Twist and stability?

[andym79]

Hi guys, I hate to keep asking questions like this!

I have come to realize that for cast shooting generally the slowest twist you can get away with is the best, I thought my 1:10 was okay and my 1:12 30-30 better, recently I got a 30 cal in 1:14 and it puts the other to shame.

The biggest part of my problem lies in that every bullet twist calculator seems to produce a different output with the same inputs. I am having a lot of luck with a 1.06" bullet in my 1:14 at about 1500-1600 fps, half the calculators said it would stabilize half did not. I came here to ask advise and confirm one way or the other, and thankfully I was advised that it would.

So I am asking the same of you about a .270cal bullet the NOE 279-165 1.170" long, in a 1:10 at about 1600fps, will it stabilize, again half say one thing half the other. What do you think?

[Ben]

The individual rifle always ends up with the final say.........

[Tom Myers]

Hi guys, I hate to keep asking questions like this!

I have come to realize that for cast shooting generally the slowest twist you can get away with is the best, I thought my 1:10 was okay and my 1:12 30-30 better, recently I got a 30 cal in 1:14 and it puts the other to shame.

The biggest part of my problem lies in that every bullet twist calculator seems to produce a different output with the same inputs. I am having a lot of luck with a 1.06" bullet in my 1:14 at about 1500-1600 fps, half the calculators said it would stabilize half did not. I came here to ask advise and confirm one way or the other, and thankfully I was advised that it would.

So I am asking the same of you about a .270cal bullet the NOE 279-165 1.170" long, in a 1:10 at about 1600fps, will it stabilize, again half say one thing half the other. What do you think?

andyM,

Using the new, Version 5, Cast Bullet Design ~Advanced software that was used to design the NOE 279-165-FN, a twist length of 1/10 and a muzzle velocity of 1600 fps was entered.

The software uses the Bullet Stability Equations of Don Miller to calculate muzzle velocity, bullet length, bullet weight, twist rate and a stabilization factor.

The minimum calculated stability factor to stabilize a bullet has been determined to be 1.5, However the minimum safe factor is usually considered as 2.5. Therefore a default value of 2.5 is used as the starting factor in the calculations.

Nothing is "written in stone" when calculating, estimating or predicting a real life, bullet stability factor but the calculations are very useful for observing the relationship between twist rate and the length-weight-velocity aspect of a particular bullet design.

The first image shows the calculated stability factor for the NOE bullet using your combination of 1/10 twist and a velocity of 1600fps




Indicating the required velocity to achieve a calculated stability factor of 2.




Indicating the required velocity to achieve a calculated stability factor of 2.5




Indicating the required twist length to achieve a calculated stability factor of 2.5




Indicating the required bullet length to achieve a calculated stability factor of 2.5




Hope this helps.

[Larry Gibson]

Don't know what calculator you're using but the Miller Formula has that .270 bullet fully stabile at 700 fps.

Back in the day before the forums, ballistic calculators and such when I didn't know any better I was shooting a 165 gr Lyman 287308 out of a Parker Hale .270 cast soft and loaded over Bullseye (Paco Kelly type cat's sneeze load) at 800 fps as chronographed with an Oehler. Accuracy was excellent. Didn't record the bullet length but it probably wasn't much different than the 165 NOE.

Larry Gibson

[Digital Dan]

I'm pretty much in Larry's camp on this with a few thoughts added. Calculators are as good as the way they are constructed and there be a few that are flawed in my opinion.

We use twist rate to impart angular momentum which provides stability in opposition to overturning moments that are a function of drag and the disparity of CG and CP. There is a lot of misunderstanding about these things.

[Outpost75]

Larry and I have had this conversation before, but the most accurate cast bullet .30 cal. barrels I have are all 14" twist.

I shoot 200-grain bullets in them. So did John Ardito, who set many of the CBA benchrest group records.

With lighter bullets up to 160-grains I have a 16" twist .30'06 which is scary accurate when bullets are driven fast enough.

Where the common 10" twist .30 cal. barrel excels is for stabilizing heavy bullets subsonic. Much of my recreational shooting is with NOE #311299 at about 900 fps with 5 grains of Bullseye in the '03 Springfield, Winchester 54 or pre-1964 Model 70.

While having too much twist doesn't hurt as much as too little, as has been discussed in Larry's posts for years, a slower twist permits higher velocity with better accuracy and freedom from leading. But if you want to go heavy, quiet and SLOW, a faster twist has great possibilities.

I used to have a 7" twist .30'06 barrel which we used mostly for testing armored doors and trunk lids with APM2 ammunition, but it also shot accurately with 125-grain Sierras driven so hard they gave a blue smoke trace all the way to the target, and the bullets would come apart completely on one layer of target cardboard. Devastating on woodchucks like a .220 Swift on steroids.

[303Guy]

I found a stability calculator that showed the stability over a range of velocity in the form of a graph. It was interesting to see how the stability went south through the transonic range. Bullet shape was taken into account. This site took the calculator away for some reason.

[andym79]

Thanks for your responses. I am building a 270 rifle and have option of a 9 or 10 twist, I would rather go for the 10 twist if it will do the job.

While appreciate no two riles are exactly the same, does anyone have a 1:10 twist 270 and the noe bullet that so that they may be able to let me know what it does at 1600fps or there abouts?

[Digital Dan]

10" will work.