I haven't read all of the responses but 223 is the smallest I would use and in fact am using due to physical constraints. Federal Premium ammunition loaded with 60 grain Nosler Partition bullets will do the job. My 1 in 12 twist S&W 1500 (Howa) stabilizes them just fine. Most newer 223 guns will be at least 1 in 9 twist which can stabilize even heavier bullets.
