How much, Im not 100% sure. But IMR 4451 was always touted as the H4350 Enduron replacement with copper fouling eliminator.... I have a bunch of 4451 but honestly Ive never tested it..
But, if this is any indication, IMR 4166 which is the Enduron Varget replacement with copper fouling eliminator... That powder is amazing. And here how close it compares to Varget..
In my 22BR with same brass, same primers, same barrel, same 90smk bullet:
28.7gr Varget 2971
28.7gr 4166 2973
And here's how the 4166 shoots.... So its an exact Varget replacement for me....
I have 8lb of this stuff on the way, Ill get a test loaded up in my 6.5x47L with actual IMR 4451 I have here as well as this new MP 579 IMR4451 equivalent. I guess we will so how similar it is.... I already have extensive data on this barrel with H4350 to compare to....