I asked this question years ago.

Using the .223 bolt, the only 6mm options would be the 6x47 or the 6mm Whisper.  If you are wiIlling to use the off the shelf 7.62 bolt, you can go with the 6mm PPC, or a .220 Russian or 7.62x39 necked to 6mm.   Problem with all of these in 6mm is heavy bullets will not seat ideally out of the magazine.  A 6PPC really needs 2.330 to maximize powder capacity with a Sierra 107 grain.

Only way to maybe squeeze a bit more with the 6x47 would be to use what were known as the LRP magazines that would seat to 2.460".  These are now sold by DPMS.

The .243 WSSM depicted is using light 6mm bullets, use a 107 grain Sierra and it will be hanging way out of the magazine.

After alot of research, I have been using the 6.5 PPC for 2.5 years. Here is why:

100-130 grain bullet weights.
2700 fps with a 128 grain bullet seated magazine length.  BC on the 128 is .560.  A 100 grain Nosler is just hitting 3000 fps.

As far as accuracy, .5 MOA is the norm and I shoot from 200-600 yards with it.   Sight adjustment from 200-600 yards is 10-12 clicks (1/4 MOA).  

For magazines, I use standard GI 20 round .223 magazines with minor tuning of the feed lips.

Other then using a 7.62x39 DPMS bolt and tuned magazines, the only thing different then a .223 Rem is the barrel.

Alexander Arms will be selling factory brass, custom magazines and uppers in the beginning of 2004.  They will be selling the 6.5 PPC under the name .26 Grendel.

To see my rifle, http://www.competitionshooting.com

The WSSM is a single stack magazine. Issues will be pressures on the WSSM are high (65,000 psi).   In addition, with light bullets, the rounds are magazine length.. Go with heavy longer bullets and they wont fit.

The .50 Beuwolf or the other one are not high pressure rounds (35,000 psi).  They do hold up well.

The WSSM's will not likely ever show up in a standard AR15 format.  The Olympic offering is the most promising as they are using both a larger diameter barrel in the chamber area and a bigger bolt/carrier/extension combination.  The 65k+ pressures of the WSSM's and the large case diameter will not computer to anything close to safe working pressures versus the tensile strength of any current barrel blanks in standard AR15 diameters. There just isn't enough meat left once the big hole for the case is reamed out. It's even an issue with 6mm PPC and those cases are smaller diameter than the WSSM's.