Forums
New posts
Search forums
What's new
Articles
Latest reviews
Author list
Classifieds
Log in
Register
What's new
Search
Search
Search titles and first posts only
Search titles only
By:
New posts
Search forums
Menu
Log in
Register
Install the app
Install
Forums
Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Stability: Fine Points to be Aware Of
JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser
.
Reply to thread
Message
<blockquote data-quote="QuietTexan" data-source="post: 2634858" data-attributes="member: 116181"><p>There is a difference between gyroscopic and dynamic stability, they're highly interrelated but not the same thing. The effect Greg is seeing is real (higher velocity = longer bullets more stable at a given twist rate) and has to do with dynamic stability and how the bullet responds to <em>external </em>inputs. A 22-250 shooting a Berger 85.5 (1.170" long 85.5gn) bullet at 3100 FPS in an 7 twist will obviously have a higher bullet RPM and higher Sg factor than a 223 Rem shooting the same bullet in the same 7 twist at 2600 FPS. That bullet will be acceptably gyroscopically stabilized when it exits the muzzle at each velocity (1.64 Sg vs 1.55 Sg), but the dynamic stability Sd factor tries to describe what happens after that point. The bullet has forces imparted upon it while flying, if the Sd is low those inputs will have more of an impact. A faster bullet (to a point) should be more resistant to external effects, and will have a resultant higher Sd. The 223 Rem shouldn't be having those 85.5gn bullets keyholing at 100 yards, but the 22-250 should also outrun it at every range.</p><p></p><p>The reason the 168 SMK suffers from transonic instability has to do with how in that particular bullet design there is a cliff in the Sd calc where the transonic velocity reduction has significantly more impact on stability than the same absolute decrease in velocity in the supersonic range. Robert McCoy wrote a book that shows the math behind Sd, it was enough for me to not worry about it and generally go with "faster is better unless the target sucks" as a mantra <img class="smilie smilie--emoji" loading="lazy" alt="🤣" title="Rolling on the floor laughing :rofl:" src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f923.png" data-shortname=":rofl:" /></p><p></p><p>Pretty much every stability issue can be solved by matching components, if a bullet is unstable in a bore either change the bullet, change the twist, or change the cartridge. No one ever sits around and goes "this is fine" when something key holes <img class="smilie smilie--emoji" loading="lazy" alt="🤦♂️" title="Man facepalming :man_facepalming:" src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f926-2642.png" data-shortname=":man_facepalming:" /> Tends to be easier to find a lighter bullet than get a new barrel though.</p></blockquote><p></p>
[QUOTE="QuietTexan, post: 2634858, member: 116181"] There is a difference between gyroscopic and dynamic stability, they're highly interrelated but not the same thing. The effect Greg is seeing is real (higher velocity = longer bullets more stable at a given twist rate) and has to do with dynamic stability and how the bullet responds to [I]external [/I]inputs. A 22-250 shooting a Berger 85.5 (1.170" long 85.5gn) bullet at 3100 FPS in an 7 twist will obviously have a higher bullet RPM and higher Sg factor than a 223 Rem shooting the same bullet in the same 7 twist at 2600 FPS. That bullet will be acceptably gyroscopically stabilized when it exits the muzzle at each velocity (1.64 Sg vs 1.55 Sg), but the dynamic stability Sd factor tries to describe what happens after that point. The bullet has forces imparted upon it while flying, if the Sd is low those inputs will have more of an impact. A faster bullet (to a point) should be more resistant to external effects, and will have a resultant higher Sd. The 223 Rem shouldn't be having those 85.5gn bullets keyholing at 100 yards, but the 22-250 should also outrun it at every range. The reason the 168 SMK suffers from transonic instability has to do with how in that particular bullet design there is a cliff in the Sd calc where the transonic velocity reduction has significantly more impact on stability than the same absolute decrease in velocity in the supersonic range. Robert McCoy wrote a book that shows the math behind Sd, it was enough for me to not worry about it and generally go with "faster is better unless the target sucks" as a mantra 🤣 Pretty much every stability issue can be solved by matching components, if a bullet is unstable in a bore either change the bullet, change the twist, or change the cartridge. No one ever sits around and goes "this is fine" when something key holes 🤦♂️ Tends to be easier to find a lighter bullet than get a new barrel though. [/QUOTE]
Insert quotes…
Verification
Post reply
Forums
Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Stability: Fine Points to be Aware Of
Top