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DCM
02-22-2011, 09:34 PM
I see this question asked an awful lot in all kinds of places on the forum.

It is usually stated in the form of what twist for X weight bullet in Y caliber.

If one studies the Greenhill formula you will see that it should be stated as what twist rate for X length bullet in Y caliber at Z velocity to achieve an RPM that will stabilize it.

The easy way to find the the minimum twist rate you need is to go to http://kwk.us/twist.html and plug in all of the values you are using including the SG.

If you plan on using varmint bullets or boolits it is possible to overspin them into fragments before they reach the intended target so it is recomended that you go with as slow of twist rate as reasonably possible.

:lovebooli

Willbird
03-08-2011, 06:07 PM
I see this question asked an awful lot in all kinds of places on the forum.

It is usually stated in the form of what twist for X weight bullet in Y caliber.

If one studies the Greenhill formula you will see that it should be stated as what twist rate for X length bullet in Y caliber at Z velocity to achieve an RPM that will stabilize it.

The easy way to find the the minimum twist rate you need is to go to http://kwk.us/twist.html and plug in all of the values you are using including the SG.

If you plan on using varmint bullets or boolits it is possible to overspin them into fragments before they reach the intended target so it is recomended that you go with as slow of twist rate as reasonably possible.

:lovebooli

If you are using "normal" bullets, not blitz, tnt, or sxpx they are pretty tough, in 1/14 twist for example it takes the likes of the Cheetah mark IV to begin to blow them up in a 1/14 twist.

The 70 grain Seirra Blitz .243 dia bullet..starts to blow up from a good 1/8 twist barrel around 3800fps.......so this is generally not as much of a concern as some might think.

Bullets that are very close to being unstable....no more twist than necessary are a bit more accurate.........more tolerant of the ever present flaws in every bullet ever made, but again, with a good bullet, and a good throat, and good brass....too much twist might add .200" to your groups at 100 yards...again not as significant as some would have you think for most uses.

Bill

DCM
11-17-2013, 01:21 PM
Another good twist rate calculator can be found here, near the top of the page on the right hand side.
http://www.bergerbullets.com/products/all-bullets/

johnson1942
11-18-2013, 12:37 AM
the green hill formula has always worked for me. if you use the number 150 in the math of it will make the most accurate length of bullet to twist possible out to about 300 yards but can loose stability after that and be no good. if you use the number 120 in the math of the formula your bullet length to twist will be good at 1000 yards. the length of the bullet to twist rate is what is important not just the weight alone. i personally use the 150 in the math and just subtract the number 3 from the final answer at that way has never failed me. ive cured a lot of socalled guns that wont shoot in to tackdrivers useing this method. cryoing the barrel doesnt hurt either.

Chill Wills
12-07-2013, 11:49 PM
the green hill formula has always worked for me. if you use the number 150 in the math of it will make the most accurate length of bullet to twist possible out to about 300 yards but can loose stability after that and be no good. if you use the number 120 in the math of the formula your bullet length to twist will be good at 1000 yards.

I agree with that. The constant 150 for our slow velocity (black powder class) rifle bullets just does not make for accuracy to 1000y in all conditions. We have played with barrels/bullets that agree with 130 and 125 as constants. These are faster than the old standard twists. It may be true that a just barely stable bullet can be very accurate and for the reasons stated above, however shooting in gusty winds and in wind conditions that move in more than one direction and have shear in the boundary require more stability. I tend to the faster twists because it is rarely calm for a whole match and when you are called to the line....... it is time to shoot. We can't wait for those special times when we can shoot a wall hanger.

Then again, twist can be over done. I am just about to un-barrel a 40-70 Sharps straight that has a 13 twist Lilja barrel. I've put a wheel barrel of powder and lead through it. Never have been able to make to perform despite rebuilding everything on the lock, stock and now I guess, the barrel! Something 14, 15 or 16 twist may be a better way to go. 13" twist might be just too fast.

