I'll leave it to others to talk about 4756, but in my schuetzen days I used a bunch of 4759, mostly in 32-40s but occasionally in 45-70 as well. I don't currently own a 38-55, but it works very well for that cartridge also. I have never used duplex loads in the old high-walls and Ballards I shoot, but I have recipes for about 10% 4759 under a case full of black for a good clean burn. Two problems with 4759 though... it doesn't meter very well in most powder measures because of the stick shape, and it leaves some debris in my bore when I shoot the most accurate loads. I had to find the proper measure for the former, and I don't worry about the latter as long as accuracy stays. I like the stuff enough that there's still a 6# jug and a bunch of pound sized cans of it in the basement!

Froggie