A standard 6mm PPC sizing die (not a bushing die) is a lot better for necking down cases, and is what I use for all of my 243 LBC sizing. Just don't run the die down to the shellholder; set it up for the correct shoulder bump.
A few loads that work well in my 24" 243 LBC: (copy and paste from my load data, hope the format isn't too hard to read)
24" BHW 243 LBC 95 Berger VLD Hunting Lapua Grendel 2.235 S&B SR CFE223 32.4 2,910
24" BHW 243 LBC 95 Berger VLD Hunting Lapua Grendel 2.300 S&B SR H4895 29.0 2,890
24" BHW 243 LBC 105 Hornady BTHP Win 2.240 WLR Leverevolution 29.5 2,795
