The Stiller Actions come with the extended magazine boxes and I haven't had a lot of feeding issues with 7MM WSM
. I have built a dozen 7MM wsm last year and with 180 bergers they get 2950-3000 without pressure signs and thats where Bergers seem to shoot well. The 7mm Mags
have worked better for me with the 160 Accubonds and I have video of one shot kills at 900 yards with that load, so either one is a viable choice.
Good luck with your build.