TWIST AND BULLET STABILITY

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  • Last Post 24 January 2017
joeb33050 posted this 21 January 2017

Bullet stability varies with twist, bullet length, bullet density, air density, bullet shape-maybe, and bullet velocity-somewhat. 

Cast bullets that are tipping = making elongated holes in the paper, are sometimes very accurate. 

I have been unable to make .30 bullets tip as powder charge/velocity decreases, they just shoot bigger groups as velocity decreases. 

Some bullets at some velocities go through the paper sideways-not tipped-sideways. These are always wildly inaccurate, so far. 

Some bullets at some velocities are wildly inaccurate, but show no sign of tipping. 

1/20/17 Savage Striker 22-250, 14” twist, 225646M, .692” long, Greenhill minimum twist required = 11” per turn, 8 SR4756.  This shot 1 off the paper, 5 into ~6”, and 3 with another aiming point into ~7”. This load and gun have been reasonably accurate in the past. If there is any tipping, it is slight. 

Without cleaning, same Striker, same shooter, same day, same range; Midway Dogtown 55 gr SP, 8.0 SR4756, .8, 1.0, .85, 1.175, .7; AVERAGE .905. 

It was in the 50s at the range, much of my shooting is in the 80s and 90s. 

This 14” twist shouldn’t stabilize the 225646M at .692”, or the Dogtown bullet at .700”; but it does, at least most of the time. 

Could the increased air density cause a marginally stable bullet to become unstable and shoot wildly, at low temperature?

 

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Ken Campbell Iowa posted this 21 January 2017

thanks again for your observations ..... i feel guilty because i just plink, and rarely see consistant cast groups under about 2 moa ... and yet i see reasonable ” testing ” groups over 8 moa .... that with jacketed would for sure be under 2 moa ...  even with factory mj rejects, i have never had a group over 4 moa in a deer rifle mj load ...

when cast go bad, they can really go bad .  

i have always thought we learn more from bad groups than from great groups ... after all ::  aren't they * supposed * to be great groups ?

i would like to see more reports of embarrassingly poor groups from reasonable loads ... and guesses as to what went wrong ....

poor bedding, loose scopes, stock screws .... don't count ... and i have managed to achieve all three ...

..... can unfriendly barrel vibrations cause 7 moa groups ??  never afftected my mj loads that much ...  something different ??

ken

 

 

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Larry Gibson posted this 21 January 2017

joe

 

Based on computations using the Miller formula the instability of your 225646 is caused by the change in temperature from 80 - 90 down to 50.  That ambient temperature change affects the stability.  Your 225646 is marginally stable on 80 - 90 degree days and unstable at 50 degrees.  Additionally the stability is further exasperated by the drop in temperature because that also drops the velocity.

 

LMG 

Concealment is not cover.........

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joeb33050 posted this 22 January 2017

joe

 

Based on computations using the Miller formula the instability of your 225646 is caused by the change in temperature from 80 - 90 down to 50.  That ambient temperature change affects the stability.  Your 225646 is marginally stable on 80 - 90 degree days and unstable at 50 degrees.  Additionally the stability is further exasperated by the drop in temperature because that also drops the velocity.

LMG

 

It doesn’t seem to be the temperature change, at least according to Miller.

Using the Don Miller rule

.225” dia. 55 gr .692” long bullet, 14” twist barrel, 1400 fps, 100 degrees F temp., S sub g =  the stability factor = 1.00. (S sub g should be 1.4 min.)

at 30 degrees F, S sub g is .87.

This using “2.6.1.3 Don Miller Twist Formula Workbook” from “Cast Bullets For Beginner And Expert”, 3rd Edition.

The workbook is available on the Yahoo Group Site at  https://groups.yahoo.com/neo/groups/CB-BOOK/info

Or ask and I’ll email it.

joe b.

 

 

 

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Larry Gibson posted this 22 January 2017

Joe

 

Thanks, for the offer to send the workbook but I don't need it.  I have the computer program along with all the equations. 

 

The Sg factor in your example dropped from 1.0 to .87 based on a temperature change of 70 degrees.  If we read a thorough explanation of the Miller Sg factors as in Applied Ballistics for Long Range Shooting, Bryan Litz, p 136 - 138 we find an SG of 1.0 - 1.39 to give marginal stability.  Below an Sg of 1 the bullet is unstable.  According to Miller “a stability factor of 1.4 or greater insures adequate stability".  However, a marginally stable bullet may still fly point forward but accuracy may be questionable.

As I stated earlier your 225646 is marginally stable at 80 - 90 degrees but at 50 degrees it becomes unstable due to the temperature drop.  On exit from the muzzle the bullet actually will be less than .224 diameter and will be slightly longer than .692.  It is those measurements I used in the calculations as they are the measurements that actually effect stability not the bullet measurements before shooting.  A subtle change yes but when on the edge of stability enough to perhaps affect the out come.

 

Just food for thought:  might consider the fact as to your quandary over the accuracy you've been getting with that bullet as posted in several threads that the 225646 as you use it in the 14” twist has been barely marginally stable.  That's why the bullets have been making round holes in the target yet accuracy was questionable with unaccounted for flyers(?).  

 

LMG   

Concealment is not cover.........

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joeb33050 posted this 24 January 2017

Joe

 

Thanks, for the offer to send the workbook but I don't need it.  I have the computer program along with all the equations. 

 

The Sg factor in your example dropped from 1.0 to .87 based on a temperature change of 70 degrees.  If we read a thorough explanation of the Miller Sg factors as in Applied Ballistics for Long Range Shooting, Bryan Litz, p 136 - 138 we find an SG of 1.0 - 1.39 to give marginal stability.  Below an Sg of 1 the bullet is unstable.  According to Miller “a stability factor of 1.4 or greater insures adequate stability".  However, a marginally stable bullet may still fly point forward but accuracy may be questionable.

As I stated earlier your 225646 is marginally stable at 80 - 90 degrees but at 50 degrees it becomes unstable due to the temperature drop.  On exit from the muzzle the bullet actually will be less than .224 diameter and will be slightly longer than .692.  It is those measurements I used in the calculations as they are the measurements that actually effect stability not the bullet measurements before shooting.  A subtle change yes but when on the edge of stability enough to perhaps affect the out come.

 

Just food for thought:  might consider the fact as to your quandary over the accuracy you've been getting with that bullet as posted in several threads that the 225646 as you use it in the 14” twist has been barely marginally stable.  That's why the bullets have been making round holes in the target yet accuracy was questionable with unaccounted for flyers(?).  

 

LMG   

 

Sunday, 1/22/17, was forecast to be in the 80s, so I went to the range and shot 9 bullets at 100 yards that couldn't be covered by a basketball. Same load/gun/rest/shooter.

At 82 degrees, it clearly ain't the temperature/air density. I don't know why these 225646M .692” long bullets shoot so wildly and Midway Dogtown 55 gr .700” long bullets average .905” for five 5 shot 100 yard groups using the same everything-but-the-bullet with a cb-dirty barrel.

Tomorrow a Shilen 9” twist 22-250 barrel is due, I'll screw it on a M10 Savage and see what happens.

I'm sorry for the blue invective resounding through the air, I've been getting a new computer cranked up. The nerd's revenge is still working.

joe b.

 

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