I might as well include my (so far limited) experience with my 6CM...
My Tube gun build has been a bit of a star-crossed experience so far (got sent a 6mm Competition reamer, a tight-necked 6-6.5x47 Lapua with a 40° shoulder instead of a 6mm Competition Match, the bolt handle I had installed wouldn't run in the tube gun slot) but the gun is shooting great, after all of that.
I ended up with a .237" bore (this seems to be very important--the .236s don't, for some reason, play as nicely) Krieger 1:7.5 twist, the short-ish throat reamer that Joe uses, and a 700 action in an Eliseo stock. The lot of Nosler brass I had was really heavy and really thick, and even with formed brass, I couldn't get more than 49.0gr of H1000 in it. (I think Whitley posted a picture here or elsewhere of formed brass and H1000 filling up into the neck--that was probably Nosler brass. It seems to be very short on capacity) In fact, the same 49.0 grains of H1000 in un-formed Win brass doesn't come up as high in the case as the same load does in formed Nosler brass.
Anyway, how'd it shoot? With the formed Nosler brass, I shot some 105 hybrids into the lands (on account of not having the capacity to seat them any deeper with any degree of consistency) at a 600 yard match, and after dropping three points in a bit of sorting out a slightly slanted firing point, the gun shot very well: 197-9, 200-14, 200-13 (97-3, 100-6; 100-9, 100-5; 100-7, 100-6) I didn't chrony the load, though.
With un-formed Win brass:
49.0 grains H1000 (thrown charges), BR2, 70gr Berger FB (I think they're discontinued, I got 1500 of them a while back for about $9/100) and using a false-shoulder so I could shoot them for rapid-sitting, the cases formed up nicely (same headspace measurement at .400" as the heavier bullet loads, but the corners were still a little rounded in the shoulders) They shot very well--but it's the 200 yard target, so nearly everything shoots well there.
With un-formed Win brass:
48.0 grains H1000 (thrown charges), BR2, 108 Berger, again with a false shoulder (necked up to 6.5, then run back through the sizer) They fed fine for 300, and while I ended up with a 200, I couldn't buy an X for a 200-8--I'm chalking that one up to being new to the bolt gun in the rapids. At 600, however, I had a 200-14, (9X on the front, 5X on the back--and if I hadn't been a chicken with the windage knob, I probably could have turned four *tight* 10s at 9 into Xs on the back 10) I also haven't run that one over the chrony, but whereas I'm curious, I don't really care what it says. That load shoots, and I wouldn't be worried about shooting it in a match anywhere.
So, with a false shoulder, fireforming loads work very well fed from the magazine.