Just got this from Scarab Arms's Facebook page:
Photo I found from my PBE trip that I forgot to post. Don't know how many of you guys have seen these yet but this is one of the new One Two One rounds that Scarab Arms is producing for mag-fed markers. Will work in any marker that currently will shoot First Strike rounds. It uses a .50 cal ball encapsulated in a biodegradable foam shell and they come complete as you see them in this pic, no assembly required :-)
So, to sum it up, .50 caliber paintballs pre-loaded into a .68 caliber projectile.