Essentially the same thing, same case; 223 Remington, necked up to 6MM. Possibly minor differences in neck length or shoulder angle after fire forming.
I was looking for a 6-TCU in the early '80s and couldn't find one and ended up with a 6 X 47. Longer neck and slightly larger powder capacity. Still got it and use it on varmints and game up to Mule deer size. It is a Contender 14" heavy barrel, custom built by a long gone gunsmith.