[bcraig]

Ballistic jell is not animal tissue however it can give a pretty good idea what a bullet is going to do when it strikes tissue.
This is why the FBI uses it so much to establish criteria for penetration and expansion.
The reason the statement and discussion we are currently having(at least for me) is not about the size of game to be hunted with the 6.8 and whether to use a different cartridge or to use a different bullet,but rather if is a way to give up a little expansion and gain a little bit of penetration using the federal xm68gd ammunition.

 

[Xman]

All bullets are Yawing in flight with some being more or less stable depending on caliber for weight and rifling twist. When a bullet is very unstable during flight and tumbling already the effect in flesh is obviously going to be a bullet striking either sideways or base first.

SOME Bullets whether FMJ or soft point will yaw or tumble upon impact depending on when in the flight the bullet strikes the target. Also where the impact takes place (bone, soft tissue etc).

Impact velocity affects the yaw or tumbling as well.

Some FMJ bullets enter nose first and basically remain nose first as can be shown with ballistic gel.

The bigger the mushroom the more likely a bullet is to diverge from its course as there is more in front to (catch) on tissue or bone to divert the course. And if the bullet is turned sideweighs it will act like a parachute and slow the bullet down more which impedes penetration.

Bullets for a particular caliber are designed from the get-go to be stable during flight so conditions of a bullet being unstable during flight, yawing, or even tumbling before impact are rare. I have, however, experienced these conditions several times when testing bullets too long/heavy for the typical 1:11 twist in the 6.8mm and even the 1:10 twist for the .270. In these situations, I have found it difficult to even hit the target and if I do, the terminal trajectory is typically unpredictable.

Unstable conditions in flight can most likely happen in very cold weather as the air becomes a more dense medium for a bullet to "fly" through. As an example, a Hornady 130 GMX fired from a .270 Win with a typical 1:10 will likely lose stability if temperature drops below 15F. The denser the medium, the less stable a bullet becomes (the advantage of having a 1:10 twist in a 6.8mm if you hunt in cold weather with heavy/longer bullets). When transitioning from air to flesh, which is mostly water, the density of the medium increases significantly. For a bullet to remain stable after impact, it must immediately shorten and or increase in diameter to remain stable. If it doesn't, it will yaw, tumble, and often swap ends exiting tail first. I've seen this repeated time and time again. FMJs do not remain nose first. You are welcome to post links to videos that show otherwise. There may be a short .224 55gr FMJ that appears to remain stable on impact if shot from a 1:7 or faster twist, but it likely has already started to yawn as it exits the gel. Also, the majority of OTM do the same and, if they have thin enough copper jackets, will break in the middle and fragment. If their jacket is too thick and maintains structural integrity, they are no better than an FMJ, e.g., .277 Berger VLDs. The 115 SMK is one of the few 6.8mm OTM bullets that doesn't yaw on impact. The ogive of its copper jacket is thin enough to flatten before it can yaw then fragment.

If a bullet leaves the muzzle in a stable condition, its stability increases as it travels down-range because its velocity decays more rapidly than its spin/twist rate. The only issue I have encountered when doing terminal performance testing at long ranges/lower velocities is, if the bullet does not shorten or increase its diameter enough, it tumbles and over penetrates usually tail first. It does not act like a parachute. The mushroom part of the bullet is now in the wake of the temporary wound cavity created by the boat-tail not doing its job. The .224 Nosler 64gr BPB is a good example of this behavior if impacts are outside of 100 yards and shot from a 16" 1:8 twist or slower. I have not seen evidence of a bullet with a larger mushroom diverting off course. Because they have become shorter and larger in diameter, their stability increases. If they expand and shorten too much, they can become unstable due to the CG shift but very few bullets exhibit this amount of expansion performance. The bullets I have seen divert off course after impact are ones that yaw having not immediately expanded and shortened enough.

Regarding if there is a way to give up a little expansion and gain a little bit of penetration using the federal xm68gd ammunition, the xm68gd has excellent weight retention and shows good penetrates. For a 90 grain bullet, it accomplishes a lot for being on the smaller side of the family of 6.8mm bullets. It may be difficult to modify this bullet to do better.

 

[bcraig]

Bullets for a particular caliber are designed from the get-go to be stable during flight so conditions of a bullet being unstable during flight, yawing, or even tumbling before impact are rare. I have, however, experienced these conditions several times when testing bullets too long/heavy for the typical 1:11 twist in the 6.8mm and even the 1:10 twist for the .270. In these situations, I have found it difficult to even hit the target and if I do, the terminal trajectory is typically unpredictable.

