AR15 is a good bet.  If you want to make sure it's a one shot target, AR10 using 308/7.62x51 is longer range and substantially more powerful.  A bit bulkier and heavier on ammo and slightly heavier, but nail somebody and it's over for them.  Either one gives semi-auto firepower.  The 15 is much gentler, but I've found target re-acquisition isn't too much different using the 308.  I really liked the 308 in Vietnam.