Barrel Twist: Over Stabilize?

[Colt191145lover]

I have been toying with the thought of a 12.5 SBR in 5.56 with a 1/6 twist to help with the 70+ gr projectiles. Interesting info here.

[TiroFijo]

I have been toying with the thought of a 12.5 SBR in 5.56 with a 1/6 twist to help with the 70+ gr projectiles. Interesting info here.

Accuracy wise I've never seen a problem with 70+ gr bullets and 1/7 twist, even with short barrels.
Are there any new bullets that require a faster twist for accuracy?
Are the terminal effects any different using a faster twist than 1/7? I've never seen a 1/6 5.56 barrel.

[Colt191145lover]

Don't have a great reason other than what Doc has already said and I had read about it from Todd Hodnett years ago. More of a experiment to me as the idea of a suppressed "short recce" has always sounded fun and practical for me, YMMV. A lot of barrel company's will do custom twist rates for a little extra .
Green Mountain Barrels have done a few .
http://www.gmriflebarrel.com/gm-m34-...ited-quantity/

[Chance]

This quote is from an AMA at The Firearms Blog with the owners of Applied Ballistics. Speaking specifically is Bryan Litz, author of Ballistic Performance of Rifle Bullets:

Regarding over-stabilization; the 'common knowledge' that you hear often is that a bullet which is spun too fast will not trace with the trajectory, in other words, it will point 'nose high' on the downrange leg of the trajectory, thereby causing extra drag and lowering BC.

This explanation comes from high angle artillery shells which are far more massive, fly to very high altitudes (thin air) and arc thru dramatic angles over their trajectories. On this scale, it's true that spin stabilized projectiles can fail to 'trace' and fall belly or base first.

However, in the realm of small arms ballistics which usually occur below 10,000 feet ASL, projectiles weight less than 1 pound, and trajectories typically only arc a couple degrees (120 MOA) max, the flight dynamics are very different.

Spinning a bullet faster has the effect of increasing spin drift, but not increasing drag. In fact, over stabilizing (I prefer the term 'super stabilizing') can have the effect of suppressing the limit cycle yaw angle of the bullet, and cause it to fly with a higher BC over long range. This is a major subject in the new book and you can read much more detail there.

One caveat is that if you have a high velocity varmint round, over spinning might lead to bullet failure. But if you're dealing with larger calibers and velocities closer to 3000 fps than 4000 fps, spinning a little faster typically won't cause structural issues.

[DocGKR]

Bryan Litz knows what he is talking about--great class if you ever get a chance to take it!

[Unobtanium]

This quote is from an AMA at The Firearms Blog with the owners of Applied Ballistics. Speaking specifically is Bryan Litz, author of Ballistic Performance of Rifle Bullets:

Here is a group fired from my 16.1" DDM4 with suppressor, 1/7 twist, at 100 yards, using a 4x scope resting the rail of the weapon on an empty plastic jug. Accuracy seems fine to me given the constraints.



Ammo used: http://www.midwayusa.com/product/290...in-polymer-tip

[mmc45414]

Another Litz article I read also pointed out that spinning the bullet faster than it needs to be spinning will also exacerbate any imperfections in the bullet.

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[Handy]

Barrel wear?