Unstable conditions in flight can most likely happen in very cold weather as the air becomes a more dense medium for a bullet to "fly" through. As an example, a Hornady 130 GMX fired from a .270 Win with a typical 1:10 will likely lose stability if temperature drops below 15F. The denser the medium, the less stable a bullet becomes (the advantage of having a 1:10 twist in a 6.8mm if you hunt in cold weather with heavy/longer bullets). When transitioning from air to flesh, which is mostly water, the density of the medium increases significantly. For a bullet to remain stable after impact, it must immediately shorten and or increase in diameter to remain stable. If it doesn't, it will yaw, tumble, and often swap ends exiting tail first. I've seen this repeated time and time again. FMJs do not remain nose first. You are welcome to post links to videos that show otherwise. There may be a short .224 55gr FMJ that appears to remain stable on impact if shot from a 1:7 or faster twist, but it likely has already started to yawn as it exits the gel. Also, the majority of OTM do the same and, if they have thin enough copper jackets, will break in the middle and fragment. If their jacket is too thick and maintains structural integrity, they are no better than an FMJ, e.g., .277 Berger VLDs. The 115 SMK is one of the few 6.8mm OTM bullets that doesn't yaw on impact. The ogive of its copper jacket is thin enough to flatten before it can yaw then fragment.

If a bullet leaves the muzzle in a stable condition, its stability increases as it travels down-range because its velocity decays more rapidly than its spin/twist rate. The only issue I have encountered when doing terminal performance testing at long ranges/lower velocities is, if the bullet does not shorten or increase its diameter enough, it tumbles and over penetrates usually tail first. It does not act like a parachute. The mushroom part of the bullet is now in the wake of the temporary wound cavity created by the boat-tail not doing its job. The .224 Nosler 64gr BPB is a good example of this behavior if impacts are outside of 100 yards and shot from a 16" 1:8 twist or slower. I have not seen evidence of a bullet with a larger mushroom diverting off course. Because they have become shorter and larger in diameter, their stability increases. If they expand and shorten too much, they can become unstable due to the CG shift but very few bullets exhibit this amount of expansion performance. The bullets I have seen divert off course after impact are ones that yaw having not immediately expanded and shortened enough.

Regarding if there is a way to give up a little expansion and gain a little bit of penetration using the federal xm68gd ammunition, the xm68gd has excellent weight retention and shows good penetrates. For a 90 grain bullet, it accomplishes a lot for being on the smaller side of the family of 6.8mm bullets. It may be difficult to modify this bullet to do better.

When a bullet is on the verge of being so unstable that it is showing some of the shots keyholing on paper then it is much more likely to tumble and have penetration and direction of travel affected in tissue.

What I said was that SOME FMJ bullets enter and basically remain nose first.
When the word SOME is used, it is to imply that not ALL bullets enter nose first and BASICALLY remain nose first but SOME do.
The key word here is BASICALLY
There is obviously a difference in the AMOUNT of YAW experienced AND there is a Difference in YAWING versus TUMBLING.
I would be interested in links to videos that show FMJ Bullets ALWAYS EXIT base first.
Obviously if the bullet was about to keyhole upon striking the tissue the effects would be to have the penetration and direction of travel would be affected in an unpredictable manner.
AND this holds true whether the bullet is a FMJ or a conventional expanding bullet
Obviously no bullet is ever spinning perfectly around its axis whether in flight or in tissue.

As far as the Mushroom of the bullet not acting like a Parachute in effect because of the temporary wound cavity.
The bullet is creating the Temporary and permanent wound cavity as well.
ANY part of the bullet whether it be the sides of the bullet exposing more of the bullet to the tissue Because of Yawing OR Tumbling will slow the bullet down impending the penetration.
The same holds true with the Mushroom of the bullet as it slows the bullet impeding penetration.
And this holds true whether the Mushroom is being PUSHED or PULLED through the tissue.
Just because there is a temporary wound channel does not mean that there is NOTHING for the bullets mushroom to "CATCH"on and slow the bullet down.

A bullet does not have a perfect mushroom to bear with equal Pressure on tissue,there will be one side so to speak of the bullet that has exposed more or less of itself to the tissue which will slow the bullet and affect the Line of penetration.

Think about a Barnes X Bullet and its penetration
Less of the front of the bullet is exposed to tissue so GENERALLY it is slowed down less, tends to penetrate deeper and in more of a straight line.

