.223 Sub Sonic

[janfred]

Twist rate does not work the same for subsonic bullets. The aerodynamic force is less than a quarter compared to supersonic flight. This reduces the overturning moment considerably.

A 1-12 should be enough to stabilise 80gr bullets comfortably, if not heavier.

Secondly, round nose bullets is much better aerodynamically speaking for subsonic flight than spitser ogives.

[Treeman]

Twist rate does not work the same for subsonic bullets. The aerodynamic force is less than a quarter compared to supersonic flight. This reduces the overturning moment considerably.

A 1-12 should be enough to stabilise 80gr bullets comfortably, if not heavier.
Secondly, round nose bullets is much better aerodynamically speaking for subsonic flight than spitser ogives.

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I was of the opinion  that the faster the bullet the less twist needed, the slower the bullet the more the twist?
This inside the usual rules for length of bullet in regard to twist, the fast you shoot a bullet the more you can back of one the twist?
Therefore if your twist is a bit on the slow side, you can sometimes over come that with more velocity.
 Or what ?

[janfred]

Bear with me.

On a long bullet the centre of gravity is behind the centre of pressure. The air pushes back from the front through the centre of pressure and the weight pushes from behind through the centre of gravity causing an overturning moment. The magnitude of this moment is directly related to the distance between the two points and the force acting through them. To overcome this, we spin the bullet. The rate we spin it depends on the size of the overturning moment, i.e. length and speed of the bullet.

Basically, the faster the bullet goes the higher the force, the more RPM you need. For the rest of the explanation to make sense you have to think about RPM and force instead of twist and velocity.

Subsonic the force acting on the bullet is much less than in the supersonic or transonic zones. Hence the less RPM required.

A standard drag curve shows it very well. In the supersonic regime, the drag is high, reducing quickly to through the transonic zone to low when subsonic.

[oafpatroll]

Thanks for that. First time I think I actually got the underlying reason for different twist rates.

[Treeman]

Thanks for that. First time I think I actually got the underlying reason for different twist rates.

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Ha ha - Ja, me not being so clever, well I am still figuring it out. This - "The rate we spin it depends on the size of the overturning moment, i.e. length and speed of the bullet "

And then the ? -Why do we use slower ie 1-14 for a 40 gr .223 but a faster twist 1 - 8 for the 70 gr .223 bullet.

I am not questioning the validity of your scientific fact, just trying to get past some mental block.

[BBCT]

Thanks for that. First time I think I actually got the underlying reason for different twist rates.

And then the ? -Why do we use slower ie 1-14 for a 40 gr .223 but a faster twist 1 - 8 for the 70 gr .223 bullet.

That holds true for supersonic bullet speeds (which is what we are all accustomed to working with). Subsonic bullet speed is a different animal and the variables like bullet shape and weight, as well as twist rate do not have the same influences relative to one another as they do at supersonic speeds.

[Ds J]

Thanks for that. First time I think I actually got the underlying reason for different twist rates.

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Ha ha - Ja, me not being so clever, well I am still figuring it out. This - "The rate we spin it depends on the size of the overturning moment, i.e. length and speed of the bullet "

And then the ? -Why do we use slower ie 1-14 for a 40 gr .223 but a faster twist 1 - 8 for the 70 gr .223 bullet.

I am not questioning the validity of your scientific fact, just trying to get past some mental block.

A heavier bullet has a larger tendency to stray and therefore needs more spin to keep it in check.

[Treeman]

Does the general mantra then hold true in your opinion, use the heaviest bullet for subsonic loads.
I have found this to be untrue unless the barrel is changed for subsonic - heavy bullets.

[Treeman]

Does the general mantra then hold true in your opinion, use the heaviest bullet for subsonic loads.
I have found this to be untrue unless the barrel is changed for subsonic - heavy bullets.

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  You posted while I was asking.

[Treeman]

past some mental block.

A heavier bullet has a larger tendency to stray and therefore needs more spin to keep it in check.

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You sound like a priest - speaking of the bullet who lost its way along the righteous path and strayed of target.

[janfred]

Does the general mantra then hold true in your opinion, use the heaviest bullet for subsonic loads.
I have found this to be untrue unless the barrel is changed for subsonic - heavy bullets.

Provided you stay away from the transonic zone, yes. Anything faster than 950fps you take your chances.

[Ds J]

Does the general mantra then hold true in your opinion, use the heaviest bullet for subsonic loads.
I have found this to be untrue unless the barrel is changed for subsonic - heavy bullets.

Provided you stay away from the transonic zone, yes. Anything faster than 950fps you take your chances.

The transsonic zone gave me trouble with my .22 and standard ammunition. For some reason, standard ammunition deviated from 940fps up  to 1200fps, especially on hot days. It gave erratic groupings - up to two flyers for every five shots.

After I changed to supersonic (high velocity) and subsonic ammunition, the groupings became more consistent.

When I loaded 173gr LRN for my 308, I stuck to 800fps to say clear from the transsonic zone.

For some reason, the accuracy is not very good yet. I still get flyers now and then.

[janfred]

Do the flyers go sideways through the target at 100m?

[Treeman]

Does the general mantra then hold true in your opinion, use the heaviest bullet for subsonic loads.
I have found this to be untrue unless the barrel is changed for subsonic - heavy bullets.

Provided you stay away from the transonic zone, yes. Anything faster than 950fps you take your chances.

The transsonic zone gave me trouble with my .22 and standard ammunition. For some reason, standard ammunition deviated from 940fps up  to 1200fps, especially on hot days. It gave erratic groupings - up to two flyers for every five shots.

After I changed to supersonic (high velocity) and subsonic ammunition, the groupings became more consistent.

When I loaded 173gr LRN for my 308, I stuck to 800fps to say clear from the transsonic zone.

For some reason, the accuracy is not very good yet. I still get flyers now and then.

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You having erratic ignition, I found my subsonics to be stupid accurate. 800 fps is just to slow, here on the coast we stay at 1000 - 1040, even the late 900+ fps. Below this funny things seem to happen.
Also, get a proper over powder wad, use quite a bit of toilet paper or cotton wool even a sheet of paper cut in a circle pushed in with a pencil, then kapok, I put a lot in,almost a compressed load, just get better ignition.
I must buy another sedate caliber rifle and play again.

[Tripodmvr]

The reason for heavy bullets is that lighter bullets need very small charges so as to not exceed the speed of sound. The extra weight of the bullet needs more oomph and charges then increase in volume and tend to give better consistency and then accuracy.