the tightest groups with the shorter lighter bullets. not so all the time but more time than not. i hope i answered your question about the numbers.
 
Catman42 said:
1.5 is ment to make a bullet stable to about 250 yards. 1.2 or 1.3 is ment to make a bullet stable as far as you want to shoot. do not have to long a point on your bullet, a round nose is better. if you have a boat tail your rifle must discharge it 3600 to 4000 ft. per second. a flat base bullet is over all more accurate and doesnt need to be pushed fast to shoot. most over the counter barrels on factory made rifles shoot the tightest groups with the shorter lighter bullets. not so all the time but more time than not. i hope i answered your question about the numbers.
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I've never heard of this reasoning. Especially when 30 caliber 250 grain boattail hollow point bullets shot 2150 fps from a 1:8 twist barrel tested about 6 inches at 1000 yards.

What rifling twist rates are best for boattail bullets leaving over 3600 fps?
 
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Bart B. said:
Bullet spin rate slows down about 10% over 1000 yards. Approximate formula developed by Geoffrey Kolbe provided by Unclenick:

Nm = the spin rate the bullet had at the muzzle
N = the spin rate of the bullet after your time of flight to the range of interest
t = time of flight in seconds from the muzzle to the current bullet position
d = bullet diameter in inches
e = natural logarithm base, 2.71828…

N = Nm × (-0.035 × t / d)^e

Good quality, well balanced bullets, even 30 caliber ones, are well stabilized by 100 yards else they would not shoot 1/10 to 2/10th inch or smaller 10 shot groups at 100. Berger and Sierra match bullets are examples when they tested at 100.

If you review benchrest aggregate records at 100, 200 and 300 yards, groups enlarge about 1/10 inch every 100 yards past the first 100.
Click to expand...
Thanks. Makes sense.
 
Catman42 said:
1.5 is ment to make a bullet stable to about 250 yards. 1.2 or 1.3 is ment to make a bullet stable as far as you want to shoot. do not have to long a point on your bullet, a round nose is better. if you have a boat tail your rifle must discharge it 3600 to 4000 ft. per second. a flat base bullet is over all more accurate and doesnt need to be pushed fast to shoot. most over the counter barrels on factory made rifles shoot the tightest groups with the shorter lighter bullets. not so all the time but more time than not. i hope i answered your question about the numbers.
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Thanks for trying to answer, but I disagree with pretty much evetything you said.
 
Another twist to this subject is what the British commonwealth learned about shooting 7.62 M80 ball ammo in 500 to 1000 yard matches. Leaving the barrel at 3000 fps, a 1:14 twist usually produced best accuracy with those 147 grain FMJBT bullets.
 
shoot what you want but i would never shoot a boat tail. the gasses from the gun envelope a boatail bullet and not a flat base one. it can make it not as accurate. you want a tiny bubble of air to follow your bullet. it acts like the feather on a arrow to keep it going straight.im not going to argue with anyone. again do what you want. im just giving years and years of experience. i like a hollow based bullet in my 45/70 bullets. they are very short and blunt in the nose also. tack drivers every one of them. ive solved neighbors problems with 25/06/s and 270/s for coyotes. shoot the smallest lightest bullet you can and of course it has to be flat based. shoot it fast and hard it will be accurate. and all my neighbors got many coyotes after we worked that out.again 120 for wayyyyyy out their and for very very good accuracy to 200 yards 150. using 150 i got nickle size groups at 200 yards with open sights. but after 250 yards the bullet flopped every wheres. then i went to 120 for the varient in the formula bullets and they were on the money. if you want to break records at 200 yards use 150 if you want to shoot to the moon use 120. paul matthews taught me this, didnt come up with it on my own. he knew some things. a lot of things. im just passing it on. i taught this to two brothers from ill. they get 3 inch groups at 500 yards with a 45 val 1/18 twist gun. every one at the shoots they go to hate them as they always win with a different bullet than every one else. it just cant happen, but these two brothers use the sciences of shooting and win every time. sorry its just shooting facts. again do what you want.
 
Catman42,

What are the record group sizes at 200 yards? What were they shot with?

Got any shock wave shadow graphs to prove your theory? All the ones of both types I have seen show similar air tubulance behind each. Air compressed shockwaves are a little different due to the bullet's shape.
 
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Catman42 said:
shoot what you want but i would never shoot a boat tail. the gasses from the gun envelope a boatail bullet and not a flat base one. it can make it not as accurate. you want a tiny bubble of air to follow your bullet. it acts like the feather on a arrow to keep it going straight.im not going to argue with anyone. again do what you want. im just giving years and years of experience. i like a hollow based bullet in my 45/70 bullets. they are very short and blunt in the nose also. tack drivers every one of them. ive solved neighbors problems with 25/06/s and 270/s for coyotes. shoot the smallest lightest bullet you can and of course it has to be flat based. shoot it fast and hard it will be accurate. and all my neighbors got many coyotes after we worked that out.again 120 for wayyyyyy out their and for very very good accuracy to 200 yards 150. using 150 i got nickle size groups at 200 yards with open sights. but after 250 yards the bullet flopped every wheres. then i went to 120 for the varient in the formula bullets and they were on the money. if you want to break records at 200 yards use 150 if you want to shoot to the moon use 120. paul matthews taught me this, didnt come up with it on my own. he knew some things. a lot of things. im just passing it on. i taught this to two brothers from ill. they get 3 inch groups at 500 yards with a 45 val 1/18 twist gun. every one at the shoots they go to hate them as they always win with a different bullet than every one else. it just cant happen, but these two brothers use the sciences of shooting and win every time. sorry its just shooting facts. again do what you want.
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The kind of groups you are talking about would finish in last place in the type shooting being discussed.
 
Bart B.,
Referring to the Cat's voodoo as a theory is very generous of you.
Without some kind of supporting principles or evidence
its really no more than his own notion.
I seem notice it more now and never like it when some speaks with an undeserved authority.

Catman42,
Yes - 1st time I've heard the notion that flat base bullets are inherently more accurate at 200 yards than boattails. BUT, I have an interest and am willing to be convinced, if you could offer up more than just your opinion.
 
In one of Sierra Bullets manuals back in the 1970's, they mentioned flat based bullets tended to be more accurate in barrels of lesser quality. Boattail bullets were more accurate in better quality match barrels.

I believe the first boattail bullets made in the USA were those tested on the shores of Daytona Beach in the 1920's. The 174-grain FMJBT bullet was intended to be the long range machine gun bullet in the 30-06 cartridge. It was so much more accurate than the other 30 caliber bullets, it was used in match grade ammo.
 
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