 

[ozarkpugs]

When a bullet is on the verge of being so unstable that it is showing some of the shots keyholing on paper then it is much more likely to tumble and have penetration and direction of travel affected in tissue.

What I said was that SOME FMJ bullets enter and basically remain nose first.
When the word SOME is used, it is to imply that not ALL bullets enter nose first and BASICALLY remain nose first but SOME do.
The key word here is BASICALLY
There is obviously a difference in the AMOUNT of YAW experienced AND there is a Difference in YAWING versus TUMBLING.
I would be interested in links to videos that show FMJ Bullets ALWAYS EXIT base first.
Obviously if the bullet was about to keyhole upon striking the tissue the effects would be to have the penetration and direction of travel would be affected in an unpredictable manner.
AND this holds true whether the bullet is a FMJ or a conventional expanding bullet
Obviously no bullet is ever spinning perfectly around its axis whether in flight or in tissue.

As far as the Mushroom of the bullet not acting like a Parachute in effect because of the temporary wound cavity.
The bullet is creating the Temporary and permanent wound cavity as well.
ANY part of the bullet whether it be the sides of the bullet exposing more of the bullet to the tissue Because of Yawing OR Tumbling will slow the bullet down impending the penetration.
The same holds true with the Mushroom of the bullet as it slows the bullet impeding penetration.
And this holds true whether the Mushroom is being PUSHED or PULLED through the tissue.
Just because there is a temporary wound channel does not mean that there is NOTHING for the bullets mushroom to "CATCH"on and slow the bullet down.

A bullet does not have a perfect mushroom to bear with equal Pressure on tissue,there will be one side so to speak of the bullet that has exposed more or less of itself to the tissue which will slow the bullet and affect the Line of penetration.

Think about a Barnes X Bullet and its penetration
Less of the front of the bullet is exposed to tissue so GENERALLY it is slowed down less, tends to penetrate deeper and in more of a straight line.

By the time a mushroomed bullet is going so slow that the temporary cavity is as small or smaller than the mushroom it is going too slow to penetrate much farther.

Sent from my Moto G Play using Tapatalk

 

[ozarkpugs]

Bullets for a particular caliber are designed from the get-go to be stable during flight so conditions of a bullet being unstable during flight, yawing, or even tumbling before impact are rare. I have, however, experienced these conditions several times when testing bullets too long/heavy for the typical 1:11 twist in the 6.8mm and even the 1:10 twist for the .270. In these situations, I have found it difficult to even hit the target and if I do, the terminal trajectory is typically unpredictable.

Unstable conditions in flight can most likely happen in very cold weather as the air becomes a more dense medium for a bullet to "fly" through. As an example, a Hornady 130 GMX fired from a .270 Win with a typical 1:10 will likely lose stability if temperature drops below 15F. The denser the medium, the less stable a bullet becomes (the advantage of having a 1:10 twist in a 6.8mm if you hunt in cold weather with heavy/longer bullets). When transitioning from air to flesh, which is mostly water, the density of the medium increases significantly. For a bullet to remain stable after impact, it must immediately shorten and or increase in diameter to remain stable. If it doesn't, it will yaw, tumble, and often swap ends exiting tail first. I've seen this repeated time and time again. FMJs do not remain nose first. You are welcome to post links to videos that show otherwise. There may be a short .224 55gr FMJ that appears to remain stable on impact if shot from a 1:7 or faster twist, but it likely has already started to yawn as it exits the gel. Also, the majority of OTM do the same and, if they have thin enough copper jackets, will break in the middle and fragment. If their jacket is too thick and maintains structural integrity, they are no better than an FMJ, e.g., .277 Berger VLDs. The 115 SMK is one of the few 6.8mm OTM bullets that doesn't yaw on impact. The ogive of its copper jacket is thin enough to flatten before it can yaw then fragment.

If a bullet leaves the muzzle in a stable condition, its stability increases as it travels down-range because its velocity decays more rapidly than its spin/twist rate. The only issue I have encountered when doing terminal performance testing at long ranges/lower velocities is, if the bullet does not shorten or increase its diameter enough, it tumbles and over penetrates usually tail first. It does not act like a parachute. The mushroom part of the bullet is now in the wake of the temporary wound cavity created by the boat-tail not doing its job. The .224 Nosler 64gr BPB is a good example of this behavior if impacts are outside of 100 yards and shot from a 16" 1:8 twist or slower. I have not seen evidence of a bullet with a larger mushroom diverting off course. Because they have become shorter and larger in diameter, their stability increases. If they expand and shorten too much, they can become unstable due to the CG shift but very few bullets exhibit this amount of expansion performance. The bullets I have seen divert off course after impact are ones that yaw having not immediately expanded and shortened enough.

