I shot several different bullets about 5 years ago with 13.0 gr of 2400 out of a 5" M625. A 240 gr tumble lubed Lee averaged 873 fps, a 255 gr Oregon Trails averaged 1005 fps and a 270 gr custom averaged 925 fps.

2400 is a lot easier to use than Unique and may well perform in an acceptable manner if it was loaded at 12.0 gr.

Some 255 gr bullets seat very deep, like that Oregon Trails, so be careful not to jump your pressure.