Paper cup wads for use under nitro powder wads have been around for decades. I remember shooting lots of Winchester ammo with them in 1960s and 1970s. I have old loading book for late 1800s that show their use. They literally function just like the plastic cup wad base of modern one-peice plastic wads. Their function is same, to seal bore against leakage of powder gases, regardless of variations in forcing cone and bore dimensions. Remember, folks have been reaming bores to remove pitts for a very long time. Paper over powder cup wads were a more universal solution than oversized nitro and cushion wads (except in thin brass hulls).
Niklas
