Hi Blusam;
Around a decade ago, due to a neck injury, I used a T/C Contender. At that time, there were 6mmTCU's, 7mmTCU's, and more rarely 6.5mmTCU's. I had the latter.
They are built upon a 223 Remington brass. The 6TCU uses the same bullets as a 243 Winchester. The 7TCU uses the common 7mm bullets. The 6.5mm bullets were scarce around my area, so I had to order them from MidwayUSA.
As my neck injury healed, I traded off the Contender towards the purchase of a 243 rifle.
My conclusion about the 6.5 TCU was that the brass was a slight aggravation to fireform, but other than that I didn't see anything phenomenally wonderful or phenomenally terrible about the cartridge.
Some fellows just feel a need to shoot something that's not common. Supposedly the 7TCU is the most popular of the TCU's... at least it was a decade ago.
Regards,
Jerry.