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Discussion Starter · #1 ·
Hornady has released a new copper bullet called the CX to replace the GMX. The new 6.8mm version is also 100 grains like the GMX it replaces. Unfortunately, the new 6.8mm 100gr CX has demonstrated degraded terminal performance at impact velocities below 2300 fps causing the CX to act like an FMJ. The details regarding terminal performance are in this thread.


The CX’s performance potential can be regained if the bullet’s new, harder ballistic tip (BT) is removed. Test results with the BT removed have demonstrated expansion below 1600 fps. I found it easier to accomplish the BT’s removal with the bullet seated in its brass case. I initially used a sharp hunting knife backed by a piece of wood to clip the tip off just above the copper. Don’t try to do this with a flimsy exacto blade. I later found a pair of wire cutters would cleanly cut the BT very close to the copper. The next step is to file the tip flat so the copper just begins to show. This will allow the remaining plastic stem to be pushed out of the way and back into the expansion cavity on impact. Or, if you want to make sure the tip is out of the way, use a 1/16" bit and start to drill into the stem with light pressure. The remaining stem will quickly be pushed into the chamber. You can stop right there, reverse the drill out, and move on to the next bullet.

With the CX’s ballistic tip removed, you now have a 6.8mm hunting bullet with good expansion out to 350 yards or more depending on your muzzle velocity. The BC (G1) will be reduced to 0.230 but the trajectory difference is less than an inch at 200 yards. I found no difference in POI or accuracy with the BT removed using 3-shot groups at 100 yards.

I would recommend removing the CX’s ballistic tip on any caliber if you want to ensure bullet expansion below 1150 ft-lbs of kinetic energy.

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Xman, these findings are amazing. Q: Do you see it more valuable to just buy Barnes bullets as opposed to putting in the work to do all the cutting/filing? I thought that would add a nice commentary to your findings.

I use Barnes and 100gr CX.

TC
 

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Discussion Starter · #3 · (Edited)
mongo, the primary purpose of this thread was for those hunters who have already purchased CX bullets vs suggesting going out to buy then modify them. But your question is a fair one.

A: Maybe if it was the 95 TTSX. Barnes makes a good bullet, but I would consider the Cavity Back MKZ as a better option for a monolithic hunting bullet.
 

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Xman, thanks for posting this. As always, very thorough and well done.

All of my factory loaded CXs, lot#3222630, has the crimp line at the tip. It also looks like there is a secondary line on the bullets in your pictures (none on mine). Is this from seating them from hand loading?

All metals, copper in particular, work hardens. In addition to the tight grip on the plastic tip, I wonder if adding the crimp makes the bullet too tough to expand properly?

I must have missed it, why did Hornady replace the GMX?
 
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I must have missed it, why did Hornady replace the GMX?
I watched the Hornady podcast about the new bullets and it appears that when they started working higher BC monolithic bullets and changed their groove design, they decided to update the older bullets as well. Clearly the techniques that worked on the new bullets didn't translate to this one. Now I wonder just what they did to screw it up so badly?
 

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Discussion Starter · #8 ·
Guts, I was asking the very same question. Why did Hornady need to replace the GMX as it was one of the better performing copper bullets on the market. It did provide Hornady with a new marketing opportunity.

"The CX™ (Copper alloy eXpanding) bullet from Hornady represents the most advanced monolithic hunting bullet on the market. Its optimized design offers extended range performance, greater accuracy, high weight retention, and deep penetration."

Improvements were made with a different copper composition, new bands that create less drag, and their new ballistic tip (BT) that didn't heat up and deform down range. Go to their website and there is a video with 4 of Hornady's employees just sitting around the table talking about it for 25 minutes. Unfortunately, that new BT is impeding expansion. I am beginning to believe that the crimp is not the primary culprit and will be testing that with other CX bullets.

Oh, there are seating stem marks on they handloads. 270 Win cases have significantly more next tension than the 6.8. My 270 has a 1:10 twist which provides more representative expansion at lower velocities used in testing.
 
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Discussion Starter · #9 ·
Nincomp, I would guess Hornady didn't find this issue in testing because they were concentrating CX development on their heavy .308 CX bullets that do not reach the 1150 lbs-ft kinetic energy threshold where the BT remains intact until beyond typical hunting ranges. Hornady also seemed to accept 2000 fps as an acceptable minimum velocity threashould which was surprising to me because just about every .308 copper bullet I've tested still has effective expansion at velocities well below 2000 fps.

