stancrist said:

I can't help but wonder if it may be possible to have one caliber that combines the advantages of both SCHV and full-power ammo into a single "have your cake and eat it" round.

yes, combining all advantages... and disadvantages. TANSTAAFL

stancrist said:

P.S. The US Army was three decades ahead of the Russians in fielding a 7.62 machine gun at the squad level.

back in around 1964 Soviet experts did a comparison of the new Soviet squad and platoon TOE (AKM/RPK) vs NATO (German, US and French squads and platoons)

Following results were obtained: It was found that at ranges of up to 400 m Soviet infantry had a distinctive edge over their NATO adversaries. On average, for units of comparable size, Soviet troops with AKM and RPK were expected to have a 20% higher hit rate than U.S. troops armed with M14 rifles and M60 machine guns, and a 100% higher rate than French troops with M1949/54 rifles and M1952 machine guns. At extended ranges, however, the tables turned in favor of NATO troops, mostly due to the larger number of rifle caliber machine guns per platoon and larger unit sizes.  At 800 m, U.S. troops were expected to perform approximately twice as effectively as Soviet troops.

However, this calculation did not include such important squad and platoon assets as the APC (BTR-60 or BMP-1 and their western counterparts)

mpopenker said:

yes, combining all advantages... and disadvantages. TANSTAAFL

Except the main disadvantages of full-power ammo -- weight, bulk, and recoil -- would be much less with 6mm SAW, while still having the range advantage.

mpopenker said:

back in around 1964 Soviet experts did a comparison of the new Soviet squad and platoon TOE (AKM/RPK) vs NATO (German, US and French squads and platoons)

Following results were obtained: It was found that at ranges of up to 400 m Soviet infantry had a distinctive edge over their NATO adversaries. On average, for units of comparable size, Soviet troops with AKM and RPK were expected to have a 20% higher hit rate than U.S. troops armed with M14 rifles and M60 machine guns... At extended ranges, however, the tables turned in favor of NATO troops, mostly due to the larger number of rifle caliber machine guns per platoon and larger unit sizes.  At 800 m, U.S. troops were expected to perform approximately twice as effectively as Soviet troops.

That seems reasonable.  I wonder if the results would be similar with the current 5.45x39/7.62x54 mix compared to 6.8x51 NGSW?

6mm Lee-Navy shall be vindicated! Hybrid cases at high pressure, let's go with a 120-125 grain boat tail instead of a 135 round nose.

stancrist said:

I can't help but wonder if it may be possible to have one caliber that combines the advantages of both SCHV and full-power ammo into a single "have your cake and eat it" round.

The 6.5x43 mm could be an option. Weight neutral compared with the 5.56 mm, but 7 magazines will carry 175 rds instead of 210 rds. The plus side is that each round will deliver +60% of muzzle energy, and +100% at 500 m.

The 260 Remington using "high capacity" steel case and firing a 95 gr class bullet is another option (7.1 g case + 6.2 g bullet + 3.3 g powder + 0.4 g primer = 17 g cartridge), but only 140 rds available instead of 210 rds, but this combination seems very effective.

EmericD said:

The 6.5x43 mm could be an option. Weight neutral compared with the 5.56 mm...

The 260 Remington using "high capacity" steel case and firing a 95 gr class bullet is another option...

Unfortunately, while 6.5x43 and .260 REM keep weight down, both still have the bulk disadvantage of 7.62 NATO/6.8 NGSW; magazines are just as tall and MG belts just as long.

The only way to get reasonably close to having the whole assault rifle "cake" is if case diameter is no bigger than that of .224 Valkyrie, and preferably smaller (a la 6x45mm SAW).

Below, l. to r. -- .224 Valkyrie / 6x45 SAW / 5.56x45

Farmplinker said:

6mm Lee-Navy shall be vindicated! Hybrid cases at high pressure, let's go with a 120-125 grain boat tail instead of a 135 round nose.

LOL.  That brings back an old, long forgotten memory. 

Way back when I was thinking up my 6mm Optimum, my first effort at making a dummy round was to take a 6mm Navy case, shorten the neck and stuff a 6mm SAW 105gr FMJ bullet in it.

The result was virtually identical to the Russian 6x49 Unified cartridge (below, middle).

stancrist said:

The only way to get reasonably close to having the whole assault rifle "cake" is if case diameter is no bigger than that of .224 Valkyrie, and preferably smaller (a la 6x45mm SAW).

So, your only option is a +P+ 224 Valkyrie in SIG hybrid case, which may be not effective enough in the support role.

The Russian 6x49 mm is an interesting round, credited with a MV of 1150 m/s from a 72 cm / 28.3" barrel.

At first glance and using the case capacity of the .25 Savage, you need 73,000 psi to push your 5 g / 6.2 mm bullet at 1150 m/s / 3770 fps.

With the same pressure and a 13.5" barrel to keep the rifle length / weight around the M4 form factor, the predicted velocity is still around 1 km/s. The bullet BC is already OK with a C7 of 0.197.

Very interesting.  Thanks, Emeric.