It is my understanding that Unique was reformulated sometime in the mid 80's and that older loading data should be reduced by 10% when using the new formulation. 

That said, Lyman 45th edition from 1970 suggests 4.5-5.5 grains of Unique and 7.0-9.0 grains of 2400 for an 86 grain jacketed bullet.

The current Lyman Cast Bullet handbook suggests 4.1-4.4 grains of Unique and 6.3-7.0 grains of 2400 for an 85 grain cast bullet.

If you are loading for a Winchester 1892, be aware that the aintique black powder guns are not as strong as the later models produced for smokeless powder.

Here are a bunch of posts from the Marlin Owners site on 25-20 WCF reloading.  I found this when looking for some loads for my new 25-20 WCF and before I found a pound of AA4100.  Buried in the info is some info on plain base bullets.  As always take any internet reloading info with a healthy dose of concern.   

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Dave

I think after McDonalds got sued for having hot coffee, the world has changed into a paranoid place where one must consult a lawyer before commenting on anything