There are two uses for any felt wad: One is to deliver lube to the barrel to keep fouling soft. Tow, is to seal gases behind the wad so they don't cut the patch and ball in front of the wad. Sealing gases also helps insure complete burning of the powder, smaller variations in velocity from shot to shot, and a smaller amount of residue left in the barrel.
If you use a very soft felt wad, or a very thin felt wad, it can allow gas to blow by it, as the gas would be pushing against a soft object( the felt wad) which is turn is NOT supported across the full width of the bore by what is in front of it. If you are shooting a flat based conical bullet, instead of a PRB, that makes a difference, and a thin felt wad, or a soft felt wad works just fine under the base of the bullet, to both lube the barrel, and protect the base of the bullet.
With a PRB, because of the curve of the surface of the ball, you probably want to use a thicker wad so that the wad itself supports its " walls" and keeps the gase sealed behind it. I have not tested the thin vs. thick wad idea over a chronograph, but in principle, a thick wad should seal better.
If you look at the OP wads sold by Circle Fly, they are made of hard paper cardboard. They do seal, even at 1/8 of an inch. I use them in my .50. I have used the T/C " wonderwads" which are felt, and about 1/8" thick in a BP revolver, and they worked well to lube the barrel, but did not seal the barrel from gas blow-by at all. If I used two of them instead of just one, I would get better sealage. I didn't try three, but I suspect using three of them would have sealed the gases very well, and reduced group sizes.
I don't see why the same thing would not happen with felt wads in a rifle.
