I always made my 7TCU from R-P .222 mag brass. I didn't like .223 that always formed short 7TCU.

I have some 222 Rem Mag brass and also 204 Ruger brass and may use that later. I did make some TCU for my TC with that brass and it formed OK but I haven't shot it enough to know how long it would last. That will be next if my next experiment if the newer FC brass doesn't pan out.

A little information for you on current production FC brass. There are basically 2 different types that you will come across. The difference being the thickness of the web. The thinner web FC brass is the one that people have problems with primer pockets because they lack the strength to hold up. These also can show more case head expansion. I have a piece of steel rod that will fit inside of a 223 case neck and this rod is slightly longer than the 223 case. I use this as a "standard" to zero my caliper. I then slide the rod all the way into a case then use the caliper to measure from the case head to the end of the steel rod. The resulting measurement reflects the case web thickness. I only use the cases with the thicker web. Some people say you can tell the two cases apart by the size of the letters on the headstamp. I have not found this method to be accurate. In fact to be really accurate in this measurement you need to be sure there are no burrs remaining inside the case from punching the primer flash hole. Richard Pickering alluded to exploring case webs in his earlier post.

All of my 7mmTCU cases come from 223 brass.
I have had splitting problems with Winchester cases and also found that some Federal cases seemed to be weak in the neck area and would deform slightly when seating the bullet, this prevented chambering.
I use RP brass exclusively now.

I should probably add that has been since 1985 and my newest brass is probably ~1998 vintage.