Another way to adapt the Greenhill formula can be to replace the constant "150" by using the anticipated muzzle velocity divided by 10. Try it. I am not sure that is perfect either but it will get you closer.

-Michael Rix

303Guy
12-10-2013, 01:40 PM
Boolit geometry affects stability. Spitzers increase stability with higher velocity while flat nose boolits loose stability with increasing velocity. Transonic and subsonic velocities have different spin requirements too, again depending on the geometry. Boat tails reduce stability quite a bit. The spin rate and BC calculator that showed this was to be found on the Border Barrel website but the last time I looked it wasn't available.

atr
12-10-2013, 08:31 PM
the formula also takes into account the specific gravity of the projectile....note that the specific gravity of a cast boolet is different than the specific gravity of a jacketed bullet.....so if you want to find out if your twist rate is good for a cast boolet don't use the specific gravity value of a jacketed bullet.
atr

johnson1942
12-28-2013, 12:03 PM
lots of good advice from everyone here. chill wills, that would be a long bullet for .40 cal 1/13 twist. at leaste a 1.5 long bullet. could even try on 1.55 or 1.6 long bullet, just like a dart. the best way for that one would be to throat the back of the barrel and use a little harder lead to stop any deforming of a long bullet. i have a throating reamer i can loan you if you want to try it if your havent already rebarreled the gun. p.m. if your interested in throating your barrel.

AlaskanGuy
12-28-2013, 01:23 PM
When i use the 2 different twist rate calc's mentioned here, the results are different....

Lets try an experiment....

Bullet length. .667
Bullet weight 61.4
Muzzle velocity 2000 fps
Temp 55
Altitude 200
This is a lead bullet.... From a 1 in 9 twist bolt gun.

Maybe i am doing it wrong, but will the bullet stabilize??? One says yes, one says no...

AG

swheeler
12-28-2013, 04:23 PM
AK Guy you're doing something wrong, both online calculators and basic(150 constant) Greenhill formula all show a stabilized bullet with 1:9 twist. At 2000 fps a 1:11 twist would be required with that boolit.

johnson1942
12-28-2013, 05:18 PM
alaskan guy, i may be dense but i cant seem to find the bullet diam. that you posted you want a twist rate for? if i missed it can you post what the bullet diam. is. ill do the math for 200 yard accracy and 1000 yard accracy if i get the bullets diam. thanks.

swheeler
12-28-2013, 06:52 PM
not AK guy but had better be .224" same as 22 cf barrel groove

AlaskanGuy
12-28-2013, 07:29 PM
Yes sir... Sorry... .224. Dont know how i missed that one...

I get fine accuracy with both my 1 in 9 and my 1 in 12 using 9 gr of 2400... I dont have a chroney, but i figure to be traveling around 1700 fps ....shoots about 1 in with my old eyes. The bullet is a 62 gr MIH boolit meant for the AR platform, and stolen for my bolt gun... Had a heck of a time finding a load that would work, and was reluctant to use 2400 as i am down to less then a pound left... But cant get it to shoot with 4064 or unique, the other powders that I have lots of....

AG

johnson1942
12-28-2013, 08:29 PM
well you got it right. a 1/11 twist will shoot that bullet very very well at 200 yards but past 300 yards their could be a problem. in fact it should be a bumblebee type group at 200 yards. the 1/9 twist will shoot way way out their and is perfect for long range shots. it will do close up also but i would put my money on 1/11 twist for under 300 yards. shooting at 1700 ft. per sec or 2000 ft. per sec. a nice flat base and a blunt round nose should work very well. the green hill always comes through for me but i only shoot cast and nothing faster than 2200 ft. per second.

AlaskanGuy
12-28-2013, 08:33 PM
That is all i shoot anymore either... If I cant cast for it, then it just sits there...

AG

Wayne S
12-28-2013, 08:52 PM
http://www.mcgowenoutlet.com/twistcalculator.html

Wayne S
12-28-2013, 08:53 PM
If life wasn't complicated enough

http://www.mcgowenoutlet.com/twistcalculator.html

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi