Alliant manual states that 5.4 grains of Unique under heavy 260gr bullet give 760 fps and 19,400 psi.
Quickload program shows that 8 grains of Unique under 255gr bullet of .64" length (approximated from the weight), which is seated to standard 1.275" OAL, give muzzle speed 1147fps and max pressure 32,300psi. This is WAY MORE than max SAAMI of 21,100. See the attached screenshot.
The manual and the program show that the load is way overcharged and should never be used.
You can see on the graph at the bottom of the screenshot that longer barrel does increase muzzle speed, but not too much, around 5% from 5" to 7" barrel. Unique burns fast, and max pressure occurs in less than an inch of bullet travel through the barrel. Therefore, with the charge max pressure does not increase in longer barrels as long as the barrel (less cartridge) is longer than one inch.