A friend told me that most of the primers of this Hornady "TAP" ammo (he didn't specify which, and I've yet to find it online) had popped when using his BCM midlength and gave me 30 rounds to test. I broke down 5 of them to get an idea of assembly consistency, then reassembled them to be foulers.





Cartridge Overall Length:
#1 - 2.203"
#2 - 2.206"
#3 - 2.202"
#4 - 2.204"
#5 - 2.198" *

*-used as a throat gauge for a suspected minimum .223 Remington chamber.


Headspace:
All at 1.457" +/-0.0005, within my margin of error measuring.

I think that's about -0.006" fit in my chambers.


Powder: ball type, maybe more flattened
#1 - 25.4gr
#2 - 25.2gr
#3 - 25.3gr
#4 - 25.3gr
#5 - 25.1gr

The spread should ideally be within 0.1gr. In reassembly, I set my powder measure to drop 25.3gr, but ended up 2.0gr short on #5.


Bullet Overall Length:
#1 - 0.833"
#2 - 0.827"
#3 - 0.831"
#4 - 0.832"
#5 - 0.831"

0.006" spread is decent to good for hollow point rifle bullets. Most of this is uniformity of the meplat. The better measurement of consistency would be base-to-ogive, but I don't have a good datum comparator.

JBM Ballistics lists the Hornady 55gr GMX at 0.832". The copper monolith is about 0.1" longer than most nontipped lead core 55gr bullets, and similar to the lead core 60gr HP and 62gr BTHP.


Bullet weight:
#1 - 55.1gr
#2 - 55.1gr
#3 - 55.1gr
#4 - 55.1gr
#5 - 55.0gr


Case + primer weight:
#1 - 95.2gr
#2 - 94.8gr
#3 - 96.6gr
#4 - 93.4gr
#5 - 96.0gr


Neck Tension: Unscientifically measured with a hand-swung inertial hammer bullet puller. No neck sealant present.
#1 - 10 whacks
#2 - 6 whacks
#3 - 15 whacks
#4 - 9 whacks
#5 - 8 whacks


Primer Comparison:
I decapped #1's primer for comparison with a Winchester SRP. The primer pocket was very snug and moderately crimped. The decapping pin pushed the anvil into the cup, deforming the cup into a dome shape.

The Winchesters are brass colored like that of the Hornady, but a little shinier, and seem thinner walled. The foil papers above the priming compunds seem to be a similar shade of green. I should've weighed, and measured the diameters and cup height.

Winchester left. "Hornady" right.


In trying to return the Hornady primer to it's case, the crimp and the dome shape conspired to ruin it. I removed it, lightly reamed the crimp, and replaced it with the Winchester. Hornady's primers are seated below flush with the base. My priming tool only reaches to just flush. Ideally, the anvil should touch the bottom of the pocket and just barely push closer to the cup, but I don't know if the Hornady's are closer to "crushed" or if my tool doesn't make the anvil "land". If "crushed", firing pin protrusion could be more likely to pierce the cup.

Replacement Winchester left. Untouched "Hornady" right.





Firearm:
Frankenbuild M4gery with a Core15 5.56 NATO 16" CL 1:7.7 barrel, PSA Premium bolt that loosely fits in a RRA semi auto carrier, standard carbine action spring with H3 buffer, freefloat quadrail, and 4.5lb 2-stage trigger. It is 0.3 MOA capable (mean radius), but I normally drive it at 0.5. No issues with stout loads like XM193, 75gr BTHP/24.5gr Varget or 25.5gr CFE223, and 77gr SMK/23.5gr H4895.



I used the above arrangement, except Prone and about 10ft forward.

I used a red dot for chrono calibration, fouling, and the first 10 record shots. The gun got a 20 minute break while I commanded the rapid fire portion of the HP match I was attending, and mounted the 3x scope. With the scope, I fired a couple shots to check zero, then fired 10 record shots. I forgot to save one for the water jug I had brought.


Visual Symptoms:
13/30 popped primers. Two more likely would have, except #1 had the primer replaced with a Winchester Small Rifle, and #5 had a 2gr lower charge. All had swipes. Few were visual; most were tactile.







Velocity:  
The .223 55gr GMX Superformance has an advertised muzzle velocity of 3275fps from a 24" barrel, so ought to be ~3000fps from a 16". Oddly the 5.56 version is advertised at 3130fps from a 20" barrel.

Past experience has shown my old Chrony F1 to read 100fps high, but it seemed to be accurate today.
Winchester M193 clone calibration: 3159, 3169, 3181fps.

I forgot to record the first 3 reassembled foulers. #4 was 3115fps and #5 with the 2gr reduced charge was 2916.


1st string: 3035, 3044, 3092, 3050, 3028, 3018, 3056, 3004, 3052, 3065fps
Avg: 3044fps
ES: 88fps
SD: 25fps

2nd string: 3071, 3076, 3022, 3074, 3011, 3011, 3045, 3017, 3078, 3023
Avg: 3043fps
ES: 67fps
SD: 29fps


Precision:
You can probably blame the high outliers and maybe a low on me, but those won't affect the mean radius much.

With red dot.



With 3x scope.



Conclusion:
Thin or brittle primers; barely suitable for .22 Hornet. Now I wonder if they're Remington 6 1/2s.

Powder has too long a pressure curve for shorter-than-rifle gas systems. Considering that even a 2gr reduced charge still swiped and wanted to pop, but the primers were otherwise round indicates to me that peak pressure is within spec, but port pressure is great and unlocking the bolt early. Seems like Superformance symptoms.

Average velocity is about right. Spread is decent for what seemed to be relatively inconsistent charges of ball powder and moderate mouth crimps.

Precision is similar to "M193". An HBAR may play better, but not likely to be better than 0.6 MOA.

Really fantastic ammo review. Only wish you had a known consistent sub MOA .223 Wylde type precision rifle with 10x or greater magnification for accuracy testing.

I know the regular non Superformance GMX I tried was really slow and no more accurate than M193. Seems like the loading itself could be the issue, or the GMX bullet variation could be the issue, as far as why the accuracy is sub par vs something like a Factory Barnes or Black Hills TSX loading.

Thanks for trying to find it.

Bullet diameter slipped my mind until I wrote this report.