Cost efficient would be a chamber in 223 Rem, 6mmARC or 6.5 Grendel on a Howa Mini platform. Plenty of aftermarket upgrades like gunstocks, magazine replacement, bottom metal replacement, etc. But for honing in one’s shooting skills a stock Howa mini rifle in those chambers with 20” or 22” barrel are nice, accurate shooters. They all have correct fast twist rates and offer excellent long range ballistic capability.