My friend built a 700 action and Hart barrel w/ brake in a 7mm ultra mag. It's a nail driver. Not sure what the stock is. Shoots one ragged hole at 100 yards. Took him almost a year to get the load right though. I can't really kick him too much, took me 5 months to get the right load for my LRH.
I did some work on a 7mm hart that bobby hart built. If its the one your talking about it is basically just a 7mm rem mag ackley improved. It uses 7 rem mag brass and fire form like any other ackley wildcat. He built it around and recommended the 140 nosler btip with 74 gr of rl22 at around 3450+. I also worked up a load with the berger 180vld at 3100+with h1000. Both of these loads were sub 1/4 Moa. I still have the reloading dies I was actually thinkin of building myself one of these for my light weight carry rifle.