Grab some of the Level 1 (Grey color code, IIRC) power driver blanks.
Pressure is near nothing, unless you block the barrel with a nail.
The Yellow ones (level 3?) dont expand enough to cause difficulty extracting them by fingernail, when fired in my Hammond Game Getter, where they are only supported by a ring around the rim end.
That's with a swaged down OO buck ball being driven down the barrel of my .308.
Cheers
Trev