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Old 01-15-2005, 12:03 PM   #1
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^.6X43mm Cartridge Performance VS 5.56mm Performance

6.8X43mm Cartridge Performance vs 5.56MM Performance

It is highly probable that the 6.8X43mm (.276 Caliber) SOCOM cartridge will replace the .223 (5.56MM) as the standard service rifle cartridge. Possubly in a new rifle or perhaps in rebuilt M16s/M4s.
.
The Army's objectives in designing the new cartridge are simple to state but difficult to achieve. They are to:
o Achieve a major increase in combat effectiveness.
o Require minimum modifications to existing weapons.
o Have a minimum impact on training and logistics.
To achieve the improved combat effectiveness, the Army decided a larger caliber bullet was required, using a larger cartridge case which would still fit in an M16 magazine and feed properly through the M16 action. Remington and Winchester participated in the program and produced the test cartridges.
The new cartridge case was based on the existing commercial .30 Remington case. This meant installing a new barrel if converting an M16 rifle.. The largest caliber bullet which could have been used with the modified .30 Remington case was the 7.62mm (.308). Several other calibers were tested using the new case. These included 6mm, 6.5mm, 6.8mm, 7mm and 7.62mm bullets.
The 6.8mm was selected because it demonstrated the best combination of reliable functioning, accuracy, penetration and terminal ballistic performance. 6.8mm is the actual bullet diameter used in the popular .270 Winchester rifle cartridge, so a wide variety of bullets was immediately available for testing.
The new cartridge had to be used in existing M16 magazines and feed into the rifle's chamber properly. This limited the length of the 6.8mm cartridge to being no longer than the .223 cartridge it would replace. The 43mm case length was selected because it functioned better than all other lengths tested. The selected cartridge is designated the 6.8X43mm.
Several 6.8mm bullet weights, ranging from 90 to 140 grains, were tested. The 115 grain bullet was selected because it gave the best performance for the planned combat role. Special purpose bullets of other weights can be used for particular combat tasks.
Comparative Energy of the 5.56mm vs 6.8mm in Foot Pounds
cartridge 5.56X45mm 6.8X43mm

muzzle energy 1158 1793 Foot pounds
100 yards 928 1467
200 yards 753 1189
300 yards 573 954
400 yards 442 759
500 yards 338 600
600 yards 258 475

The 6.8mm clearly outperforms the 5.56mm, delivering superior striking energy at all practical combat ranges.
 
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Old 01-15-2005, 12:29 PM   #2
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7.62 also outperformes it all all practical combat ranges too, right?

Any idea how easy it would be to get it subsonic?
 
Old 01-15-2005, 02:52 PM   #3
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Any thoughts as to the rifling twist recommended for 6.8mm barrels with standard/milspec loadings?I'm thinking that to go sub-sonic ,at the barrel/bullet end of things, a heavy for caliber bullet(long+extra sectional density),and a moderately increased rifling twist form the super-sonic standard COULD be a possible 'transit system"...as to powder charges,since I'm clueless on the current loading specs;i'd guess something faster burning,but metered to prevent dangerous pressure spikes or detonation MIGHT be ther formula.Who's the powder gurus on this board?
 
Old 01-15-2005, 03:46 PM   #4
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Ever hear of the 7mm Waters? A necked down 30/30? It is funny that they are basicly going to a rimless necked down 30/30. Nothing really new under the sun.
 
Old 01-15-2005, 05:04 PM   #5
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Djetace,was the 7mmWaters also known as the 7-30 Waters?my brain is skipping gears at the moment...must be theleftover Christmas cookies
 
Old 01-15-2005, 06:11 PM   #6
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Yes I think it is the 7-30 Waters, gipper! Used in Wincheser 94s.
 
Old 01-16-2005, 12:24 AM   #7
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Thanks Hard Ball. That was a very good, informative post.

+one ...
 
Old 01-16-2005, 12:32 AM   #8
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I think faster burning powders on a M16 style rifle would be desired, for two reasons: (1) to limit the occasion of gas tube fouling with unburned powder, and (2) to reduce muzzle flash signature.
 
Old 01-16-2005, 05:58 AM   #9
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Here is some 6.8X43mm reloading data

RIFLE DATA from Hogdon

NEVER Exceed the Loads Listed Here

BULLET POWDER GR VEL PRESSURE
---------------------------------------------------------


6.8 MM REMINGTON SPC
CASE: REMINGTON

BBL: 24"
PR: REMINGTON 9 1/2
TWIST: 1:10"
TRIM: 1.676"

90 GR. SPR HP (Starting Loads)
DIA. .277
COL: 2.255"
H335 30.0 2699
37,600 PSI
Benchmark 29.0 2705
38,900 PSI
H322 29.0 2765
42,100 PSI
H4198 26.0 2810
47,700 PSI
Lil'Gun 20.0 2784
46,500 PSI

90 GR. SPR HP (Maximum Loads)
DIA. .277
COL: 2.255"
H335 33.0 2987
51,900 PSI
Benchmark 31.0C 2853
45,900 PSI
H322 31.0C 2929
49,500 PSI
H4198 28.6C 3012
54,000 PSI
Lil'Gun 22.7 2923
53,500 PSI

100 GR. BAR X (Starting Loads)
DIA. .277
COL: 2.260"
Benchmark 28.0 2588
42,000 PSI
H322 27.0 2502
37,700 PSI
H4198 24.0 2542
40,500 PSI

100 GR. BAR X (Maximum Loads)
DIA. .277
COL: 2.260"
Benchmark 30.0C 2755
51,200 PSI
H322 30.0C 2803
52,300 PSI
H4198 26.5 2791
52,900 PSI

110 GR. HDY V-MAX (Starting Loads)
DIA. .277
COL: 2.260"
H335 27.0 2487
41,700 PSI
H4895 27.0 2335
33,900 PSI
Benchmark 27.0 2518
44,000 PSI
H322 27.0 2571
47,600 PSI
H4198 23.0 2431
40,600 PSI

110 GR. HDY V-MAX (Maximum Loads)
DIA. .277
COL: 2.260"
H335 29.5 2694
53,600 PSI
H4895 29.0C 2550
42,400 PSI
Benchmark 28.8C 2665
52,000 PSI
H322 29.0C 2697
53,600 PSI
H4198 25.3C 2651
52,000 PSI

115 GR. SIE HPBT (Starting Loads)
DIA. .277
COL: 2.260"
H335 27.0 2415
40,800 PSI
H4895 27.0C 2302
34,300 PSI
Benchmark 26.0 2400
42,400 PSI
H322 26.0 2421
43,500 PSI
H4198 22.0 2413
47,800 PSI

115 GR. SIE HPBT (Maximum Loads)
DIA. .277
COL: 2.260"
H335 29.0 2569
48,500 PSI
H4895 29.0C 2495
42,800 PSI
Benchmark 28.5C 2581
51,800 PSI
H322 28.2C 2608
53,300 PSI
H4198 24.0C 2534
52,300 PSI

C = Compressed Charge
 
Old 01-16-2005, 11:08 AM   #10
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While we're on this subject, if the 6.8 SPC was designed to replace the 5.56 with "minimum" modifications, why doesn't the 6.8 have a rebated rim, so that it can use the same bolt as the 5.56?
 
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