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Guts, I was asking the very same question. Why did Hornady need to replace the GMX as it was one of the better performing copper bullets on the market. It did provide Hornady with a new marketing opportunity.

"The CX™ (Copper alloy eXpanding) bullet from Hornady represents the most advanced monolithic hunting bullet on the market. Its optimized design offers extended range performance, greater accuracy, high weight retention, and deep penetration."

Improvements were made with a different copper composition, new bands that create less drag, and their new ballistic tip (BT) that didn't heat up and deform down range. Go to their website and there is a video with 4 of Hornady's employees just sitting around the table talking about it for 25 minutes. Unfortunately, that new BT is impeding expansion. I am beginning to believe that the crimp is not the primary culprit and will be testing that with other CX bullets.

Oh, there are seating stem marks on they handloads. 270 Win cases have significantly more next tension than the 6.8. My 270 has a 1:10 twist which provides more representative expansion at lower velocities used in testing.
The GMX did have a propensity for copper fouling, but by all accounts was a good performer. It seems to me, the CX was designed for the 270, 270 WSM and 6.8 western. The 6.8 SPC loading is strictly an afterthought. At least in .277 caliber anyways. Any bullet that gets a bad reputation for failing to adequately expand below 2200 fps is hard to shake. Terminal performance was the biggest issue in my testing copper bullets 25 years ago. Apparently it still is, with the CX.

A 100gr 6.8 SPC isn’t fast enough to warrant a hard heat resistant BT. If the tip was fragable it might help?
 
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Nincomp, I would guess Hornady didn't find this issue in testing because they were concentrating CX development on their heavy .308 CX bullets that do not reach the 1150 lbs-ft kinetic energy threshold where the BT remains intact until beyond typical hunting ranges. Hornady also seemed to accept 2000 fps as an acceptable minimum velocity threashould which was surprising to me because just about every .308 copper bullet I've tested still has effective expansion at velocities well below 2000 fps.

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Xman, do you think higher centrifugal forces will have much affect on the CX’s terminal performance? I have 6.8 SPC barrels with 1:8, 1:10 and 1:11 twist rates. I don’t think the 1:10 and 1:11 will have any significant difference, but the 1:8 might. However, I’m reluctant to test terminal performance with the BTs in place on my upcoming hunt after Jawjaboy’s experience.
 

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Discussion Starter · #13 · (Edited)
A 100gr 6.8 SPC isn’t fast enough to warrant a hard heat resistant BT.
Bullseye! I can't believe that Hornady would design the 100gr for the bigger-cased 270s when they have a dedicated 130gr bullet for those chambers.

Higher centrifugal forces will have a significant effect on the any monolithic/copper bullet's terminal performance. Even the difference between 1:11 and 1:10. This is when I first learned how important twist rate was for a copper bullet's terminal performance. Hunters with bolt-action 6.8mm SPCs were getting effective terminal performance with 110 TTSX while others were not. The bolt guns were 1:10 twist while the others were 1:11. Terminal performance testing showed more expansion with the 1:10 twist at the same velocity. I then conducted tests with .224 bullets using different twist rates. The lead-core bullets (62gr Fusion and Nosler's 64gr BPB) came out like clones of each other, i.e., no difference in expansion, though faster spin rates did help stability after impact. The 55gr GMX had significantly more expansion with faster twist rates as shown in the graph below.

However, I don't believe the faster twist rate will make the CX open up at a lower velocity as the BT has to "removed" on impact before the expansion cavity can perform its job. Once the bullet starts to expand, then the faster RPM will perform its magic. As the diameter of the expanding bullet increases, so does the spin forces effecting its expansion.

BTW, I shot another POI/accuracy comparison last weekend at 200 yards with and without the tips. I couldn't see a difference in POI except for slightly more wind drift. Plus the group size was actually smaller with the tips clipped and filed flat. I too would be reluctant to hunt with an unmodified 100gr CX. I would, however, encourage you to clip the tips, file them flat, and take them on your hunt - you will experience excellent expansion.

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A 100gr 6.8 SPC isn’t fast enough to warrant a hard heat resistant BT. If the tip was fragable it might help?
Exactly. On a podcast, the Hornady reps said that the "heat shield" tip was only useful on bullets that were to be shot beyond 500 yards and had a BC or over 0.5. It makes no sense why it would be on the 100gr CX.
 

