I've had the poly-wads case bulge with new, unfired S'line brass also.
My fired cases look polished inside and out, from using Dave Mauer's media.
I know I'm compressing a goodly amount. Just never thought a 'plastic' wad would bulge a case wall.
Think I was try'n to fix somedangthing that wasn't broke.
Live n learn.....
I'm back to .060 Walters fiber wad.
Gary
I'm run'n Old 'E' 2F in both my .45-70 and .45-90 Shilohs.
The 500 yd targets said that .370-.380 compression was the 'sweet spot' for groups with my PJ C'moor bullet.
G.