I would suggest that conjecture has probably stopped many a useful conclusion from becoming reality.

The best answer seems to come from doing and then drawing the conclusion. If the M855 was an unsafe combination with the Mini-14, I suspect this would be a well known fact by now. In any case a quick examination of the fired cartridges could identify entrance into a chamber pressure danger zone; the primer would be flattened with very sharply squared off edges. When they are sharp, you're running the gun with too much pressure.

My rifles can properly stabilize 69gr and some 75g match loads in a 1:9" barrel. When the 69gr Match shoots a smaller group than the 75gr match, you know the rifle has too slow a rifling twist for the longer bullet.

My unsubstantiated guess would be that the M855 will stabilize in the 1:10", but that it could also simply be an inaccurate round. This has been my experience (stabilizing = round bullet holes; wider grouping = inaccurate load). I'm guessing that the multiple component bullet core may lead to an offset rotational axis during flight.

And I could just as easily be totally wrong.

When considering long range bullet stability, it is important to note that there is more than one type of stability. Two types are static stability and dynamic stability.

Static stability originates in the barrel and endeavors to keep the bullet's rotational axis pointed in the same direction from start to finish of the trajectory. It is most similar to the stability toy gyroscopes and tops possesses.

Dynamic stability originates and changes continuously after the bullet enters the environment from the muzzle, and endeavors to aerodynamically realign the bullet's rotational axis with the airflow as the trajectory arcs downward. It is most similar to the kind of stability arrows and weather vanes possess.

Dynamic stability has its greatest influence out at the far end of the trajectory, where the the trajectory plunges steeper and steeper in relation to the bore axis. The static stability actually tends to yaw the bullet in relationship to the true trajectory because it delays the aerodynamic forces from realigning the bullet's rotational axis with its true plunging path.

I have tested this characteristic by firing 120gr 6.5mm bullets at 1000yd with a 1:8" twist barrel. The bullet's drop is greatly increased (on the order of 10-15MOA) due to the yaw (for the purposes of ballistic computation, pitch and yaw are viewed together as yaw), and the bullet holes appear to be somewhat elongated with the tips always appearing to point straight upward. While drop is magnified, accuracy (in MOA) still appears to be nearly the same as would be expected from a more properly chosen twist. A slower twist would reduce static stability, allowing the dynamic stability to better conform the bullet's rotational axis in relationship to the trajectory's increasing drop. The result will be less drag, more resulting velocity, and less drop. This is why bullets derive their better performances from a twist which nearly, but not completely, slower than one may expect to be ideal.

In the preceding example, it would appear that bullets can be overstabilized, and that the general result is increased drag at the greater distances. IMHO, the question becomes whether or not a given bullet is long enough for best performance in a given twist. The 75gr HDY BTHP Match is long enough, where the 75gr A-Max may be too long for the 1:9" twist, in my personal experience; and the 69gr BTHP Match might well benefit from a slower twist than 1:9".

Greg

 

My 1911 Swedish Mauser, LIR( Long Infantry Rifle) M96 cal. 6.5x55 has been sported down to a 16 in. barrel and fitted onto a Mandlicher stock. I call it the "Prom Queen" because she's barely legal. Others call it the "Bat Mobile" because of the blue flame roaring out the muzzle every time I touch it off. That barrel was 29" long before the blade. I'm guessing it at a 1/12, 1/14 twist. What would I qualify it at now? It still drops a 1 1/2 minute MOA(100 yds) I know this is the AR thread, but I thought it OK to cut in with a carbine barrel length, twist question. Thanks, come back.

 

Great post! I have an older Bushmaster AR-15 M4/A3 with a 14.5" chrome lined barrel with a pinned Izzy flash suppressor. It has a 1x9 twist. I've found 55gr V-Max is perfect for 100 yards. I usually use 62gr green tip for a bit further out.

 

Just read this thread. Pretty good read. Required twist is not dictated necessarily by bullet weight though. It's dictated by bullet length. What's here works if you use conventional lead core bullets. Start using monolithic bullets and you might find things don't quite work out. For example, copper bullets for a given weight in a given caliber are considerably longer than lead core bullets of the same weight and caliber.

 

Not to argue, but to try to add to the conversation,

Bullet length is what matters in stability. Different types of bullets, secant or tangent ogives, hollow points, etc do change the formula to some degree, but length is THE parameter that matters most.

The old wisdom that heavier bullets need faster twists was formed around the idea that heavier bullets for a given caliber are longer in length, and until recent years, that was generally true. However, things have changed a lot. You've probably noticed all the manufacturers now making their "VLD", or "ELD", or "XLD", or whatever kitchy acronym they've decided to label their new extra long bullets. Weight is no longer an accurate consideration.

For example, in the 6mm/.243 caliber, Nosler makes the 100 grain Partition Spitzer and it is 1.065" in length. That bullet will stabilize well in a 1:9.25 twist rate Savage barrel (the twist rate they put on many of their .243 rifles).

Now, take a look at the Barnes 95 grain ELD-X. That bullet is 5 grains lighter, but it is 1.27" long, a hair less than a full quarter inch longer. This bullet won't even come close to stabilizing in a 1:9.25 twist. In fact, a 1:7 twist puts stability right at the line between stable and marginally stable.

If you want to shoot the modern extra long bullets that seem to be the new "thing" you'll most likely need an aftermarket barrel as most factory barrels aren't made in fast twists for most calibers.