Regarding if there is a way to give up a little expansion and gain a little bit of penetration using the federal xm68gd ammunition, the xm68gd has excellent weight retention and shows good penetrates. For a 90 grain bullet, it accomplishes a lot for being on the smaller side of the family of 6.8mm bullets. It may be difficult to modify this bullet to do better.

Kinda off topic here , Last year I shot a small doe almost completely broadside on level ground with a fusion ( have to go back and check which one) and it broke the left shoulder took out the left lung and heart and split the belly open perfect for field dressing ,the deer ran 50 yards down hill and litteraly gutted itself . One in a million bullet performance for sure .I had previously read where another shooter claimed he had a fusion turn 90° half way through a deer and thought he might have been confused about the angle but I'm 100% sure on mine . Have you experienced that behavior .

Sent from my Moto G Play using Tapatalk

 

[bcraig]

By the time a mushroomed bullet is going so slow that the temporary cavity is as small or smaller than the mushroom it is going too slow to penetrate much farther.

Sent from my Moto G Play using Tapatalk

It Is all relative and incremental as any contact between bullet and tissue slows the bullet.

 

[bcraig]

Kinda off topic here , Last year I shot a small doe almost completely broadside on level ground with a fusion ( have to go back and check which one) and it broke the left shoulder took out the left lung and heart and split the belly open perfect for field dressing ,the deer ran 50 yards down hill and litteraly gutted itself . One in a million bullet performance for sure .I had previously read where another shooter claimed he had a fusion turn 90° half way through a deer and thought he might have been confused about the angle but I'm 100% sure on mine . Have you experienced that behavior .

Sent from my Moto G Play using Tapatalk

Not Xman but I can share an experience.
I handgun hunted exclusively for over 12 years
I shot a 150 pound small buck at about 75 yards With A Thompson Center Contender 44 magnum Using a Corbon 280 Grain bullet(before I started making my own bullets)
The Buck was Quartering to me and I hit him right on the shoulder,he ran less than 30 yards and dropped
I found the bullet about 3 inches behind his ears laying along the spine !

That is when I tried Hard cast Water Dropped wheelweights 300 Grain LBT Wide Flat nose mold.they would penetrate almost the full length of Mature deer but often times ran over a Hundred yards before dropping. Then started using a softer nose cast 2 piece bullet out of the same mold for more tissue destruction .MO Better !

 

[Texas Hog Man]

If you don't want a rifle bullet to tumble, and you want extreme penetration just turn it around backward when loading....this used to be an old handloaders trick many many moons ago for increasing penetration and often times it actually increases accuracy.

A nose first rifle bullet is actually "wrong" in the aerodynamic world... pointy end forward is kind of the opposite of ideal for stability when it comes down to it. But design wise (for expansion) that's how they work.

Also, as shown in multiple tests, a flat frontal area (after the "mushroom" forms or petals break off) actually creates a more devastating wound chanel

Each bullet has it's own unique design --- some fragment for a quick energy dump with a large diameter wound chanel, some are designed to mushroom for larger frontal area and deeper penetration, some are designed to mushroom but retain all weight, some designed to tumble and "shred", some designed to stay 100% intact as is for deep (read that as 3-4 foot) penetration, some designed to expand drasticly for more of a "cutting" action.

Different bullet designs for different applications is why x-man started his bullet testing thread, that way we can all see real world testing of bullets and choose it for our own unique application.

 

[ozarkpugs]

If you don't want a rifle bullet to tumble, and you want extreme penetration just turn it around backward when loading....this used to be an old handloaders trick many many moons ago for increasing penetration and often times it actually increases accuracy.

A nose first rifle bullet is actually "wrong" in the aerodynamic world... pointy end forward is kind of the opposite of ideal for stability when it comes down to it. But design wise (for expansion) that's how they work.

Also, as shown in multiple tests, a flat frontal area (after the "mushroom" forms or petals break off) actually creates a more devastating wound chanel

Each bullet has it's own unique design --- some fragment for a quick energy dump with a large diameter wound chanel, some are designed to mushroom for larger frontal area and deeper penetration, some are designed to mushroom but retain all weight, some designed to tumble and "shred", some designed to stay 100% intact as is for deep (read that as 3-4 foot) penetration, some designed to expand drasticly for more of a "cutting" action.

