Bullet Damage from Rifling Engraving - Potential Impacts?

[LRShooter101]

I am starting this topic in response to some discussion that took place in another topic:
http://www.snipershide.com/shooting...-300-norma-magnum.6869034/page-2#post-7288108

The question came up about the "proper" twist rate in a .300 NM, followed by discussion about why one twist rate was or was not working?

My response to that line of questioning is below:

I am going to go out on a limb here, as I am by no means an expert on this subject, but hopefully it will get the conversation started, and maybe some more qualified experts will jump in.

There is talk here about barrel twist rates, and its impact on various parameters. Running through those:

Physical Impact on Bullet = Decreased BC - I have been arguing for years that the physical damage done to bullets could be a factor for consideration. If you look at how "hypothetical" BC numbers don't always work in External Ballistic Calcs, the bullet being damaged as it travels down the barrel could result in an altered/decreased BC.

Physical Impact on Bullet = Decreased Stability - Like the decreased BC, I have also been arguing that the physical damage done to a bullet can impact its physical makeup which can impact stability. If there is enough force/torque to distort/move the jacket in relation to the core, it would obviously impact stability.

I believe the 2 reasons above are why some shooters may be seeing less than optimal performance when they go to a faster twist rate. Basically the increased "twisting forces" does more damage to the bullet which has a negative impact.

Talking to Bryan Litz, he stated this should NOT be an issue with a "properly designed bullet" in a "reasonable twist rate" fired at an "appropriate speed". The key then becomes defining:
"Properly Designed Bullet" - this comes down to the jacket being strong enough, and the jacket and core maintaining their stability. It is a well known fact that a thin jacket bullet can be "blown up" by firing them out of a fast twist rate barrel at very high velocities.
"Reasonable Twist Rate" - I have no idea on this one? I am not sure if going from something like a 10 twist to an 8 twist in some situations could be enough of a change to produce negative consequences? The impact of this variable would obviously be heavily tied into the influence of the other 2 variables.
"Appropriate Speed" - I would normally say this could come into play if it is above 3,000 fps, but I am once again not really sure of the answer?

Increased Twist Rate = Increased Pressure - this is actually one that I DO NOT believe is an issue. If you look at Quickload there is no parameter entry for barrel twist rate. From my understanding that is because barrel twist rate has no impact on Internal Ballistics. Barrel Twist Rate also has almost NO impact on Muzzle Velocity, which would also support the argument that increasing the twist rate does not impact the Internal Ballistics.

IMHO, if people are seeing negative results by going with a faster twist rate in a .300 NM, it is because they are firing a bullet that is being damaged by the faster twist rate to the point that it is impacting performance. Obviously a thin jacket bullet, or a bullet with a poor jacket to core bond, could be susceptible to this.

I wish someone could fire some bullets with varying twist rates and speeds in a water tank so the physical damage to them could be analyzed and compared. I suspect if they did, there would be something there?

*EDIT - besides bullet construction, I also suspect that the Bullet Bearing Surface also comes into play here. The greater the Bearing Surface, the more interaction with the rifling, and the greater the potential for the issues listed above to come into play. In the pursuit of building better Long Range Bullets, the Bearing Surface sizes have gone up significantly.


The pictures below, show the bullet damage that I am referring to.

So, the Million Dollar Questions:
- Could a change in twist rate, change the amount of damage occurring to a bullet?
- Is the damage occurring to the bullet having a significant impact on its performance?
- Does that potential negative impact only occur at certain flight speeds, or at all flight speeds?
- And any other questions that someone else wants to throw into the mix?

Please let the discussion begin. Thanks!

 

[Jeffd]

Just my humble Mechanical Engineering opinion....I cannot see where 1-10 compared to 1-8 twist, for example has any impact. We are only talking about 1 360 degree twist in 8" as compared to 10 ".

 

[CoryT]

In most cases, the rifling marks are inside the boundary layer and have basically zero effect on drag. Exceptions exist for 'driving band' bullets, where the band itself can setup another Mach wave, just as a cannelure can, which is one reason match bullets don't have one.

What twists, crowns and muzzle brakes and suppressors can do is change the yaw angle of the bullet, which will in turn alter the BC by some fraction. This is why you can shoot the same load through two different rifles and have a slightly different BC line up correctly in your ballistic software for each rifle.

 

[LRShooter101]

In response to the above:
- Bryan Litz talks about "Energy Balance" which is made up of "Linear Kinetic Energy" and "Rotational Kinetic Energy". I would speculate that if you increase "Rotational Kinetic Energy" that you could also increase the potential damage caused by the rifling engraving? The Energy Balance and Rotational Kinetic Energy in the case of something like a 300 NM would be relatively high given that you are launching bullets around 3,000 fps. Question, what is the correlation in resulting damage between between Rotational Kinetic Energy and Twist Rates? Assuming the Rotational Kinetic Energy is the same, would a faster twist rate cause more engraving/damage to the bullet?

