Has anybody taken the 5.6x50 magnum case and necked it up to 6mm and kept the rest of the dimensions the same as the 6mm tcu except .2" longer? My thoughts are that it would surpass the 6 tcu in velocities, but with the small case head it would be very friendly to the contender frame with minimal backthrust.

The 6mm tcu is very efficient, would this case actually improve upon 6 tcu performance or is the 6tcu already optimum for contender length barrels?

I currently have a 16" 223 and 14" 7x30 and I'm looking for something that falls in the middle for performance and I'm intrigued by the inherent accuracy of the TCU.

Yes, but they usually improve it while they are at it.
5.6x50R Imp, 6x50R Imp, and 6.5x50R Imp.
Typically, for the contender one prefers the rimmed version.
It outruns the TCU's by a good margin.

Building a 6.5x50R at the moment, I also have the 6.5TCU. As soon as I get things rolling I'll be able really get a feel for what these 2 cals can do.

I have two hints to give you on this cartridge.

The first is a suggestion to give the gunsmith making it for you. Have 'em cut the neck tight and then neck turn the brass. Even without neck turning, the neck of the 5.6X50R brass will get worked quite a bit, as it thins out when expanded up to 6mm plus. Also, the tough RWS brass should be annealed before necking it up - this sometimes makes the brass thickness uneven and will cause some bullet runout. Bellm did me a 6.5X50R a few years back, and I wish I had known to ask for a tight neck.

Also the 6TCU or 7TCU may be a better choices because I believe you can get Redding custom bushing dies for the 6 and 7 but not the 6.5. Check on this because on the Hirtenberger brass the necks were too thin, and when sized they did not grip the bullet. A bushing dies will allow you to adjust to the brass.

The 50R wildcats are great performers for the Contender, but you will be happiest with it by following these suggestions.

Thanks for the replies, can anybody give me any idea as to how much of a velocity increase I could expect out of the 6x50 AI rimless?

Also would it be an advantage to use the rimmed version of the cartridge?