I shot the challenge again today, but with my 223. It's a Rem 700 with 28" HV Bartlein, sitting in a Manners T6A. NF NXS MOAR reticle on top. I'm shooting Lapua brass with 24.5 gr of Varget behind a 80 gr SMK.
I measured outside to outside and subtracted .224. Groups were 1.866, 2.211, 1.755...