- I am personally having an issue getting my head around how a Driving Band or a Cannelure can be potential issues when it comes to drag, but somehow the engraving/damage would not be? If the engraving/damage is supposedly inside of the Boundary Layer, then why are the Driving Band or a Cannelure not inside the Boundary Layer, given they are located in the same place on the bullet?

- Given we are talking ELR, I would have to say performance would have to be looked at across the board and not just in supersonic flight. If we are talking about flight below supersonic, I would speculate that the engraving/damage could have an impact in both drag and stability?

- I would throw out that in terms of stability, by moving/redistributing material around on the outside of the bullet, that it would have to impact the spin stability of the bullet. You are taking a bullet that the manufacturer has gone to great lengths to make everything symmetrical, balanced, etc, and that is being changed by the engraving process resulting in something that is no longer symmetrical, balanced, etc. It is very difficult to try and spin stabilize something that is out of balance.

Like I said, I am not pretending to have the answers, I am just trying to open throw all of this around to see what shakes out.

Thanks for the responses, please keep them coming!

 

[LRShooter101]

Let me also throw these out there:

CONCEPT 1
Variable 1 - Barrel Design (Rifling Type/Profile, Reamer Specs, etc.) + Variable 2 - Bullet Design (Profile, Jacket Thickness, Bearing Surface Size, etc.) + Variable 3 - "Rotational Kinetic Energy" (see post above) = Outcome - Bullet Damage Due to Rifling Engraving

If you change anyone of the parameters that makeup the 3 variables, then you may also change the outcome, which is the amount and type of damage that is occurring to the bullet by the Rifling Engraving.

What parameters have the biggest impact on bullet damage?
- What types of chamber design and rifling creates the most bullet damage?
- What bullet types/designs are the most susceptible to damage?
- How much of a change in Rotational Kinetic Energy is required to change the amount of damage done?

CONCEPT 2
Not all Rifling Engraving, and resulting bullet damage, is created equally. Damage could range from something that may be inconsequential to something that is significant.

What Type / How Much bullet damage is required to be significant?
- Size/Depth/Type of damage?
- Only jacket damage, damage to jacket & core, jacket & core separation?

CONCEPT 3
Significant Rifling Engraving, and resulting bullet damage, can have an impact on External Ballistics.

How / When will significant bullet damage impact External Ballistics?
- Will it only be an issue at certain speeds, and not at others?
- Will it just decrease BC, or will it also impact stability?

Once again, I am not a Rocket Scientist , Engineer, or anyone with any formal education in any of this. I am just trying to work through all of this to see that if it all gets broken down, if there may actually be something there that is significant? Shooting inside of 1,000 yards, none of this was ever an issue. But when you start shooting at much longer distances, every little single nuance obviously matters.

Also once again, this issue came out of the discussion on why a slower twist rate seemed to be performing better than a faster twist rate. New research has shown that for ELR shooting that faster twists rates have increased stability and performed better, so why would a faster twist rate not produce better results? My Theory was that in SOME cases, the faster twist rates are causing more bullet damage, which is why there was a drop in performance. Is there a scenario/point where a faster twist rate may cause more bullet damage, and negatively impact performance because of that damage? This issue may simply turn out to just be minutia, or inconsequential noise, or it may help to explain why the bullets are not going where the Ballistic Calculator says that they should be going.

 

[THEIS]

Hi,

Great Topic

IMO this exact topic would have to be broken down to individual projectile types/designs.

For example:

7 twist 338 seems crazy fast, now you pair that twist with high MV of the new 33CX and you would think it would "eat" projectiles for lunch. And quiet possibly correct UNLESS you were using some of the solids that actually have less bearing surface than their lighter weight jacketed cousins.

7 twist 375 that is using a modified BMG case to get close to 4k fps you would also think destroys projectiles...but same as above.

The interesting sidenote on the above examples is both are running "P" rifling profiles from Schneider.....would be interesting to test other rifling profiles . Because it matters more than people realize. There is a reason back in the original ELR days that Lawton had a different rifling depth for the Lost River Ballistics Projectiles due to how hard that projectile alloy was and squeezing them in normal bore profiles had severe pressure problems.

Way more to come but time to get ready for my "You have been pre-selected for additional security screening"....will catch back up from flight.

Sincerely,
Theis

 

[whiskeysierra762]

Bartlien transitional rifling (gain twist) could be something to look at if you need a fast twist rate at high velocity.

 

[THEIS]

Hi,

For some light reading on this subject from Dr. Kneubuehl researchgate page:
https://www.researchgate.net/profile/Beat_Kneubuehl

Here is photography of a particular projectile at different velocities. You can see where the air pressure/weight/flow starts to "impact" the body of the projectile.

