And how did that turn out? Obviously, you're still alive to talk about it! 
Most of the specs I've seen, like Hornady's proposed .300BLK ammo for 2012, have ca. 210/220 gr for subsonic, and half that for supersonic. Kinda curious as to how something in between the 2 limits would work in supersonic.
It really was a sweet shooter.
No recoil, no noise (sub-sonic with a 'can'), and accurate to minute of beer can.

I don't remember the brand name of the powder he used, or how many grains (sorry, I may have posted it in the original thread I had about shooting it, so I'll try and find it).
**EDIT: found the thread and I was mistaken on bullet weight, they were 170gr (possibly either Berger or Lapua).http://www.downrange.tv/forum/index.php?topic=17024.0It was subsonic out of a 16.5" barrel the 170gr bullets bounced at about 250-300 yards and was pellet gun quiet through a suppressor.
As soon as we both get free time at the same time, I am going over and we plan to do more in-depth ballistics testing. From what he has posted on here, Bafsu has way, way more trigger time with it than me....and in several guns.