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Discussion Starter · #601 · (Edited)
There have been several prior posts on 100 CX Testing (#573-582, 596) that have led to this conclusion.

Thank jawjaboy for pulling 100 CX bullets from factory Hornady ammo (Lot # 949179) and contributing them for testing. His lot of factory 6.8mm SPC ammo had the same crimp/finishing marks just below the ballistic tip as the component bullets. Macatac also confirmed his factory Hornady ammo (Lot # 3221532) had the crimped just below the ballistic tips. Testing with jawjaboy’s bullets, minimum velocity expansion was consistent with the component 100 CXs tested so far where velocities 2210 fps and below did not expand and yawed like an FMJ. Testing at a higher speed of 2290 fps, the 100 CX finally did open up with expansion deep into the expansion chamber. The expanding petals did break away but I will have to test further to determine if that is due to the CX being a softer copper than the GMX or the testing method used. Note, a 6.8mm CX on-line review from another shooter stated he was getting failure to open at mid-range velocities and loss of petals when speeds were high enough to effect expansion.

So, what is going on. After impact, a bullet must shorten and expand to a larger diameter to remain stable. Anything the delays expansion, as is happening here, will cause the bullet to be unstable in the denser medium, yaw, and fail to expand. The previous tests with the BTs removed showed proper expansion at speeds below 1600 fps so the expansion cavity is not causing any issues. The remaining factor(s) that could be delaying expansion would be the bullet’s copper gripping the BT to tightly and/or the increased hardness of these new tips. I found some .308 150gr CX bullets that do not have the crimp and will test them and then smaller calibers to further determine the details of why Hornady’s new CX bullet is not providing optimal expansion.

Meanwhile, what does a hunter do with a good bullet that turns into an FMJ after 150 yards (or closer to 100 yards with a shorter barrel). I suggest one of two options. You could resort to shoot hogs through the shoulder blade to create bone fragments that will improve terminal performance (not easy to do if they are running). Or, remove the obstruction that is preventing expansion, e.g., the bullet’s ballistic tip. This will give you a bullet with good expansion to 350 yards and beyond depending on your muzzle velocity. Good medicine for hogs and even better for coyotes. I tested two configurations: 1) with the BT removed but stem still in-place, and 2) BT removed with the stem pushed into the expansion cavity. As you can see in the test results below, leaving the stem in place did not impede expansion. Even with the lower BC (G1) of 0.230 vs 0.303, there is less than an inch difference in trajectory at 200 yards. I did a quick accuracy test with two 3-shot groups at 100 yards. POI did not change and accuracy was not degraded with the tips removed. Below is a link to a new thread on how to accomplish BT removal if you want to recover the performance potential of your 6.8mm SPC 100 gr CX bullet and ammunition.

(38) Hornady’s New CX Bullet: How to Restore Terminal Performance | 6.8 SPC Forums (68forums.com)

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The chrono results for the lot of ammo (Lot # 949179) from which I supplied the pulled bullets for Xman's test is attached.

The particulars are:

Rifle: PSA 6.8 SPCii 18" 1:11 twist
Suppressor: SilencerCo Omega 300
Chronograph: Oehler 35p
Distance from muzzle: 10 feet
Altitude asl: 295 feet
Temperature: 51 deg F
Humidity: 58%
Pressure: 30.10 inHg

I will add my anecdotal observations from the field on several hogs and a coyote with the Hornady 6.8 SPC CX 100 grain are consistence with Xman's findings. The one deviation with my observations from his results is it appears that the 6.8 SPC CX 100's failure to expand may occur at shorter distances, under 150 yards, possibly because hogs / coyotes flesh may have less obstructive media effects, from Xman's test media, on the CX 100 hence less expansion and lethal terminal performance. Once again this is anecdotal but to my eye I've noticed a difference in shot hogs' and coyote's reactions from Hornady 6.8 CX 100 versus both S&B 6.8 SPC PTS (Red Tip) 110 grain and IMI Systems Razor Core 5.56 77 grain HPBT-Match (Sierra MatchKing) ammo. They just seem more like a poor shot placement or FMJ pass through than with the other ammo I've used.