To further the discussion of this topic in regards to ELR......the biggest take on this topic would be as it has been said for decades now. IF you are building a dedicated ELR platform then do so with more rimfire philosophy than centerfire. Do not build a rifle then pick a projectile....PICK your projectile and then build your ELR platform purely for that projectile.

Sincerely,
Theis

 

[CoryT]

As you can see here, the rifling is still in the boundary layer, while the cannelure is a deep enough and sharp enough transition to create another Mach cone. It's a circumferential defect, where the rifiling is more linear and intermittent. As the layer separates at the base, the 'burble' formed is a good deal of the drag, which is why boattail angle and length matter. Base drag is far more of a concern than rifling. If it really mattered that much we'd all go polygonal.

Very high twist rates can cause the core to lag in a jacketed bullet or just strip the rifling altogether. This is why the long solids may benefit from gain twist. While it's generally better to overspin, at some point the bullet will just give up.

As Theis states above, it may be best to pick a projectile and then design the launch platform for it.

 

[LRShooter101]

Thanks to everyone for the replies and feedback!

QUOTE: "As you can see here, the rifling is still in the boundary layer, while the cannelure is a deep enough and sharp enough transition to create another Mach cone"

- I would argue that the damage caused by SOME Rifling Engraving could be substantial enough to have the same impact that the cannelure is causing above. If you look at some of the examples that I posted above, the damage is fairly significant, deep, and pretty much circumferential. Some bullets will initially engage the rifling, but the engraving is not deep enough to hold, so it will "skid" across the surface (until it goes far enough to actually catch), causing a defect that does go around the bullet. This can be impacted by how the chamber is cut, and the transition angle into the rifling. It can also be impacted by the profile of the bullet at the transition from the ogive to the bearing surface. You can see some of that in the bullets below:

- The picture posted addresses the Supersonic flight, but once again since we are talking ELR, this is not just a Supersonic flight issue. I am pretty sure this issue is going to exacerbate the issues that occur when a bullet goes Transonic, and will also cause issues once it goes Subsonic. I believe this issue has never really been looked at before because its impact in 95% of real world shooting situations is nonexistent. Most bullets are not fired hard/fast enough to cause the severe engraving issues. Most bullets hit their intended target will being Supersonic. However that can all change when you start talking about ELR shooting.

Please keep it going. Thanks Again!

 

[Ledzep]

As far as CD increasing (BC decreasing) due to increased damage to the jacket from faster rates of twist...

From my understanding with my discussion with a couple of the Hornady guys with their doppler, the trend is that higher stability factors typically result in higher BC. If you look at the 225gr ELD-M, for example in the link below you'll notice a slightly higher BC from the 1:7" twist vs. the 1:10.

https://www.hornady.com/support/ballistic-coefficient

Also from the conversations I've had with them, every barrel has the potential to produce different CD/BC values from the same bullet. The effect is not dramatic, but it is measurable. We're talking two rifles, same barrel length, same barrel maker, same twist rate, same bullet, same velocities, two slightly different CD profiles. So how much you can "design" for it, I don't know. I think not much.

 

[THEIS]

As far as CD increasing (BC decreasing) due to increased damage to the jacket from faster rates of twist...

From my understanding with my discussion with a couple of the Hornady guys with their doppler, the trend is that higher stability factors typically result in higher BC. If you look at the 225gr ELD-M, for example in the link below you'll notice a slightly higher BC from the 1:7" twist vs. the 1:10.

Hi,

That is correct but completely dependent on projectile design/alloy/etc.
Monolithics are well known to perform better with faster twist rates.
Jacketed projectiles can very much be a hit or miss (pun intended lol) with faster twist rates.
The best explanation of this is Jim Boatrights paper on Hyperstabilization Theory.

Edited To Add:
In regards to discussion of ELR situations of this topic.
We also need to NOT forget that projectiles typically have a maximum MV they are designed to operate under, just as they have a minimum MV they are designed to operate over. With MVs being pushed faster and faster, we need to keep that in mind when picking our intended projectile.

Sincerely,
Theis

 

[Jeffvn]

Given the limited engraving area on most of the currently available solid projectiles, I suspect they need to be considered separately from jacketed pills when considering this issue. I have a 12" to 5.4" exit gain twist .338 barrel. Suffice to say it tears jacketed pills to pieces - quickly (some of the pieces may get to 100 yards, but they start falling out of the sky at 60-70 yards) , but solids fly from it with no problems.

Not going to venture into the velocity discussion issue here, but heat resistant materials become critical when dealing with sustained velocities above Mach 3 and distances beyond 2,500 yards. This is a topic for a different thread....

Jeffvn