I believe the 340 gr will fit in the Blackhawk, with little room to spare. I went to Great Northern Guns to buy some for my anniversary SBH, and the salesman steered me to the Cor-Bon 320. He claimed to have tested both rounds through his SBH and settled on the shorter 320, prefering the slightly lighter recoil and extra room to allow for crimp jump.
I fired several of the 320s one and two handed from the 6.5" barrel, and found it stout but not unpleasant. A shorter barreled 'hawk might come back and smite you between the eyes though, if you tried to one-hand it.
I checked BB's website, and I stand corrected, they state the ammo can be fired through the super blackhawk. That said, looking at their claimed performance of 340 gr @ 1478 fps out of a 7 1/2" redhawk has me scratching my head. Published max loads for the 44 mag run 300 gr @ 1300 fps from a 7 1/2" revolver. Running nearly 15% heavier bullets nearly 200 fps faster is a serious increase!!! Recoil asside, that's really straining things in a 6 shooter super blackhawk.
Gotta make those .454 Casull guys sweat a little. Here is Buffalo Bore's hottest offering in that caliber:
360 gr. L.W.N.G.C. (1425 fps / M.E. 1623 ft. lbs.)
(Big game up to 2000 lbs.)
To be fair, they offer this bit of wisdom:
The .454 Casull is one cartridge that BBA does not load to it's full pressure limit. When loaded to it's full pressure potential the .454 Casull can become unreliable and unpleasant to shoot. At upper pressure levels, heavy bullets tend to jump crimp - tying up the cylinder. (wouldn't this be great in an emergency) Also, fired brass can stick in the chambers. BBA feels that a 360 gr. bullet (or 325 gr. bullet at 1525 fps) at 1425 fps is still very powerful, comfortable to shoot, and TOTALLY RELIABLE. At these levels, these bullets give up nothing in killing performance.
I am sure some of you .454 guys who roll your own will disagree with BBA on this.
we just tried that load in a super blackhawk this weekend. It is too long. With some fumbling and tilting of the gun it will fire, but it did hang up. Crimp jump was the suspected culprit as the first 2 rounds went off with out a hitch.