Which is better to make into an optimum round?

[Evil Monkey]

The 5.56x45mm is flat shooting, accurate, has low recoil. But it's lacking against certain barriers, looses alot of energy from short barrels, can't always rely on fragmentation.

The 7.62x39mm is great for punching through certain barriers, isn't affected by short barrels in comparison to 5.56mm, has more energy. However, it's trajectory may be frowned upon.

So which round has more room for improvement? Necking up the 5.56mm to a heavier 6.xx or necking down a 7.62x39 to a lighter 6.xx?

 

[jerkface11]

Just try 250 savage.

 

[MutinousDoug]

I'd go with the one that leaves you with the most powder capacity after you have modified (i.e. wildcatted) it.
5.56mm being pretty much maxed out at maybe .24 cal (6mm), I don't know anything about the potential ballistics of the 7.62x39 but I think you can put more powder in there.

 

[elmerfudd]

Load the 7.62x39 with soft points and you'd have a fantastic cartridge for combat within 300 yards. Beyond that it would be at a real disadvantage, but still could be used out to around 500 yards.

 

[R.W.Dale]

Some of the most accurate rifles in the world are chambered for two cartridges based on 7.62x39, 220russian and 6mm PPC both of which offer truly impressive preformance

I think that the 7.62x39 could become an even better cartridge is a modern high pressure version were standardized.

30-30 Winchester 42,000 PSI

7.62 x 39 45,000 PSI

270 Winchester 65,000 PSI

As you can see the little Russian round is somewhat downloaded compared to what modern sporting and military rifles can handle. This is just speculation, BUT I think an extra 300 FPS worth of muzzle velocity (2400fps to 2700) could be achieved within a 65,000PSI pressure limit. Couple that with a slippery bullet and you have a cartrige that is comparable to many of the new BOUTIQUE AR-15 rounds.

My pet handload launching a 125grn Nosler ballistic tip at 2500 fps is surprisingly flat shooting and hard hitting.

 

[rbernie]

So which round has more room for improvement? Necking up the 5.56mm to a heavier 6.xx or necking down a 7.62x39 to a lighter 6.xx?

The answer can be seen by looking at the battle betwen the 6.8SPC and the 6.5 Grendel.

The Grendel, based heavily on the 7.62x39 case, uses a short OAL for the brass (39mm) to facilitate the use of bullets with a long ogive - giving it good BCs and excellent retained energy downrange. It's a bit fatter, as well, to buy back some of the case capacity lost to the shorter length. It's shape is reported to cause occasional feed issues in certain semiauto weapons, and it clearly 'thins' the bolt face of the AR15 bolt.

The 6.8SPC, based on the 30 Remington case, uses a 43mm case length. It requires bullets with a blunter, less aero shape to 'em to fit inside the OAL imposed by the AR15 magazine well. It does not retain energy as well downrange as a result, but it reportedly (and in my experience) is quite reliable in terms of feeding and it's smaller case diameter takes less 'meat' out of a standard AR15 bolt face - potentially giving the bolt a longer life.

So which is better?

Depends on what you want to do.