Does anybody have experience with 6.8, do they use same mags or are they modified i.e. feed lips , followers ect..
why did it not work in Iraq

If you are referring to the 6.8 SPC the answers to your questions are:

Needs a new magazine, no it isn't the lips. The magazines for the 5.56X45 N are too narrow and the sides bulge outward after you load 4 or 5 6.8 SPC cartridges that the magazine will no longer fit into the magazine mortise of the AR lower.

I believe the militaries issue with this cartridge is one of logistics.

With a 24" tube you can get 2700 to 2750 fps with a 115gr HP if you are interested.

I have a Bushmaster w/16 inch barrel and muzzle break.Really fun to shoot no recoil or muzzle lift.You can fire at target and see where your round hit and continue fireing.As understand it most are going to Afghanistan.w/regards

The 6.5 Grendel beats the 6.5 SPC. The 6.5 bullet is more ballistic coefficient and even beats the 308. Remington has since reduced their velocity because it was found the 6.8 was loaded too hot.

Yes you need a different magazine and yes the barrel extension feed ramps are different, they are wider.

Joe

As the 6.8 SPC and the 6.5 Grendel are two rounds desiged for different applications I dont see how you can fairly compare them....Ar15.com head to the AR variants section and do some reading...its all broken down in a tact thread at the top of the variants section.