Different bullet designs for different applications is why x-man started his bullet testing thread, that way we can all see real world testing of bullets and choose it for our own unique application.

Which is why the Accubond is so good it has a hammer shaped mushroom and makes me wonder if the cb bullets designe may have more going for it than just powder capacity . I have not read all of x man's tests but don't remember seeing any comments about round mushrooms going off course more than others . Evidently it has been experienced by more people than just me .I have seen it in pistol bullets but thought it was the result ricocheting because of round mushrooms .I really don't think fusion bullets are designed to change course and I'm not saying they are the only bullets that do .

Sent from my Moto G Play using Tapatalk

 

[41bear]

Xman you are truly "The Man!" This is a wonderful job of bullet performance the likes of which I've not seen before. Thank you for your time and effort.

 

[Xman]

Thanks, 41bear. I have been branching out to test .224 and .308 bullets but I'm traveling more at work and, now that I have a hunting lease, there doesn't seem to be enough time in reserve to do much testing.

I have been switching over to faster twist barrels (1:10 in 6.8mm and 1:7 or faster in the 5.56mm) because the monolithic bullets have measurably more expansion when they are spinning faster.

 

[41bear]

Thanks, 41bear. I have been branching out to test .224 and .308 bullets but I'm traveling more at work and, now that I have a hunting lease, there doesn't seem to be enough time in reserve to do much testing.

I have been switching over to faster twist barrels (1:10 in 6.8mm and 1:7 or faster in the 5.56mm) because the monolithic bullets have measurably more expansion when they are spinning faster.

When/if you get around to writing that book let me know, I'll be the first in line.

 

[ozarkpugs]

Hopefully cbb will come out with factory load to test

Sent from my Moto E (4) using Tapatalk

 

[sfs13]

Thanks, 41bear. I have been branching out to test .224 and .308 bullets but I'm traveling more at work and, now that I have a hunting lease, there doesn't seem to be enough time in reserve to do much testing.

I have been switching over to faster twist barrels (1:10 in 6.8mm and 1:7 or faster in the 5.56mm) because the monolithic bullets have measurably more expansion when they are spinning faster.

You have done some great work with the 6.8 bullet test. Have you or will you post the .308 and .224 test results somewhere on the forum?

 

[Googa]

100 gr Accurbond @ 2,800 FPS . 100 yard quartering away. The bullet entered the back of the rib cage, exited the front of the rib cage, and passed through the backside shoulder. It was caught by the hide.

 

[Xman]

Nice buck you harvested, Googa. Expansion and terminal performance expected of a 100 AB.

 

[Psaudowatt]

95gr ttsx

95gr ttsx at 2723ft mv.
Rabbit at 10 yards. Fist size exit hole.
Bullet was recovered in soft soil/Moss and might have expanded a little extra in that secondary media.

 

[northwoods]

Been thinking about getting a 6.8 barrel - this kind of information is awesome...thanks for your effort!

 

[Xman]

I have been asked the question, "Have I seen a copper bullet fail?" I thought this would be the appropriate thread to provide an answer. However, like most things, the answer is not always simple and can depend on how you define "failure". So, I've created three different "failure" scenarios for this discussion:

• failure of design,
• failure to use a copper bullet in a situation where it works best, and
• failure of the hunter.

First Category - Failure of design.

• I have tested three monolithic bullets that can be put in this category; early 6.8 Controlled Chaos, Cutting Edge, and Fox bullets. The early 100 grain 6.8mm Controlled Chaos bullet was a monolithic fragmentation brass bullet that had multiple issues when I tested it. The fragments were too small in mass and too few in numbers to cause significant trauma. The depth of the bullet's expansion cavity was also not deep enough resulting in the remaining bullet shank being too long to remain stable in the terminal phase. The shank would flip 180 degrees and penetrates tail-first transferring very little energy. These bullets blew through my bullet traps like they were going through butter including my plywood backstops. This performance is specific to the 6.8mm bullet and may not apply to other calibers. Only one other bullet I have tested, has performed this poorly - the Cutting Edge 120 and 130 grain 6.8mm copper CNC bullets. Their performance and shortfalls were identical to the 100 grain 6.8mm Controlled Chaos bullet which may no longer be available. The Lehigh Defense website now shows a 112 gr copper variant which I have not tested.

