EdS
My material came from McMaster-Carr, which is local for me and I can pick it up. (They also ship everywhere). The gray PVC sheet was #8747K12, 1/16 x 48 x 96 inches and cost $44.52. They also have other sizes; a 12 x 12 is #8747K111 and is $2.72.
They carry LDPE sheet also. Their #8657K911 is 1/16 x 48 x 96 inches and costs $35.77. The LDPE is a bit cheaper. I looked up the data sheets for it and it looks like LDPE is more flexible and has low tensile strength.
You are right. The plastic wad did NOT hold the bullet to a “hoped for” overall cartridge length. The bullet pushed back into the case even with the wad.
As a way of changing wad sizes without altering the die, I tried heating the strips while punching. When the wads returned to room temperature, they were a bit smaller than when punched when the plastic was at room temperature. I also tried the other way by putting strips in the freezer, then punching. When they warmed up, they were a bit larger.
The .30 caliber people may not have as much problem with gas checks pulling off as I have had with the 6mm. That's why I was trying to size the case necks a bit less, with less tension.