I use an over size 58 cal lube saturated wad in my 54 cal GPR with a PRB.
The purpose for using the wad is to add bullet lube to the powder fouling to keep the fouling soft to make easier to push my .535” ball, patch and wad combo down the bore.
Just like a cast bullet cartridge shooter adds lube to the lube grooves of his cast bullet to keep the fouling soft.
I do the same by having a lubed wad between the powder and PRB.
Using a slightly oversized wad causes the wad to form a slight concave dish shape when pressed into the muzzle nicely cradling the base of the PRB.
I have no doubt when the rifle is fired the wad melds to the weave of my patch firmly and is right away firmly taken into the rifling to take all the spin up RPM the rifling has to offer. I like to think the wad also provides a better gas seal for less blow by of the PRB.