Here are the links to full reports.
6.8 SPC Bullet Performance

6.8 SPC Bullet Performance

• Another example is Fox's 130 grain .277 bullet which is produced in Europe (I have had hunters from Europe and Africa send me bullets to test). It is also a brass bullet. The .277 130gr Fox bullet's expansion is minimal and it loses its small petals at a high energy state when shot from a .270 Win. It basically performed like the Controlled Chaos and Cutting Edge bullets with little energy transfer and over-penetration except when impact velocity dropped to 2200 fps. At this lower energy state, it maintained its structural integrity with a perfect mushroom and typical penetration for a deer-sized target. However, at 1800 fps, the bullet would not open up and tumbled on impact. IMO, having a 400+ fps operating window for terminal performance is a failure in design.
• Lead-core bullets can fail, too. There are two lead-core bullets I've tested that fit this category: first the .277/6.8mm Berger VLDs. Their copper jackets are too thick and maintain their structural integrity unless they impact bone. I've even clamped a 130 gr VLD in a vice with no deformation, cut its tip off to expose an 1/8" hollow point, and still couldn't get it to expand shot from a .270 Win. The .277 VLDs share the same copper jacket with the 7mm VLDs that some shooters are annealing to make them soft enough to fragment on impact.

Here is the link to the full report.

6.8 SPC Bullet Performance

• Another lead-core bullet that could be designed better is Nosler's 64 grain Bonded Performance Bullet. In close, it functioned as designed, but at approximately 100 yards shot from a 16-inch barrel or 200-yards shot from a 24", the bullet expanded but did not shorten enough or increase its diameter enough to remain stable in the terminal phase where body tissue is more dense than air. The bullet would swap ends and penetrated tail first causing a significant reduction in energy transfer. The Federal/Speer Fusion MSR bullets had superior expansion and reduction in length.

Don't get me wrong, a bullet like these above can still be lethal. However, the probability of the animal running off increases compared to other hunting bullets that transfer their energy to the animal more effectively.

In closing out this category of failure, I wanted to add that I used to believe that a copper bullet had failed if its kinetic energy was so high that it couldn't maintain its structural integrity and would lose its petal leaving the bullet shank to continue penetration. This could happen at short range with a magnum caliber or with .224 monolithic bullets when high velocities generate greater than 1100 lb-ft of KE. However, after hunting with .224 E-Tips and MKZs shot from Valkyrie and 24-inch .223, I have experienced great terminal performance on coyotes and hogs at energy states where their pedals are lost. The petals have enough mass to continue penetration and expand the wound channel while the bullet's shank continues to penetrate.

 

[Xman]

Second Category - Failure to use the bullet in a situation where it works best. I think this is where many hunters claim that a copper bullet has "failed" them.

• Let me preface, I have found that twist rate dramatically effects the expansion of copper bullets. I don't believe this is common knowledge. My first experience with the twist effect was with the Barnes 110 TTSX fired from a 6.8mm SPC. There are several reports of this bullet being ineffective in a 6.8. The 110 TTSX was my first go-to bullet in my .270 Win, so I had some to test in my 6.8mm. When I posted the results, an experienced hunter sent me a picture of his recovered, beautifully expanded 110 TTSX he shot a deer with and told me my testing was flawed. When I inquired about his rifles configuration, he said he had a bolt action 6.8mm SPC with a 1:10 twist. So, I downloaded my .270 to match the velocity out of my 18" 1:11.25 twist and tested again. That bullet expanded just like his bullet. So, if you are shooting copper bullets - light weight and fast with plenty of twist is the situation where copper bullets work best and how an 85 TSX can appear to outperform a 110 TSX in a 6.8mm. Here is a graph of bullet expansion vs twist rate with the Hornady's .224 55 grain GMX.

• Thus, with all the 16" 1:9 twist 5.56 ARs out there, I can believe there are lots of cases where hunters claim a copper bullet "failed" them. If you want to hunt with an AR and copper bullets in a .224 caliber, use a 1:7 twist or faster, 1:8 minimum. Copper bullets really wake up and expand with 1:7. As an example, here is a pronghorn's exit wound from a .224 78 grain TSX. The bullet was shot from a Valkyrie (1:6.5 twist) at a range of 130 yards.
  
  
  
• Note a 24-inch .223 adds 100 yards more effective range compared to a 16-inch barrel which also contributes to the failure impression of those hunting with monolithic bullets.
• The only copper bullet I have tested where a slower twist rate does not appreciably limit expansion is CavityBackBullet's MKZs. Regarding lead-core bullets, I've tested 62 gr MSR and 64 gr BPB, and found no measurable difference in expansion across 1:6.5 to 1:12 twist rates.