Most of what your seeing is not un-burnt powder but good old Carbon , a bi-product of the powder burning , just like charcoal from wood .
It should not affect how well the gun shoots in general , lead & copper fouling will be a greater concern for the most part , and most Handi's like a dirty barrel anyhow .
As said either H or IMR 4895 do wonderful in the 45/70 , also don't overlook Reloader 7 , it too does a great job .
stimpy