Longer bullets need faster twist because of the moment arm created by the distance between the "center of pressure" and "center of gravity." Without getting too deep into the weeds, the center of pressure is at the nose of the bullet...that's where the air is putting the most force on the bullet as it travels down range. The center of gravity is the "balance" point of the bullet. Typically, and especially with long, secant ogive type bullets, most of the mass is toward the rear of the bullet. The longer the distance between the two, the less force it takes to upset the bullet (like a long vs a short lever arm).

Since bullets spin, the gyroscopic forces work to keep wobble, or instability at bay, so in order to keep perturbations down, you have to spin it faster and faster as length increases.

There are three types of projectile stability: Static, gyroscopic, and dynamic. Static stability means the projectile wants to right itself, gyroscopic is spin stabilization (bullets), and dynamic relies on some form of fin to dampen wobble.

There are five types of instability: yaw, nutation, precession, epicyclic, and gyroscopic drift. Yaw is lateral deviation from the flight path, think "tail wagging", nutation is when the projectile "nods", but since it is spinning, the rotation appears to be somewhere along its mid length, so the nose and tail are what rotate around an axis, precession is a "wobble" that increases as rotational speed decreases, epicyclic is also known as "static instability", it's the most common form of bullet instability and is caused by bullets that aren't perfectly balanced radially (think of it like an out of balance car tire), and gyroscopic drift is a an angular movement caused by the spin.

Bullets also tend to destabilize when they pass into the transonic speed range, yet the same bullet can and will be stable in subsonic regimes. This is because at transonic speeds, the bullet itself is subsonic, but portions of the bullet are still experiencing supersonic air flow. This serves to change the center of pressure in relation to the center of gravity and destabilization occurs.

 

Not to argue, but to try to add to the conversation,

Bullet length is what matters in stability. Different types of bullets, secant or tangent ogives, hollow points, etc do change the formula to some degree, but length is THE parameter that matters most.

The old wisdom that heavier bullets need faster twists was formed around the idea that heavier bullets for a given caliber are longer in length, and until recent years, that was generally true. However, things have changed a lot. You've probably noticed all the manufacturers now making their "VLD", or "ELD", or "XLD", or whatever kitchy acronym they've decided to label their new extra long bullets. Weight is no longer an accurate consideration.

For example, in the 6mm/.243 caliber, Nosler makes the 100 grain Partition Spitzer and it is 1.065" in length. That bullet will stabilize well in a 1:9.25 twist rate Savage barrel (the twist rate they put on many of their .243 rifles).

Now, take a look at the Barnes 95 grain ELD-X. That bullet is 5 grains lighter, but it is 1.27" long, a hair less than a full quarter inch longer. This bullet won't even come close to stabilizing in a 1:9.25 twist. In fact, a 1:7 twist puts stability right at the line between stable and marginally stable.

If you want to shoot the modern extra long bullets that seem to be the new "thing" you'll most likely need an aftermarket barrel as most factory barrels aren't made in fast twists for most calibers.

Longer bullets need faster twist because of the moment arm created by the distance between the "center of pressure" and "center of gravity." Without getting too deep into the weeds, the center of pressure is at the nose of the bullet...that's where the air is putting the most force on the bullet as it travels down range. The center of gravity is the "balance" point of the bullet. Typically, and especially with long, secant ogive type bullets, most of the mass is toward the rear of the bullet. The longer the distance between the two, the less force it takes to upset the bullet (like a long vs a short lever arm).

Since bullets spin, the gyroscopic forces work to keep wobble, or instability at bay, so in order to keep perturbations down, you have to spin it faster and faster as length increases.

There are three types of projectile stability: Static, gyroscopic, and dynamic. Static stability means the projectile wants to right itself, gyroscopic is spin stabilization (bullets), and dynamic relies on some form of fin to dampen wobble.

There are five types of instability: yaw, nutation, precession, epicyclic, and gyroscopic drift. Yaw is lateral deviation from the flight path, think "tail wagging", nutation is when the projectile "nods", but since it is spinning, the rotation appears to be somewhere along its mid length, so the nose and tail are what rotate around an axis, precession is a "wobble" that increases as rotational speed decreases, epicyclic is also known as "static instability", it's the most common form of bullet instability and is caused by bullets that aren't perfectly balanced radially (think of it like an out of balance car tire), and gyroscopic drift is a an angular movement caused by the spin.

Bullets also tend to destabilize when they pass into the transonic speed range, yet the same bullet can and will be stable in subsonic regimes. This is because at transonic speeds, the bullet itself is subsonic, but portions of the bullet are still experiencing supersonic air flow. This serves to change the center of pressure in relation to the center of gravity and destabilization occurs.

Necro thread. Did you notice you are responding to a 2017 thread?

 

Ahhhh....yeah, I do now. Sorry.

 

Bartlein has done some extensive testing and work with gain twist barrels. Theirs are very popular for high power and service rifle competition. They sell what they call "Radical Gain" barrels that go from something like 1:14 at the barrel entry to 1:6.8 near the muzzle. The advantages are good stability for lighter weight 75-77 grain bullets (.224 caliber) for the short range portions of the competition and shooters can step up to 90 grainers for the long range portion. Gain twist has been shown to allow for hotter loads without pressure issues, and also increased velocities for the same loads.

Many gain twist shooters claim accuracy is phenomenal. The science says that gain twist reduces damage to the jacket or lead shift inside the jacket which causes instability or decreases in BC.

I suspect it isn't common on factory firearms because it's more difficult to manufacture (i.e. more expensive).

 

Thank you Hydro.
I didn't think anyone made it any more. I learned some thing new today.
Jon

 

Yeah, they're out there, and by all accounts their performance is established, but they're custom barrels for sure.