Bottom line: I will be removing the tip from the CX 100. When I run out of these I'll be using the S&B Red Tips until I get my reloading online with the Hornady 120 SST bullets.


JBM - Calculations - Trajectory JBM - Calculations - Trajectory 1/1

Trajectory
Input Data
Ballistic Coefficient:0.303 G1Caliber:0.277 in
Bullet Weight:100.0 gr
Muzzle Velocity:2648.0 ft/sDistance to Chronograph:10.0 ft
Sight Height:3.00 inSight Offset:0.00 in
Zero Height:1.75 inZero Offset:0.00 in
Windage:0.000 MOAElevation:0.000 MOA
Line Of Sight Angle:0.0 degCant Angle:0.0 deg
Wind Speed:10.0 mphWind Angle:90.0 deg
Target Speed:10.0 mphTarget Angle:90.0 deg
Target Height:12.0 in
Temperature:51.0 °FPressure:30.10 in Hg
Humidity:58 %Altitude:296.0 ft
Vital Zone Radius:5.0 in
Std. Atmosphere at Altitude:NoPressure is Corrected:Yes
Zero at Max. Point Blank Range:YesTarget Relative Drops:No
Mark Sound Barrier Crossing:NoInclude Extra Rows:No
Column 1 Units:1.00 inColumn 2 Units:1.00 MOA
Round Output to Whole Numbers:No
Output Data
Elevation:9.678 MOAWindage:0.000 MOA
Atmospheric Density:0.07709 lb/ft³Speed of Sound:1107.8 ft/s
Maximum PBR:315 ydMaximum PBR Zero:272 yd
Range of Maximum Height:159 ydEnergy at Maximum PBR:718.5 ft•lbs
Sectional Density:0.186 lb/in²


Calculated Table
RangeDropDropWindageWindageVelocityMachEnergyTimeLeadLead
(yd)(in)(MOA)(in)(MOA)(ft/s)(none)(ft•lbs)(s)(in)(MOA)
0-3.0***0.0***2658.12.3991568.60.0000.0***
501.42.70.30.62509.02.2651397.60.05810.219.5
1004.54.31.21.12364.82.1351241.50.12021.120.1
1506.03.82.81.82225.32.0091099.40.18532.620.7
2005.72.75.12.42090.51.887970.20.25544.821.4
2503.41.38.23.11960.41.770853.30.32957.922.1
300-1.1-0.312.23.91835.51.657747.90.40871.822.8
350-8.2-2.217.14.71716.01.549653.70.49286.623.6
400-18.2-4.423.15.51602.51.447570.10.583102.624.5

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Further comment to my post above. From a quality standpoint, excepting the bullet expansion issue, the chrono results and accuracy, sub MOA, demonstrates Hornady is really producing a fine product. This level or consistency in chrono results is approaching long range match quality.

So don't think I'm bashing Hornady with my comments about the CX 100 terminal performance. If they can fix the bullet expansion issue, I wouldn't hesitate using it and be very happy doing so.


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Discussion Starter · #605 ·
I think just going back to the softer GMX tip would return the CX to better performance. I'm trying to test with a GMX tip installed but 1) its difficult to get the harder CX tip and stem completely out. I've been able to get a tip completely out from one of the .308 150gr CX bullets but 2) I cannot get the GMX tip to fit into the CX expansion cavity. The opening to the CX's expansion cavity is 0.084" and I cannot get the GMX tip to fit which is a 0.088" diameter. The harder CX tip has a larger stem diameter of 0.090" but is tapered. The tight, compressed fit of the CX tip is leading me to believe that the crimp, or finishing mark as Hornady calls it, is not the primary cause of the lack of expansion in the 6.8mm 100gr CX bullet.
 
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I think just going back to the softer GMX tip would return the CX to better performance.
You are probably right. I am just curious to see if a softer tip would further reduce the velocity needed for expansion. I remember stories of hunters who would fill "hollow points" of bullets with vasoline to increase expansion. Hornady, for example, has been using the soft FTX polymer in a fair amount of pistol and subsonic ammo for rifles.
 
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