So I guess nobody worries about the old stories of small charges of fast powder in large cases causing detonations?
I would not use Unique in a full size rifle case.
Yes, but Unique isn't all that fast a powder.
Unique is a double base powder like Red Dot and Bullseye, but the burn rate is medium by pistol powder standards. It is no where near as fast as Red Dot or Bullseye. It is in between HP-38/ Win 231 and Power Pistol and it works great in a wide range of handgun loads, as well as in light cast bullet rifle loads.
Unique is a popular cast bullet rifle powder as it does very well at low load densities. As noted above it is quite happy in the .38-55 case and a wad isn't needed until you're up in the .45-70 case capacity range.
No one if forcing you to use it in a full size rifle case of course, but there is a lot of published load data out there for light and medium power cast bullet loads.
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I suspect that the reason why some folks think Unique is a "fast" powder is that it is a faster than the "slow" pistol powders that are more commonly used in cartridges like the .357 Magnum and .44 Magnum - Blue Dot, 2400, H110/Win 296, IMR 4227, etc.
Truth be told however, I prefer Unique in my short barrel .357 Revolvers. 9.0 grains of Unique under a 125 gr XTP will deliver 1300 fps in a 2 1/2" revolver, the same or at worst not significantly less than a full charge of Win 296 - with about 2/3rds the recoil impulse at the same bullet velocity. And 9 grains of a faster flake powder is much easier on the forcing cone than a 21 gr charger of a slow burning abrasive colloidal ball powder like Win 296.