Help with .223 bullet choice

[aOWENc]

Hey everyone! This fall were going to wyoming and we are wanting to kill some Prarie dogs, I plan on taking my S&W mp15OR, before i did some thing s to this rifle I wouldn't have dreamed of shooting Prarie dogs with it but after replacing the 3-9 leupold with a 4-16 millett, adding a timney trigger, changing out the wobbly stock with a magpul acs, and swapping out the handguard with a free floated one I feel it is now more than accurate enough for 400 yard shots with factory hornady ammo. I really want to be able to shoot lighter bullets, in the 40 grain range. This rifle has a 1-9 twist and a 16 inch barrel. I'd love to get some information on a good sierra blitzking or vmax load that this twist rate will stabilize at longer range, thanks!!