Bullets Performance vs Twist Rate

[nksmfamjp]

I'm old. Back in the day, short range benchrest was dominated by 6PPC launching light flat based bullets fast, but near the edge of stability for the twist rate. If memory serves me….60-68gr FB bullets at 3200-3300fps with a 14-15 twist bbl.



So, now I'm more thinking hunting and long range. I've heard 2 things related to twist and long range that I'm wondering if there is any testing on.



First, twist vs bc.. It is my understanding that many marginally stable bullets are accurate enough, but that the bc is somewhat lower due to not enough twist. Have you ever seen this? I know my last BC truing required up'ing my G7 BC on the 178 ELD-X to match up to my actual flight path….like 10%. MV was 2950ish



Second, I see some mono's, mostly hammers, but others too that exit the gel block in testing where something like ELD-X stays in the block. Vortex's high speed camera showed the HHT bullets tipping over and going off path in gel. …to be clear, after petal shear. So, Hammer always says you need more twist for expansion performance. Is this what is needed to fix the crazy bullet paths after expansion or bad analysis on my part. I'm nervous about Hammers because they often don't go straight in gel.

 

[choprzrul]

Follow the minimum twist recommendation on Hammer's page for the bullet, and manually verify the exact twist rate of your barrel. Hammer's twist recommendations are based on terminal performance, not ballistic performance.

 

[Mikecr]

There are two kinds of stability; Gyroscopic and Dynamic.
We can calculate gyroscopic stability (at the muzzle), which can compensate for marginal dynamic issues further down range.
FB BR bullets can release at low gyroscopic stability (as calculated) while being very stable dynamically.
We're missing half of stability calculation here, so the remainder is simply tested for (not predictable).

 

[nksmfamjp]

I'm not sure if I'm familiar with this dynamic stability you speak of. I probably could understand it if you knew how to explain what forces are affecting it stability wise down range. It's confusing to me. The idea that something can leave the barrel marginally stable and get better. It's not just me. It's even true of ballisticians who speak on the topic. I'll bet I'm not understanding what you're saying.

 

[CRP003]

@Petey308

 

[Petey308]

Good questions. You're not far off, but a couple things are getting blended together.

On twist vs BC:

What you're seeing is real, but it's not that a difference in twist rate "changes BC." A marginally stable bullet (low SG) tends to fly with more yaw/coning, which increases drag. When you true it, that makes it appear like a lower BC. Bump the twist (or stability margin), and the BC lines up better because the bullet is flying cleaner—not because the BC itself changed. That's just looking at it real technically.

On monos and gel behavior:

That's not really a twist issue. Once a bullet enters tissue, gyroscopic stability matters a lot less. What you're seeing with some monos is usually post-expansion imbalance—petals shedding unevenly or the bullet deforming asymmetrically. That shifts the center of mass and causes yaw/tumbling. More twist doesn't fix that, it just changes when it occurs, the amount, etc., but more or less velocity has a similar effect as well (as well as increases or decreases the RPM).

Big picture:

• Twist rate: affects flight stability and drag (apparent BC)
• Terminal path: mostly driven by bullet symmetry after expansion, not twist

If you want consistency, run enough twist for a solid stability margin in flight, and choose bullets that stay balanced after expansion.

 

[Phil Rizzo]

There was a local cult following here that used the term "rotational" velocity often, talking about their 257WBY/87gr sierra SP loads and the expansion it helped initiate and damage it caused. I suppose the higher rpm's and a bullet on the edge of disentigration at high speed does cause massive damage. Not sure what their twist rates were , but definitely faster than 1-12" I'm certain. Wasn't that factory? Or was it 10tw. I'm referring to the 80-90's.
But I've never has an issue with any rapidly expanding bullet in normal twist rates at any velocity. A softball sized tunnel is adequate.

 

[Mikecr]

I'm not sure if I'm familiar with this dynamic stability you speak of. I probably could understand it if you knew how to explain what forces are affecting it stability wise down range. It's confusing to me. The idea that something can leave the barrel marginally stable and get better.

Dynamic stability is beyond my full understanding. Even scientists can't predict it with precision.
So bullets, missiles, etc., must be field tested and measured for it.

Gyroscopic stability climbs with distance of bullet travel -provided the bullet was stable when released.
This, because spin decreases slower than velocity, so effective twist rate (overturning displacement per turn) get's tighter.
Yet, some bullets still become unstable through transonic velocities.
That's a dynamic instability.
I believe the cause of that is an abrupt release of the sonic wave, but a flat base adds to the issue somehow.

 

[dashaw]

In 20 years of ELR shooting I have never seen any issues with stability in the transonic zone with bullets that were 1. on the heavier side for a specific caliber and 2. Designed with extended range shooting from the start. Meaning improved BC's.
30 cal. example. Has anyone ever heard of problems with a 190 gr. SMK? I haven't. Not a new design but between weight and design there are no problems at extended ranges.
Fast twist barrels require better bullets to achieve the same level of accuracy. That is based on my definition of accuracy which may be different than yours..

