Either.223 AI or jump up to a larger bolt face. (.22-250 with custom 1:8/7 twist, .224 Valkyrie, .22 BR, etc.)
Great write up here: http://www.6mmbr.com/223Rem.html
Or stick with .223 Rem because components and barrels are cheap and plentiful.
Small bore, heavy bullets, with any decent velocity...