Guys,
There are a ton of take-off remington barrels available in .243 and the chamber end is large enough to machine down to AK specs.  That would get you into an accurate cartridge that boxer brass is available for.  I have seen balistics for the 6 PPC upward of 4000fps.  It is nearly the same as the 5.45x39.  Comments?

The rim of the 5.45x39mm is .393",chamber pressure ?but the 222 Remington has a 50,000psi max chamber pressure launching a 52 grain bullet at 3100fps. Which is what the military load does in the 5.45x39mm(54 grain bullet at 3084.14fps).
The rim on the 6mmPPC is .443" and the chamber pressure is 60,000psi.
The rim on the 7.62x39mm is .443" and the chamber max chamber pressure is 45,000psi.  
The  rim on the 6mm Remington is .473" and max chamber pressure is 65,000psi. The empty 6mm Remington case is .003" longer than a loaded 5.45x39mm or 7.62x39mm round.
The rim on the 223 Remington is .378" and max chamber pressure is 55,000psi.

So as you can see the 6mmPPC cartridge would have to be downloaded but is the only one that would not require lathe work on the bolt face of a 7.62x39mm rifle. The downloding would work out fine as the less powder the more room to seat heavier bullets and still fit them in the magazine.
While a cheap take off barrel sounds good the lathe work is going to cost some bucks as besides chambering the AK barrel has 6 steps in it. So the barrel will have to be turned to 6 different diameters.
www.k-var.com/product.asp?0=216&1=219&3=143

You would probably be better off buying a custom die set in .243/5.45x39 and rolling your own.

This would solve the "mystery" high capacity magazine problem that the guys that want to build 6.5 Grendel AK are having as the 6mm PPC and the Grendel have all most the same case taper IIRC.

Quoted: You would probably be better off buying a custom die set in .243/5.45x39 and rolling your own. This would solve the "mystery" high capacity magazine problem that the guys that want to build 6.5 Grendel AK are having as the 6mm PPC and the Grendel have all most the same case taper IIRC.

Could you explain this a little more?

Quoted: Quoted: You would probably be better off buying a custom die set in .243/5.45x39 and rolling your own. This would solve the "mystery" high capacity magazine problem that the guys that want to build 6.5 Grendel AK are having as the 6mm PPC and the Grendel have all most the same case taper IIRC. Could you explain this a little more?

The 6.5 grendel is based on the 6mm PPC, the 6mm PPC is based on the same Russian case that the 7.62x39 was made from.
So they all can use the same bolt.

The problem with many modern rounds including the 6mm PPC is that they often have higher chamber pressures than used in the AK. The AK is 45,000 and the PPC is 50,000+ IIRC
That could cause the bolt or trunnion lugs to sheer off and that could be Very Bad for the shooter.

On re-reading your post it might be better just necking down the 7.62 case.
Someone may already make this die set()

Here at Guno.net is a 22 page thread by guys discussing how to convert AK's to 6.5 Grendel and possibly fix the magazine problem.

So how would the chamber get made for this kind of wildcat of a 7.62 case necked down to 6.5mm?

A custom rifle maker can do it easy enough (for him).
I don't know any so i can't give you a recommendation.

For a 6.5x39mm you would have to have a custom chamber reamer and headspace gauges(go and no-go) made($260). www.mansonreamers.com/pricelist.htm  
Then use the reamer to cut the chamber and headspace gauges to set the headspace. www.ch4d.com/ makes 6.5x39mm reloading dies for $75. Also you will need at least one case forming die for $36 to get the 7.62x39mm case neck down to 6.5.