 

[ButterBean]

I'm old. Back in the day, short range benchrest was dominated by 6PPC launching light flat based bullets fast, but near the edge of stability for the twist rate. If memory serves me….60-68gr FB bullets at 3200-3300fps with a 14-15 twist bbl.



So, now I'm more thinking hunting and long range. I've heard 2 things related to twist and long range that I'm wondering if there is any testing on.



First, twist vs bc.. It is my understanding that many marginally stable bullets are accurate enough, but that the bc is somewhat lower due to not enough twist. Have you ever seen this? I know my last BC truing required up'ing my G7 BC on the 178 ELD-X to match up to my actual flight path….like 10%. MV was 2950ish



Second, I see some mono's, mostly hammers, but others too that exit the gel block in testing where something like ELD-X stays in the block. Vortex's high speed camera showed the HHT bullets tipping over and going off path in gel. …to be clear, after petal shear. So, Hammer always says you need more twist for expansion performance. Is this what is needed to fix the crazy bullet paths after expansion or bad analysis on my part. I'm nervous about Hammers because they often don't go straight in gel.

Hammer has a good Pill but they have their problems, lots of folks having great success with them but there are much better options.
McGuire Ballistics and Apex is where I would look

 

[hawlejr]

There was a local cult following here that used the term "rotational" velocity often, talking about their 257WBY/87gr sierra SP loads and the expansion it helped initiate and damage it caused. I suppose the higher rpm's and a bullet on the edge of disentigration at high speed does cause massive damage. Not sure what their twist rates were , but definitely faster than 1-12" I'm certain. Wasn't that factory? Or was it 10tw. I'm referring to the 80-90's.
But I've never has an issue with any rapidly expanding bullet in normal twist rates at any velocity. A softball sized tunnel is adequate.

My early '70s .257 Weatherby had a 1:12 twist. Supposedly this was Roy Weatherby's favorite with a 100 gr Nosler Partition bullet. He was definitely of the "speed kills" persuasion in designing his cartridges. The rifle would not stabilize 120 gr bullets, so I sold it. Sometime later ....'80s? the company changed it to 1:10 twist. Meanwhile I had standardized my .25 cal rifles to 25-06 with a 1:10 twist which was (is) standard. Today, I would like to have a 1:7.5 twist and ream a bit out of the chamber shoulder to make it a 25-06 AI (for you kids, that's Ackley Improved, not artificial intelligence!!). I know longer bullets are coming, and I like BOTH speed and bullet length/weight....but then again, I am a belt and suspenders old codger! I'd be able to use the same 25-06 neck/trim die to neck down all the 30-06 brass I have in the cave, fire it once, and then neck size from there. This is a recurring dream I have.....just have to shoot out the current barrel on the old M77 to give me the excuse I need to follow my dream!

 

[Phil Rizzo]

My early '70s .257 Weatherby had a 1:12 twist. Supposedly this was Roy Weatherby's favorite with a 100 gr Nosler Partition bullet. He was definitely of the "speed kills" persuasion in designing his cartridges. The rifle would not stabilize 120 gr bullets, so I sold it. Sometime later ....'80s? the company changed it to 1:10 twist. Meanwhile I had standardized my .25 cal rifles to 25-06 with a 1:10 twist which was (is) standard. Today, I would like to have a 1:7.5 twist and ream a bit out of the chamber shoulder to make it a 25-06 AI (for you kids, that's Ackley Improved, not artificial intelligence!!). I know longer bullets are coming, and I like BOTH speed and bullet length/weight....but then again, I am a belt and suspenders old codger! I'd be able to use the same 25-06 neck/trim die to neck down all the 30-06 brass I have in the cave, fire it once, and then neck size from there. This is a recurring dream I have.....just have to shoot out the current barrel on the old M77 to give me the excuse I need to follow my dream!

Don't wait!
I've run standard 25/06. The AI version, and now I'm equipped with the 25ss, 25Sherman LA (both 72gr capacity) and the 25 mega at 83grs.
Overachievers in every way, comfort included. But a 24" barrel is short to Me.
Get with it, you won't regret it.

 

[ButterBean]

Don't wait!
I've run standard 25/06. The AI version, and now I'm equipped with the 25ss, 25Sherman LA (both 72gr capacity) and the 25 mega at 83grs.
Overachievers in every way, comfort included. But a 24" barrel is short to Me.
Get with it, you won't regret it.

I'm in the same boat as Phil and paddling as hard as I can

 

[hawlejr]

Thanks Phil. Being the cheap ol' codger that I am, I'm going to wait until that barrel starts throwing 1" groups. Right now it's a "one-holer". Agreed that I want a 26" barrel, which is a compromise over a 28" barrel. I like a bit more thickness than the thin profile, so 26" med-heavy probably is a good compromise. Right now, it's a standard 1974 M77 heavy varmint barrel, so I'm used to muzzle heft. I also have a 7mm mag mauser which I'm trying to wear out! That would have the larger bolt face, but I've been thinking about just changing to a .264 WM with a 7.5" twist on that, too when it goes. Again, I'd be able to take some of my 7mm brass and just run it through a necksizing cycle. Thanks for your thoughts, you must be an adventurous young man to be pushing those modern rock stars! Take care,
Jack Hawley