Since you asked about "Smokless in a Big Bore", I'll not limit my answer to just single shots but include repeating rifle results as well.
1. My original, smokeless proofed, 45-70, M'86 Winchester does it's best with a 385 grain lead, gas checked bullet cast from Lyman No. 2 alloy, 36.5 grains of Hodgdon Varget powder in Winchester cases with a Winchester WLR large rifle primer. This load in this rifle will shoot 3", 100 yard, 5 shot groups, from a rest with iron sights. The barrel on this rifle has some minor pitting and is a bit rough in places.
2. My Large Frame Colt Lightning, rebarelled to 45-70, does it's best with a 385 grain lead gas checked bullet cast from Lyman No. 2 alloy, 30.5 grains of IMR 4198 powder in Remington Nickel Plated cases with a Winchester WLR large rifle primer. This load in this rifle will shoot 2-1/4", 100 yard, 5 shot groups from a rest with iron sights.
3. My 40-70 Sharps Straight, built on an original Remington Rolling Block action, does it's best with a 410 grain plain based lead bullet cast from 20:1 alloy, 23.5 grains of Accurate XMP 5744 powder, in cases formed from Norma 9.3 x 74R brass with a Winchester WLR large rifle primer. This load in this rifle will shoot 2", 100 yard, 5 shot groups from a rest with iron sights.
As you can see, individual guns are individuals and it takes a bit of experimenting to see what works best for your particular firearm. There are a good variety of powders, primers, cases and bullets out there and after a lot of testing these are what worked best for me in these rifles.
I don't shoot smokeless powder in my 45-70, single shot match rifle so I can't comment on it. It is strictly used with Black Powder.
I hope this gives some guidance as to where to start experimenting.
HHW