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Question regarding "over-stabilization" and twist ratios...

14623 Views 38 Replies 8 Participants Last post by  unclenick
Kind of a technical question, but...

I understand that a barrel has to have a sufficient twist ratio in order to stabilize bullets in flight. Other than a bullet coming apart and fragmenting mid-flight, how is a bullet "over-stabilized".

Example... Let’s say one 308W bullet stabilized with a 1-10 Twist, and another is stabilized with a 1-12 twist. Why would it hurt stabilization and accuracy if you lowered the twist of both rounds as long as they don’t fragment mid-flight?

My understanding is that different gr bullets need different twist ratios due to their length. And it makes sense. But it doesn’t make sense that a bullet spinning too fast can be less accurate. (As long as it doesn’t come apart)
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The bullet needs to turn its nose into the air so that it continues to fly point first. This is often referred to as the bullet “going to sleep”. The aerodynamic shape will cause this unless it's over stabilized.

If a bullet leaves the muzzle with a tiny bit of yaw (and they almost all do) and it's spinning too fast to aerodynamically correct its flight, you will lose accuracy and lower the ballistic coefficient as it's not presenting the most effecient frontal shape to the oncoming air.

At longer ranges, generally past 300 yards, the bullet needs to turn nose down as the flight path begins to curve down so that it continues to fly point first. An over spun bullet will resist turning into the flight path and the nose will remain turned upward relative to the ballistic curve. Once again, the less efficient shape presented to the oncoming air will lower the ballistic coefficient making the bullet lose velocity faster and more prone to drift.

You probably won’t notice an over stabilized bullet unless you’re shooting competitive benchrest or shooting out past 300 yards.

It's not just length but also aerodynamic profile that determines how much spin a bullet needs to properly stabilize.
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Because the 1 in 7" twist of the M16A2 over stabilizes the 63 grain M855 round.
It does seem odd that a fairly light bullet, such as the 155gr. Palma, can shoot to 1,000 yards yet a heavier bullet such as the 168gr. MatchKing cannot stay supersonic to that range. Guess there are some very subtle things about bullet design that are not obvious to the common folk..... ?
Look at the difference in velocity due to barrel length. The Palma shooters use very long barrels. Under most conditions the 168 grain will stay supersonic out to 1,000 out of a 26" or longer barrel in .308 Win.
I thought some shooters were reporting tumbling beyond 600 yards with the 168gr. bullet?
The 168s are generally stable out to 800 yards out of a 22-24" barrel .308. At my altitude, even out of a 24" barrel they'll go out to 1,000 most days. I think it's the jump from the 600 yard targets to the 1,000 yard targets you're thinking of- yes, most people don't use the 168s past the 600 yard targets as they'll frequently start to tumble.
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