Powderhombre
First I offer a CAVEAT: "Detonation" is a myth. Any propellant has X Jules of stored energy and cannot exceed that output. Light powder charges cannot of themselves destroy a gun.
There is however, a phenomenon called "Premature Shot Start." Insufficient gas pressure to propel the bullet through the Throat or down the barrel. The powder charge is just sufficient to "Start" the bullet out of the case, and then the bullet STOPS at the throat. The bullet becomes a Bore Obstruction. This confines the expanding gas to too small a volume and sends pressure through the roof. This can and will blow out a cylinder in some guns.
Each bullet reacts differently to a given amount of powder. The powder manufacturers pressure test extensively to arrive at the recommended load level. It may not at first glance, make sense to you, but care must be exercised when reducing loads or running beyond maximum. Don't get too wrapped up in the differences in recommendations. Also, powder formulations can and do change over time, changing the recommendations. Relax. Have fun.
You nailed it, CM! What generally happens is that the excessive pressure, exceed the ultimate tensile strength of the cartridge brass (about 35,000 psi) and the case ruptures sending hot gases out in a jet like a cutting torch! This burns through the cylinder walls and the cylinder bursts, generally taking the top off the gun!
This is particularly true in Colt's design guns in .45LC, which may have sloppy chambers. I showed photos in my old book "Trailrider's Guide to Cowboy Action Shooting" (no longer in print).
The question of the loads of Unique in the .44 Magnum depends on what you intend to use the load for...CAS or hunting? For CAS loads 6.5 gr with a cast bullet would probably be about CAS maximum (1,000 ft/sec). I base that on using 7.5gr of Universal...which is slightly faster than Unique behind a 213 gr bullet in .44-40, which has more case capacity (about 6.7% more) than .44 Magnum brass.
One thing that helps to prevent Premature Shot Start is to get good neck tension on the bullet, and a FIRM ROLL CRIMP INTO THE CANNELURE OR GREASE GROOVE!