Help need with a 16fcss .243

As you can guess I have a 16FCSS which I like very much but I'm having trouble getting good consistent groups ie some days I get a ½” inch group at 100 yards then the next a 1” then 2" and back to 1/2" group.
The rifle has a Ase Ultra SE moderator fitted and I’m using a Sierra 70gn blitzking #1507 with 41gn of Vit V140 COL 2.670 cci standard primmer.
It seems if I wedge a piece of .03” plastic under the foreend between the barrel and stock the rifle shoots a ½” group all the time and stays at the same zero point
The scope is a Leupold VX-111 57170 on warne mounts which have no movement at all.
Any ideas ?