Effects of over stability

[Berger.Fan222]

I prefer no more torque to the gun than needed to reach Sg at ~1.5(heavy, BT), or ~1.3(light, FB).

Mike
Interesting, never considered the extra torque with a faster twist. Wonder what the increase would be on a 180gr at 2800fps by going from a 9 to 8.5 twist
Bob

I just made that change on my 284. Torque change is imperceptible in the bags.

You're probably not really observing lack torque from the faster twist, but rather the lack of observable motion from a noticable net torque. The way the rifle is held and supported may well be countering the torque so that the net torque is too small to be noticable. A different rifle or a different support may leave the torque of the rifling twist unbalanced.

I would think the easiest way to measure the torque in a rifle would be to suspend it on strings along the mid line and use video to record the resulting radial motion when triggered remotely. Conservation of angular momentum suggests that the angular momentum of the rifle when the bullet leaves the barrel will be equal and opposite to that of the bullet.

Of course, high speed video would also be useful to determine how much a rifle is moving rotationally before the bullet leaves the barrel when held by conventional means. Most likely, some holds do a better job counterbalancing the torque spinning the bullet than others.

 

[Grimstod]

I have been doing some test with an improved case and at first I tried a 168gr berger bullet. Its SG factor was well over 2.0, think it may have been as high as 2.4. Long story short, my targets never yielded good result with it and they never truly stabilized. Examining targets revealed that the bullet tip was always striking paper at the slightest angle.
So I switched to a 190gr VLD and that put my SG below 2.0. I think it was 1.98 or something. Anyway with these I got much better accuracy. Bullets also stabilized faster and stayed stable. Bullet holes were always perfectly round and the tips dead center.
That was my experience. Testing was done at 200 and 300 yards using a 28in barrel and 1:10 twist.

 

[Bryan Litz Ballistics]

I have been doing some test with an improved case and at first I tried a 168gr berger bullet. Its SG factor was well over 2.0, think it may have been as high as 2.4. Long story short, my targets never yielded good result with it and they never truly stabilized. Examining targets revealed that the bullet tip was always striking paper at the slightest angle.
So I switched to a 190gr VLD and that put my SG below 2.0. I think it was 1.98 or something. Anyway with these I got much better accuracy. Bullets also stabilized faster and stayed stable. Bullet holes were always perfectly round and the tips dead center.
That was my experience. Testing was done at 200 and 300 yards using a 28in barrel and 1:10 twist.

I'm not sure why the 168's weren't working for you, but I can assure you that stability was not the problem from a 1:10" twist. We've shot many .30 cal bullets from 155 thru 185 from 1:8" twist barrels and had good precision with SG's well over 2 and even 3. Sometimes a bullet won't work well in a particular chamber/rifle set up and it can be difficult to diagnose the reason why. In your case, both the 168's and 190's have plenty of stability; the 168's being more stable so the cause of your observation is due to something other than stability.

Take care,
-Bryan

 

[Grimstod]

In a ruff sence of the word they are plenty stable. They just are not ever put to sleep. You look at your bullet holes to determin this. A bullet that is not asleep yet even sounds diferent.

 

[Bryan Litz Ballistics]

In the literal sense of the word, the bullets are plenty 'stable'.

If the bullets are flying with enough pitching and yawing that you can see it in the bullet holes at close range, it's not due to lack of stability, it's due to something else; possibly off-axis engraving from a long jump, or something else that's introducing a dynamic imbalance.

The .30 cal 168 Hybrid is known to be stable in 1:12" twist barrels. That means the bullet typically flies with minimal yaw and impacts paper straight on with good precision at the level of stability it gets from a 1:12" rate of twist.

If you're shooting the bullet from a faster (1:10") rate of twist and you're observing problems, those problems cannot be related to stability because the same bullet is plenty stable in a slower twist.

Higher rates of twist and stability are known to dampen pitching and yawing more rapidly (make the bullet 'go to sleep' faster) than slower twists and lower stability.

I'm not questioning your observations with the 168's. But the explanation you're offering goes counter to numerous other established facts.

-Bryan

 

[Laurie]

............. but I can assure you that stability was not the problem from a 1:10" twist. We've shot many .30 cal bullets from 155 thru 185 from 1:8" twist barrels and had good precision with SG's well over 2 and even 3. [Bryan Litz]

If bullets don't work at Sg values between 2 and 2.4, then a lot of people had flukey scores at Raton 20 months ago in the last F-Class World Championships + US F-Class L-R National Championships! With many using 1-10" twist barrels and shooting the 155.5gn Berger Fullbore at 3,100 fps and above, Sg for Raton (8,500 ft ASL) and 95-deg F on occasions sees Sg values at 3.25 or thereabouts.

The 185gn Juggernaut at 2,850 fps in a 1-10", probably the single most popular combination amongst the home FTR competitors at that event is also at or above 3 in those conditions. Drop the twist rate to 12 which is likely the slowest used by international level FTR competitors building a rifle for this bullet and the calculated Sg was still 2.09 under these range conditions.