Unique has been a staple in the 45 LC for, like, forever. I don’t know when it first came out, but it must have been close to WW1. Normally I use 8.5 grains with a 250/255, I have shot thousands of rounds that had 8.0 grains, and based on the previous poster, 7.5 grains must work great. At least here are some velocities in a SAA.
5 1/2" USFA Rodeo
255 LSWC (.452") 8.0 grs Unique thrown, lot 6/21/1993, Starline cases, WLP primers
25-Mar-07 T = 80 °F
Ave Vel = 864
Std Dev = 28
ES = 75
High = 898.8
Low = 800.1
Number Shots 24
I only tried Bullseye once, it was a long time ago, I recall it shot well and a 255 grain bullet going 776 fps is still pretty potent.
M25-7 5” barrel
255 LRN (.452") 6.3 grs Bullseye thrown R-P cases WLP (nickle) primers
26-May-97 T ≈ 75-80 ° F
Ave Vel = 776
Std Dev = 11
ES = 28
Low = 765
High = 793
N = 6
If you really want reduced velocities I would recommend Trailboss. I have not used it, but from everything I have read, the powder is extremely bulky and was made so Cowboy action shooters could load their 45 LC’s so lightly that the recoil is not more than a pop gun.
I am going to get off my butt one day and load some balloon head cases I purchased off a very nice forum member. I read that loaded with 40 grains of black powder, you can push a 255 L at 1000 fps out of a 7.5" barreled revolver. That is the real Cowboy load, and that would be very powerful and extremely low pressure.