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Discussion Starter · #15 ·
LOL!! Nincomp, you have more patients than me to sit through talk that long with no action in it. However, that is a very relevant point.
 
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LOL!! Nincomp, you have more patients than me to sit through talk that long with no action in it. However, that is a very relevant point.
I have to spend a lot of my time immobilized, so I have more time than most people to watch these things. I also watch a lot of them at 1.5x -2.0x speed!
I truly wish that I could go out shooting like you!
 

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Bullseye! I can't believe that Hornady would design the 100gr for the bigger-cased 270s when they have a dedicated 130gr bullet for those chambers.

Higher centrifugal forces will have a significant effect on the any monolithic/copper bullet's terminal performance. Even the difference between 1:11 and 1:10. This is when I first learned how important twist rate was for a copper bullet's terminal performance. Hunters with bolt-action 6.8mm SPCs were getting effective terminal performance with 110 TTSX while others were not. The bolt guns were 1:10 twist while the others were 1:11. Terminal performance testing showed more expansion with the 1:10 twist at the same velocity. I then conducted tests with .224 bullets using different twist rates. The lead-core bullets (62gr Fusion and Nosler's 64gr BPB) came out like clones of each other, i.e., no difference in expansion, though faster spin rates did help stability after impact. The 55gr GMX had significantly more expansion with faster twist rates as shown in the graph below.

However, I don't believe the faster twist rate will make the CX open up at a lower velocity as the BT has to "removed" on impact before the expansion cavity can perform its job. Once the bullet starts to expand, then the faster RPM will perform its magic. As the diameter of the expanding bullet increases, so does the spin forces effecting its expansion.

BTW, I shot another POI/accuracy comparison last weekend at 200 yards with and without the tips. I couldn't see a difference in POI except for slightly more wind drift. Plus the group size was actually smaller with the tips clipped and filed flat. I too would be reluctant to hunt with an unmodified 100gr CX. I would, however, encourage you to clip the tips, file them flat, and take them on your hunt - you will experience excellent expansion.

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Thanks again for all the great testing and info.

I once did the calculations for centrifugal forces on a spinning 6.8 bullet. IIRC, centrifugal forces were increased by about 10-ish% when going from 1:11 to 1:10. Pretty close to the graph of expansion in your real world testing. According to your graph it looks like expansion might be linear, 1:11 to 1:8 would be at least 25% more force. Pretty significant.

I wonder if the increase in centrifugal force would be enough to trigger expansion at lower velocity on impact with the ballistic tips in place? I’ve tested bullets of a different caliber going from 1:36 to 1:24 and the increase in centrifugal force was enough to pull jackets apart in flight. I forget exactly where you are, if not too far, I might loan you my 1:8 6.8SPC for testing. How far are you from the AZ/NM border?
 

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I shot another POI/accuracy comparison last weekend at 200 yards with and without the tips. I couldn't see a difference in POI except for slightly more wind drift. Plus the group size was actually smaller with the tips clipped and filed flat.
Interesting. Was there a difference in drop between the two?
 

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As far as twist rate effecting expansion goes it can cause expansion before the bullet strikes the target . Cbb recently tweaked its 195 sub because they opened right out of the muzzle with fast twist barrel . I did not witness it or have an issue but supposedly it caused damage to at least one suppressor . I wonder if the changes to the bullets changed the minimum velocity it would expand . Of course even if it cut the distance in half that is still farther than most will be shooting suds .
 

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As far as twist rate effecting expansion goes it can cause expansion before the bullet strikes the target . Cbb recently tweaked its 195 sub because they opened right out of the muzzle with fast twist barrel . I did not witness it or have an issue but supposedly it caused damage to at least one suppressor . I wonder if the changes to the bullets changed the minimum velocity it would expand . Of course even if it cut the distance in half that is still farther than most will be shooting suds .
I could be mistaken, but I believe the CX was designed for the 6.8 Western. Twist rates for the Western are 1:8. Along with the much higher muzzle velocity creates a very fast spin rate. Bullets need to be tougher to withstand the centrifugal forces and need a hard heat resistant ballistic tip. I believe Hornady loaded the CX in the 6.8 SPC as a stopgap.

There are two ways to increase bullet spin. Drive the bullet faster down the barrel or increase the twist rate of the barrel. By increasing both, the 6.8 Western compounds the forces. At shorter distances, the 6.8 Western also had higher kinetic energy at impact.
 
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