Protoolman,
1340-fps with a 320-grain hard-cast bullet from a 4 5/8 inch Blackhawk is patently unsafe!!! As commercial ammo builders, we are frequently running into handloaders who push the envelope excessively, and this is a good example of it! The Blackhawk should absolutely not be pushed beyond 30,000-psi, and there quite simply is no combination of gunpowder, bullet, seating depth, crimp, primer and case that can come anywhere near that performance level from a short barreled 45 Colt and not exceed 30,000-psi by a large margin. Also, the practice of substituting components is dangerous and should not be indulged where maximum loads are concerned! It is true that cast bullets will very often give higher velocity than jacketed bullets of the same weight (but as you admit Protoolman, you are substituting a 320-grainer for a lighter bullet), however, pressure testing does not support the notion that cast bullets give higher velocity without increasing pressure. It is commonly believed that they will, but they don't. That's why pressure testing equipment is required before people start "improving" upon published reloading data. Indeed, although cast bullets will generally give higher velocity with a given powder charge, pressure testing clearly indicates that they also give higher, often much higher, chamber pressure.
Handloaders be warned, loads such as the one just described are not safe! We are fully conversant with those that build the various six-shooters, those that write the reloading manuals, those that do pressure testing professionally, and those that write about such practices in the media, and none of them endorse the kind of load just discussed. If you want more power than you can get with a given caliber and/or handgun, get a different gun, but don't tempt fate by exceeding reasonable loading practices!!!
Best regards, Randy Garrett
www.garrettcartridges.com