The new 6.8 seems to be all the rage with its increased capability over .223

But why develop a new cartridge when we have the .308 and 7.62x39 (AK)?

The big advantage the 7.62 has is that it is readily available and lots of cheap plinking abounds.  Wouldnt it make more sense to develop and 7.62 AR rather than a 6.8?

I'm certainly not an expert on this and I'm not trying to pick a fight just looking for some knowledge.

for me, i think the best round is the 6.5 Grendel.  The .308 is good for long range, but for short distances, like CQB, the .223/5.56 is better because it is more controllable.  The .308 is also heavier and bigger, which means less ammo you can carry.  The military should just switch from using the 62 grain to either the MK262 Mod 1 or the 5.56 75 Grain TAP as the primary ammunition.  Just my 2 cents though.  

6.8 could be made to a similar overall length compared to 5.56.  that kicks .308 out of the question.

6.8 has much better terminally performing bullets and greater range than 7.62x39

sounds like good reasons to me.. thanks!

One of the key points is the usability of existing platforms (lowers) and the reliability of function for the operator.  The 6.5 grendel is the ballistically superior round, but function is questioned.  I would personally choose the Grendel, but have had some discussions with knowledgable people (Dave Embry - Hornady, and others)  which lead me to believe that the 6.8 may soon be more readily available.  Additionally,  controling their development for a price by Alexander Arms will in the long run hurt them IMHO.  If reliable mags were available for feeding the 7.62 X 39, that round would be the first World Ammo for all nations... a true first!

7.62 is not known for its terminal or external ballistics. 6.8 is the best combination of those two that the developers of the 6.8 could fit into an AR platform.

7.62x39mm and 7.62 NATO are both outdated and relatively inefficient compared to what can be designed now.  Even in the Armalite design, the AR-10 came first in 7.62 NATO.  But, the AR-10 is bigger and heavier than the AR-15 and fires bigger, heavier and harder-recoiling cartridge, and nothing is going to change that.  These are the reasons why we moved away from the M14, so I don't see any point in going back there again.

The idea behind 6.8 SPC and 6.5 Grendel was to design the largest and most powerful cartridge that could fit into the existing AR-15 platform.  6.8 SPC is a rather old-fashioned looking cartridge: to me it looks like a 7.62 scaled down.  Grendel goes for a longer, more slender bullet, which seems more in line with modern thoughts on cartridge design.

I would like to see the 6.5 Grendel become a success. . .  Unless somebody finally makes caseless ammo work.

Tony, while it is true that more efficient cartridges can be made than 7.62 NATO and 6.8 SPC, those cartridges are more suited to a competition environment than to war. The tapered case of the military cartridges aids extraction, and the dimensions are compatible with belt feed systems, whereas 6.5mm Grendel will probably not work well in belt fed weapons.

Call me old fashion but I still love the Little 5.56mm/223

IM NOT SAY ONES BETTER THAN THE OTHER

I guess you could neck down the 7.62 short and add a  long and slim 100gr bullet ?
and have close the velocity as the 6.8

The Enhanced Rifle Cartridge (ERC) project's goal was to provide optimum terminal performance from an M4 with minimal changes to the weapon. The SPC was designed from the ground up to provide increased energy, barrier penetration, and incapacitation from the Mk12 SPR, from contact distance to 500 meters.

-z

Why not?

Quoted: 7.62x39mm and 7.62 NATO are both outdated and relatively inefficient compared to what can be designed now.

Out dated and inefficient   ??  so much so that the US is using the 7.62 nato  in several rifles and killing bad guys everyday with it.  

"The big advantage the 7.62 has is that it is readily available and lots of cheap plinking abounds. Wouldnt it make more sense to develop and 7.62 AR rather than a 6.8?"

Can you say, "AR10"?

As stated above, the 7.62 NATO cartridge, is too long to fit in an AR15.