Yeah a 3-9x40 scope would be to much for an M16 a 6x32 would be good but be sure to get a riser mount or you wont be able to see over the front sight. Or you could get a non magnifying scope like a red dot or an ACOG or if you have the money a HOLO sight. Its really up to you but you will have to buy a carry handle mount or riser scope rings to see over the front sight. But if you get an ACOG or a HOLO sight you wont need riser mounts.