If you don't mind reloading, the 6mmX45 conversion is just a barrel swap. It uses 223 brass necked up to 6mm. My father in-law has the 50 beowulf upper and likes it. Pretty much if the round will fit in the magazine and there is a compatable bolt for it, someone has made an upper in that caliber. They're like Lego guns in that its easy to change out parts.